Astronomy/Questions Answered by Expert Courtney Seligman

SubjectDate Asked
A Pure-Energy Universe?2/16/2017
  Q: If all of the matter universe were converted into energy (or if it began as energy and never became ...
  A: Since the total mass of the Universe is only a fraction of the so-called "critical mass", converting ...
Rates of Rotational and Revolution of Planets2/9/2017
  Q: I recently had a series of curious thoughts that eventually led me to your site. Succinctly, they ...
  A: I'm glad that you found the discussion of the Rotation of Mercury useful and interesting; but your ...
Retrograde of Venus2/1/2017
  Q: I understand that the retrograde of Venus lasts approximately 40 days. Is this something that can ...
  A: The beginning and ending of Venus' retrograde motion CAN be seen with the "naked" eye, but due to ...
Supermoon time frames11/16/2016
  Q: The last supermoon of comparability to 2016 happened in 1948. The next one is predicted to happen in ...
  A: Actually, so-called supermoons occur all the time. All they are is full moons that happen when the ...
Telescopes11/14/2016
  Q: Is lens size important to magnification or more so distance between lenses and degrees of convexity? ...
  A: Most "beginner" telescopes have a simple telescope design, but a not terribly useful or easy-to-use ...
Moons role in earthquakes9/22/2016
  Q: Does the moon have an effect on the lava beneath the earths crust like the tidal effect? I have ...
  A: It appears that lunar tides can slightly increase the number of microquakes in the lower regions of ...
Rain in other planets.8/31/2016
  Q: Is there a possibility of rain occuring in other 8 Planets? Thanks Prashant
  A: In general, the other planets cannot have rain in the sense that the Earth does, but there could be ...
Capturing Rainbow Images.8/31/2016
  Q: Can we capture Rainbow Images using both a Telescope and a Digital Camera? Thanks Prashant
  A: You can use a digital camera to photograph rainbows, but you should not use a telescope to do it. ...
AStronomy8/31/2016
  Q: Good Evening, my 9yo daughter asked me a curious question about the EArth travelling around the Sun ...
  A: The time that the Sun rises and sets WOULD change by two hours per month, if we used the rotation of ...
Red dwarf star planet8/25/2016
  Q: A new article describes a planet around a red dwarf star that might have life on it:- ...
  A: Another discussion of the planet says that it "is tidally locked, so one side probably faces the Sun ...
Celestial Coordinate Conversions8/23/2016
  Q: Seligman, When looking at a sky map that shows both galactic coordinates and the plane of the ...
  A: Here is a link to a detailed discussion of how to use spherical trigonometry to convert from ...
Gauging the distant to stars7/27/2016
  Q: I am aware of stellar parallax method. But my problem is that I don't understand how astronomers ...
  A: As you obviously know, the stellar parallax method becomes unreliable for stars at great distances, ...
Black Holes and Neutron Stars4/10/2016
  Q: Neglecting differences in mass, what are the observable differences between black holes and neutron ...
  A: For stellar-mass black holes, there aren't many differences. The normal way of observing a neutron ...
Earth centric?3/25/2016
  Q: Although we've long established that the planets travel around our sun, is it physically possible ...
  A: No, it is not. The mass required for an object to become even a very small star is much larger than ...
planets rotation3/14/2016
  Q: What relationship do you see between a planet's speed of rotation and its length of day? was a ...
  A: Your answer was more nearly correct. The teacher's answer was almost completely wrong. The correct ...
Astronomy/Our Milky way way galaxy1/1/2016
  Q: Our Milky Way galaxy which is a spiral one that supposedly has a black hole at its center in which ...
  A: Although no light can escape the interior of a black hole, even the most massive black holes are ...
How to compute planetary conjunctions, in particular planet-to-star?8/20/2015
  Q: What's the best way to compute planetary conjunctions? Specifically, cases where two planets appear ...
  A: The easiest way is to use planetarium software. My favorite is Stellarium, which is a free download ...
is this meme factual7/26/2015
  Q: A few follow up questions to my first regarding same image. (My bad for not including these with the ...
  A: As noted in my answer to your first question, we can see the Andromeda Galaxy and the Magellanic ...
What galaxy is this?7/26/2015
  Q: the galaxy image attached is identified, albeit indirectly, as the Milky Way. I did an image search ...
  A: The image you attached is an artist's conception of what our Milky Way Galaxy might look like if ...
Space queries6/26/2015
  Q: Hey Courtney No problem Dear. That was a difficult question on a field less known. However, I've ...
  A: I'm not sure what you mean, so will give two possible answers. (1) If you mean do we know what more ...
Mars atmosphere5/28/2015
  Q: With the lowest portions of Valas Marinaris being 7 miles deep, what would the estimated atmospheric ...
  A: The scale height on Mars (the change in altitude required to cause a change in atmospheric pressure ...
Moon's travel5/25/2015
  Q: I live in NJ USA which is about 41 degrees above the equator. I usually see the moon in the ...
  A: The furthest north the Moon can ever get is about 28 1/2 degrees north of the Celestial Equator, so ...
Name of star4/26/2015
  Q: Standing in the Villages,Florida (30 miles south of Ocala, FL) at 11:30 PM on April 24th, there is a ...
  A: My earlier answer was completely wrong (sorry about that). I was in a hurry and tried using some ...
resolve apparent irrationality of gravity4/17/2015
  Q: Why does gravity Appear to violate the logic of the basic physical laws, by generating identical ...
  A: In Newton's version of the answer, the force of gravity is proportional to the masses of the objects ...
Mars / earth oppostions and mars 686 day earth year2/24/2015
  Q: I am trying to understand how to determine Mars' earth year of 686 days. through observation of 33 ...
  A: By a lucky chance, I just finished writing and posting a page about how the synodic period (which is ...
Climate Change9/6/2014
  Q: Earth's orbit around the sun has eccentricity of 0.017 which is close to circle. So if we consider ...
  A: You are correct. The tilt of our axis is much more important than the change in our distance from ...
Planets' Atmospheres7/31/2014
  Q: I have sent this question earlier to Patrick Weiler but he couldn't answer my question and I was ...
  A: The primary reason that Mercury and Mars do not have atmospheres is that they are too small to hold ...
Cameras7/22/2014
  Q: I was watching the new Cosmos TV Series (not a fan of Neil Degrasse Tyson) and it was brought to my ...
  A: It's true that the main thing that keeps us from seeing stars in the city is simply all the ...
stars6/21/2014
  Q: How we know the number of stars in our galaxy and weight of stars and planets
  A: Both answers usually depend upon Kepler's Third Law of planetary motion (see ...
North Star5/29/2014
  Q: Please can you tell me if the location of the North Star is in fact pure North. That is to say is it ...
  A: Polaris is less than three quarters of a degree from the Celestial Pole, which defines true north. ...
Dark sky in Utah area5/25/2014
  Q: I plan to visit Bryce Canyon in March 2015 to take pictures of the Milky way. I'm trying to find out ...
  A: The Moon will be at first quarter on Feb 25 and Mar 27 2015, full on Mar 5 and Apr 4, at last ...
Stars5/15/2014
  Q: What causes the difference in heat and brightness in stars?
  A: For most stars, called Main Sequence stars, their mass determines their brightness and temperature. ...
Getting a grip on gravity!4/20/2014
  Q: Is it correct to think of gravity as a filter/separator of sort allowing the heavier stuff to ...
  A: Gravity just pulls everything downward, regardless of its mass or density (the mass per unit ...
planet core temps4/18/2014
  Q: I enjoyed your article on how Core Temps of planets decline, recently I heard that they had actually ...
  A: There is no evidence that the actual temperatures have increased at any time in recent history. What ...
limiting magnitude3/22/2014
  Q: I have measured the limiting magnitude of a star in the NGC 869 cluster with a B and a V filter. I ...
  A: Limiting magnitude represents the faintest thing you can see with the instrument you're using and ...
Solar System2/20/2014
  Q: Why do all pictures of the solar system show the planet's revolving around the sun in approximately ...
  A: They all go round in nearly the same plane because they were formed in a rotating disk of gas and ...
Astronomy, specifically Earth to Sun relationships1/2/2014
  Q: I understand that if the Earth were to cease revolution around the Sun, it would then be pulled into ...
  A: The only thing that could stop our revolution would be a catastrophic collision with an object ...
constellations12/19/2013
  Q: I know this is probably a silly question, but could you please explain why some constellations ...
  A: It's because as we move around the Earth, we look at the constellations from different angles. When ...
Why does the Hill Sphere work?12/15/2013
  Q: According to Wikipedia, Hill Sphere is : the volume of space around an object where the gravity of ...
  A: I can't remember whether I sent you an answer, or merely promised to reply when I had time; but it ...
Light through the universe12/9/2013
  Q: If an object supernovas 4 million light years from Earth, the light we see is 4 million years old, I ...
  A: You're correct in assuming that the onetime supernova is long "gone" by the time we see it. In fact, ...
When is the sun in zenith in my place?11/10/2013
  Q: I live in south of Thailand, 7 deg 19 min North. I have tried to calculate when the sun will be in ...
  A: For 2014, the answer is April 8 or 9. To find out the answer, you need a table of the Sun's ...
cool temperatures in planet Mercury ?9/17/2013
  Q: Seligman, It would be possible, considering the low axial tilt of the planet Mercury (2,1 ...
  A: There isn't a latitude where normal temperatures are "moderate". Even at the Pole, if you were in ...
Exo-Planets9/12/2013
  Q: I've been keeping up with all the exo-planet news and have been particularly interested in the news ...
  A: (1) Kepler detected possible planets by looking for changes in the brightness of a star as a planet ...
"Roche limit" in astrophysics9/8/2013
  Q: Seligman, I would like to know why the "Roche limit" ( or "Limit of Roche" ), doesn't work in ...
  A: In the case of Enceladus, there seems to be some kind of misunderstanding. It is twice as far from ...
Solar System9/2/2013
  Q: Has anyone ever made a model of the solar system to scale on earth? I have been doing some 3D ...
  A: There is one in the (Rose?) planetarium lobby in New York that is to scale in terms of the orbital ...
Astronomy - Suns8/20/2013
  Q: I saw on a show on the Science Channel that when a sun produces iron, it begins to die. Does the ...
  A: When very massive stars start to produce large quantities of iron they "begin to die", because iron ...
big bang7/28/2013
  Q: Would you mind briefly stating the evidence for the big bang. Specifically, does the CMB truly rule ...
  A: The Cosmic Microwave Background (CMB) was the beginning of the end for the Steady State theory, as ...
Big Bang7/10/2013
  Q: I believe 'we' feel we know nucleosynthesis quite well in stars. Are we as confident about the ...
  A: The Big Bang didn't produce elements heavier than helium in significant amounts because you need a ...
planetary astronomy7/6/2013
  Q: Over the following billions of years how much more massive will Earth have become through accretion?
  A: Even over billions of years, the Earth is not likely to gain any significant mass. Most of what we ...
stars5/21/2013
  Q: If a star burnt out, at several million light years away, how long would it take for humans to ...
  A: Exactly the same number of years as its distance in light years. So, for example, if we see a star ...
Constellation Perspective5/21/2013
  Q: Mr. Seligman, I have but one question, and that is about the perspectives at which we view ...
  A: They would look completely different. If we moved through space, at first very distant stars might ...
Black holes5/16/2013
  Q: My question for you today is about how big does the remaining core of a supernova have to be to ...
  A: The limiting mass is determined by that for neutron stars, which cannot be more than 2 to 3 times ...
How do Stars Shine?5/15/2013
  Q: I know that stars shine because of a process called fusion, but I don't know what fusion is?
  A: Fusion is the combination of smaller atomic nuclei to make larger ones. The energy source for most ...
Stars5/15/2013
  Q: Can a Super Nova wipe out a planet?
  A: Yes, if the planet is in orbit around the star that becomes a supernova, and is reasonably close to ...
stars5/15/2013
  Q: So i asked a question about if the sun was hotter on the inside or the outside, and you said normal ...
  A: Perhaps I should have said that <i>all</i> stars increase in temperature as you go inward. However, ...
universe5/14/2013
  Q: how old is space
  A: Our Universe was formed a little less than 14 billion years ago (calculations of the exact age ...
sun5/14/2013
  Q: Is the sun hotter on the inside, or the outside of the sun?
  A: The Sun is much hotter on the inside. At the surface it is only about ten thousand Fahrenheit ...
sun spots5/14/2013
  Q: do sun spots effect temperatures here on Earth?
  A: Individual spots rarely if ever affect surface temperatures. But flares and coronal mass ejections ...
Astronomy5/10/2013
  Q: Why is too much sun bad for our skin? I thought the vitamin D that the sun gives off is great for ...
  A: The vitamin D produced by exposure to sunlight is good for us. But it only requires a few minutes a ...
Black Holes5/10/2013
  Q: If black holes are so small, how can they contain all of the matter they pull in?
  A: Simply because they <i>are</i> so small. Any object with mass exerts a gravitational force on ...
Astronomy5/9/2013
  Q: Are sunspots harmful to our skin?
  A: No. The higher-energy blue, violet and ultraviolet radiation that penetrates our atmosphere and ...
Stellar Aberration4/6/2013
  Q: How did Bradley allow for the effect of the earth's rotation about its own axis? The star's posiotn ...
  A: Bradley didn't have to consider the rotation of the Earth, because his telescope could only make ...
Target list by size for astrophotography3/30/2013
  Q: I use a variety of telescopes and cameras and am looking for an extensive list of targets such as ...
  A: I still haven't found a list of objects sorted by size, but there are a number that are sorted by ...
CMBR2/21/2013
  Q: I understand that the hotspots in cosmic background radiation are said to be where matter first came ...
  A: (Sorry for the delay. I was busy yesterday afternoon and evening, and didn't receive your note until ...
is black hole new dimesion?2/20/2013
  Q: so iv come to a couple conclusion that inside the black hole is another universe,and our universe ...
  A: When black hole theory was very young the mathematical theories involved required simplifying ...
black hole2/19/2013
  Q: i have a question on black hole,some prof says blackhole is a new dimesion some say its empty what ...
  A: Black holes are regions of essentially normal space, except for one thing. At their center they have ...
Lunar Sightings2/5/2013
  Q: I live in New Zealand. Manukau to be precise. It is now 10.15 am, I can see the sun to my right and ...
  A: Yes, it is normal. The only difference in England would be that the directions are reversed. You ...
The "look-back time" concept2/1/2013
  Q: I am a chemistry undergraduate student. I have recently encountered the concept of "Look-back time" ...
  A: I have to leave in a few minutes and won't be back until this evening. At that time I'll send a more ...
visible light1/29/2013
  Q: Courtney, I understand that we see the cosmic background in microwave(CMB)because, due to the ...
  A: The amount of redshift depends upon how close to the CMB you are looking. At well over 13 billion ...
Question Galaxy1/29/2013
  Q: Good day I recently read that there are more than 100 billion galaxies in the universe. 1. How and ...
  A: The number of galaxies in the Universe is estimated from counts of galaxies in a small part of the ...
Supermassive Black Holes1/27/2013
  Q: Is it possible that supermassive black hole at the galactic core would eventually swallow all matter ...
  A: I just noticed this question in the Question Pool (that is, the list of questions that were not ...
Light from distant objects..1/8/2013
  Q: Heres a problem that Ive been struggling with for some time: We know that when we look at a distant ...
  A: In the context of this question, the Earth was in the same place two billion years ago as right now ...
Meteorite?1/7/2013
  Q: My daughter picked up this rock while rock hunting in Lake Havasu, Arizona. It is 3 inches long and ...
  A: It could be a stony meteorite, but there is no way to tell except having an expert take a look at ...
The object inside the circle is GRB 090423, the earliest supernova explosion observed as of April 23, 2009.1/4/2013
  Q: Fascinating subject. The fact that we see this object at all must mean it has receded from us on ...
  A: Before starting, I should note that the numbers on the webpage in question ...
NGC pics1/12/2012
  Q: Following up on my last question. The software I'm writing is for use on my personal laptop. The ...
  A: Here is the page with information about accessing the Sloan Digital Sky Survey database: ...
black holes12/5/2011
  Q: What happens to the objects that are swallowed by black holes
  A: They fall into the singularity (the point of infinite density, temperature, and gravity) at the ...
big bang/void11/29/2011
  Q: When the big bang occurred, with all the universe and space/time in it, what was outside the big ...
  A: Presuming the origin of the Big Bang is as generally thought -- a single point of space-time in some ...
can meteors change direction?10/30/2011
  Q: I am having an argument with people who think a particular video shows a UFO, when I think that it ...
  A: Meteors (or more accurately, the meteoroids that produce meteors) can change their path, usually as ...
A thought experiment10/20/2011
  Q: I wonder if you could attempt an answer to this thought experiment I am presently struggling with. ...
  A: I'm afraid the answer is no, we can't look into our past. When we say that we can look "into the ...
the whole shabang10/17/2011
  Q: As an amateur cosmologist I come across much talk of multiverse, dimensions etc. It seems to me that ...
  A: (Note added partway through answering yourThe start of this answer is about a definition of hidden ...
Lunation through the ages?9/20/2011
  Q: Greetings! Basically my question is if a real month (the time between let's say 2 consecutive full ...
  A: The tidal interaction between the Earth and Moon is gradually slowing the rotation of the Earth (by ...
titans9/10/2011
  Q: Titan, one of Saturn's many moons, has an atmosphere, but our own Moon does not. What is the ...
  A: One reason is that Titan is ten times further from the Sun, and as a result, far colder. This allows ...
Energy and the expansion of the universe8/29/2011
  Q: Okay, so I have had a question that has been bugging me about all energy in the universe being ...
  A: As it happens, the solution to Olbers' Paradox isn't that the Universe is expanding, but simply that ...
Relationship of distance and time8/27/2011
  Q: I have read and understand your answer of 8/13/2011 to the question about the age of the universe ...
  A: (Sorry for the delay in replying. I wrote an answer several hours ago, but when I tried to 'send' ...
BIG BANG8/20/2011
  Q: i was watching this documentary on BIG BANG on discovery channel and there i heard a couple of ...
  A: "hello sir,i was watching this documentary on BIG BANG on discovery channel and there i heard a ...
Universe8/17/2011
  Q: From what I know (which is probably wrong), the way we have discovered that the universe is 13 ...
  A: In its simplest form, the age of the Universe is determined from the rate of its expansion, compared ...
age of universe8/13/2011
  Q: How is it possible that the light from the cosmic horizon is older than the age of our universe?
  A: It isn't possible. Wherever you heard that has to be wrong. The cosmic horizon represents the time ...
mars and earth7/28/2011
  Q: Sir, If there is any orbital changes between earth and mars and mars comes close to earth. Is any ...
  A: It would depend upon how close it came. At the moment, the closest it can get is about 35 million ...
astronomy7/25/2011
  Q: sir, I wont to know what is the difference between astronomy,astrophysics and cosmology? and ...
  A: Astronomy is the entire field of astronomy. It involves anything that has to do with the sky and its ...
Elliptical Orbits7/21/2011
  Q: Why all the planets and stars revolve around everything in an elliptical orbit rather circular? If ...
  A: The expansion of the Universe that started with the Big Bang is not related to the orbits of planets ...
Habitable planets7/15/2011
  Q: I have been using the build your own solar system gimmick on this website:- ...
  A: The numbers are "off", as are those for a 4-planet system more or less identical to Mercury, Venus, ...
orbits5/17/2011
  Q: You recently posted a question to Patrick Weiler in category Astronomy:hello..in his first law, j. ...
  A: Just wanted to let you know that I have not forgotten you, and am working on a page about orbital ...
The earth5/17/2011
  Q: We would like to ask a question. If there is no air in space, how come there is air on earth? ...
  A: When the planets formed, there was a lot of gas moving around the Sun, in what is called the Solar ...
Venus5/13/2011
  Q: I've been trying to get information about Venus' gross atmospheric loss rate. Specifically, how ...
  A: The rate of loss of low mass atoms, such as hydrogen, is accelerated by the effects of the solar ...
View of Earth from Moon4/25/2011
  Q: I enjoy doing little brain exercises, but I cannot find enough info on the web to satisfy my ...
  A: To a first approximation, the Moon moves north and south during one orbital period (27.3 days) the ...
Satellite tracking4/18/2011
  Q: Just saw 2 satellites pass overhead (both visible with naked eye simultaneoulsy) at approx 22.55 ...
  A: Try Heavens Above, at http://www.heavens-above.com/ You just have to enter your position to ...
Astronomy4/16/2011
  Q: Do scientists know yet what started the planetary straight line movement and do scientists know what ...
  A: We know how gravity works (in a simple way, as described by Newton, and an equally simple but ...
universe4/15/2011
  Q: I just wanted to know what is the biggest things right away? I thought that would a universe.
  A: It depends upon what you mean by "thing". The Universe isn't an actual object, in the normal sense ...
Coriolis and the Foucault Pendulum?4/15/2011
  Q: Almost every reference explains the apparent rotation of the Foulcault Pendulum on the coriolis ...
  A: I take it from the way you phrased your question that you understand the situation at the Poles -- ...
astronomy4/13/2011
  Q: Why orbits of all planets are elliptical?
  A: Your question doesn't make it clear whether you want the physics behind the answer, or just why they ...
keyholes4/4/2011
  Q: I have a question regarding magnetic keyholes. This will sound crazy at first but please read all of ...
  A: I believe you mean gravitational keyholes, not magnetic; but either way, the only thing that affects ...
Planetary possibilities in a binary star system3/27/2011
  Q: I had asked this question of another expert, but he mentioned his knowledge of orbital mechanics ...
  A: You're basically asking if you could have a planet orbit between two binary stars at the same rate ...
Gas Giants3/26/2011
  Q: I was curious, what exactally is a Gamma Ray Burst? I saw them on a television documentary once, but ...
  A: A gamma-ray burst is a short-lived emission (hence a "burst") of extremely high energy photons (or ...
super moon3/25/2011
  Q: Mr. Seligman: I was hoping you could speak to me about the relationship between the moon and tides. ...
  A: The super full moon (or for that matter, any full moon which takes place near perigee) is only about ...
More Questions3/25/2011
  Q: Its me again, the one who asked all those questions about astronomy before ...
  A: If the two stars are close together, you could have planets far away from them, which orbit them ...
Filament Universe, Millennium Simulation3/15/2011
  Q: Seligman, What does your expert mind conjure when viewing the Millennium Simulation produced by the ...
  A: I haven't seen the simulation you refer to, but will look it up, as it sounds very interesting. ...
Computation of Planetary longitudes3/13/2011
  Q: , Read a lot of information on the recent earthquakes and shift of earths axis etc. I wonder if ...
  A: It will have no effect on planetary longitudes. The orbital motions are calculated in comparison ...
Questions3/10/2011
  Q: I love the universe, and think that it is the most interesting subject out there. Out of curiosity, ...
  A: (Added answers to last two questions, at the end) Iron is the most stable element. When atoms ...
Earth's spin and Sun2/26/2011
  Q: The Earth has a sidereal rotation t=86164.09 seconds and when Earth makes 360° around its axis, it ...
  A: "The Earth has a sidereal rotation t=86164.09 seconds and when Earth makes 360° around its axis, it ...
rotating universe follow-up2/25/2011
  Q: I completely understand your last 2 paragraphs. It is virtually impossible to have access to ...
  A: I've been ill recently, and haven't had a chance to take a look at any of your recent posts (I ...
Apparant Magnitude2/24/2011
  Q: I need some help on a question in my astronomy class, I've tried to understand this and figure it ...
  A: Somewhere in the textbook discussion of magnitudes it should point out that each magnitude ...
A balloon pops, a gun fires -- Our Universe more similar to2/19/2011
  Q: Weiler, Why is the structure of the Universe like a cone. Why is it not in 360. Why isn't ...
  A: Apparently Mr. Weiler put this in the Question Pool, which is where I just noticed it. Answering ...
Expansion vs closure2/16/2011
  Q: In your 10/27/2009 answer about expansion of the universe you used terms like 'the center' and 'the ...
  A: Reviewing the answer I think you are referring to, I don't see any reference to a center or an edge, ...
Electromagnetic radiation and heat2/14/2011
  Q: I picked up a NASA brochure on electromagnetic radiation at the recent AAS conference in Seattle. It ...
  A: Individual photons of light do have more energy, the shorter the wavelength of the light. But the ...
galaxy and planet rotation2/13/2011
  Q: like our northern and southern hemisperes in relation to tornados and cyclone rotation.do planets ...
  A: No, they don't. The Coriolis effect which leads to the phenomena you mention on Earth is due to the ...
Cosmic inflation2/9/2011
  Q: Courtney, My understanding is that the inflation theory related to the big bang was devised ...
  A: First, a summary of how the Inflation theory was developed: The original theory of the Cosmic ...
hydrogen fusion on the sun2/7/2011
  Q: The sun fuses hydrogen to make helium on the sun. Does the helium fuse with other helium or hydrogen ...
  A: Lithium is produced in the Sun (and other stars and, for that matter, in the Big Bang); but it has a ...
the universe2/6/2011
  Q: We know that our universe is expanding. Into what kind of infinity (rhetorical)? What do you think ...
  A: The most commonly used example of how the Universe works is to imagine the surface of a balloon as ...
Earth-Moon-Sun alignement2/6/2011
  Q: since Earth's and Moon's orbital inclination compared to ecliptic oscillates with time, what's the ...
  A: To have a lunar or solar eclipse, all three objects have to be in line. At such times, it doesn't ...
Time1/29/2011
  Q: I have a young child that keeps asking me when time began and when it will stop. As I'm not a ...
  A: You are correct in assuming that (according to current scientific theory) time began with the Big ...
Weight at pole and equator1/27/2011
  Q: Does our technology enable one to measure in a decent way the difference of weight (that's provoked ...
  A: The difference is about a third of a percent of the weight of the object, so you just need something ...
origen of water1/26/2011
  Q: Courtney, I very much appreciated a previous answer from you. My question today is related to water ...
  A: If there were water molecules in space, they would be blown away from our orbit by the solar wind, ...
Sunrise-sunset changes1/21/2011
  Q: Beginning at the winter solstice. Why do the change in sunrise and sunset differ. In Seattle, after ...
  A: Added note, in response to feedback: I'm sorry that my answer wasn't as clear as it could have been. ...
Moon sidereal period1/19/2011
  Q: An mere observer can know how long the Moon takes to do 1 synodic period by looking its phases,but ...
  A: To measure the sidereal period, note exactly where the Moon is relative to the background stars on ...
Astronomy1/14/2011
  Q: So many societies have used astronomy to establish time and calendar. I have read, that circa ...
  A: The quote you have read about 12/21/2012 is a hoax. The planets will not be lined up with anything, ...
Eccentric Orbits on Earth-like Planets12/17/2010
  Q: In the old 60s TV show "Lost in Space" the Robinsons landed on a planet with some pretty extreme ...
  A: No, there's no way that orbital effects could cause such changes. You could have very high ...
Hypothetical Earth-Moon Planet12/14/2010
  Q: Mr Seligman I enjoy your answers so much I wish I could have you as a teacher. Your classes must be ...
  A: As for your question, I'm afraid you're going at things backward, starting with a combined radius. ...
Asteroids and Magnitudes12/12/2010
  Q: again. I've just finished reading about 'zodiac lights', and I was wondering... I know that ...
  A: The key to seeing the zodiacal light (and the counterglow, or gegenschein, which is opposite the ...
Jupiter's influence12/12/2010
  Q: Mr Seligman. My question has two parts. First, it concerns Jupiter. I understand that the ...
  A: The eccentricity of an orbit is due to a planet's current motion. If outside influences (anything ...
follow-on to question entitled 'cosmology!'12/3/2010
  Q: some further points,if i may. You gave me a run-through of quantum mechanics,which i understand ...
  A: (Sorry about the delay in answering you. I've been swamped with work for a project that was due ...
why does the night sky (universe) look black11/29/2010
  Q: it seems to me that since black is the result of all light wavelengths being absorbed, and since ...
  A: The basic problem with your statement is that black is not the result of all light wavelengths being ...
Question about a certain cluster of stars/constellation.11/27/2010
  Q: I've been outside in the past few weeks and I've seen this same cluster of stars in the sky, and I ...
  A: That sounds like Orion, which is rising nearly due east in the early evening. If so, there would be ...
Morning sky observation11/23/2010
  Q: I live in Southern CA; this morning I got my scope out to look at what I thought was Venus in the SE ...
  A: Venus goes through phases, just like the Moon. A few weeks ago it was more or less in between us and ...
Moon orbit VS Earth angular velocity11/22/2010
  Q: If a tiny ball is stuck thanks to a glue on the border of a spinning disk, the quicker the disk ...
  A: I haven't seen the forces involved expressed as actual forces, but as accelerations (that way, you ...
Sun influence on Moon11/19/2010
  Q: So since Sun's acceleration on Earth is equal to Sun's acceleration on Moon, does this ensure that, ...
  A: That's correct. Whether the Sun exerted a force on the Moon or not, the Moon's basic orbit would be ...
galaxy11/15/2010
  Q: I have been reading a lot on the origin of the universe and often come across the reference to the ...
  A: The redshift is caused by the expansion of the space the light is passing through, on its way here. ...
cosmology!11/10/2010
  Q: I have a number of points that i would like your comments on please. Dark matter has invoked various ...
  A: (Finally have time to finish the answer. See the additional material below the dashed line.) As far ...
Influence of velocities11/7/2010
  Q: Let's suppose a motionless planet called A and its satellite B. B revolves around A at the velocity ...
  A: The orbital speed of the planet does not affect the orbital speed of a satellite. This is a ...
big bang11/4/2010
  Q: When scientists talk of the big bang they say expansion started from a size 'smaller than a spec of ...
  A: You are correct in saying that we only know the size of the observable Universe, not its absolute ...
irregularities in space logic11/3/2010
  Q: How is it that meteor craters are perfectly formed with regular sides . It stands to reason ...
  A: (Note added afterward, in response to a followup comment about the effects of a lack of atmosphere: ...
Doppler effect and light10/29/2010
  Q: I am a lawyer with a healthy interest in science. I read the popular science books and magazines. ...
  A: The expansion of the Universe is caused by the expansion of the empty space within it. On the ...
northern latitude visibility for alpha/beta Kentarus10/18/2010
  Q: I am doing a research project that involves ancient Egypt and Greece. In my Starry Night program ...
  A: The stars are not visible from Athens at this time. Athens is nearly 38 degrees north of the ...
Big Bang10/4/2010
  Q: I hope I'm right in thinking that when an explosion takes place, there is an inwards force as well ...
  A: With a "bomb" of finite size, the central portion could be compressed by the force of the outer ...
asteroids10/1/2010
  Q: Courtney, I often hear of asteroids as being pebbles and rocks and boulders that never became a ...
  A: In the early solar system, any rocky body more than a couple of hundred miles across would have ...
Galaxies Speeds etc9/25/2010
  Q: So say Alpha Centuri is 4.5 light years away and if i was to travel there at light speed its a case ...
  A: You are correct in that when you travel at the speed of light, time "seems" to stand still for you, ...
black holes9/20/2010
  Q: Has a star been observed falling into a black hole and disappearing?
  A: No. We have seen ordinary black holes emitting high energy jets of material, which must be the ...
Planetary Science9/12/2010
  Q: Assume there are two planets at equal distance from their sun. Planet #1 is markedly hotter than ...
  A: The main factor would be what kind of difference there is in their atmospheres. If they had the same ...
Effect of atmospheric friction on low Earth orbits9/6/2010
  Q: Why does atmospheric friction increase the speed of a satellite in low Earth orbit?
  A: The direct effect of the friction is to slow down the satellite. But this makes the orbit "decay", ...
The gravitational relationship between the Earth and the moon9/3/2010
  Q: I am currently watching the program 'Universe' on the Tv and i am always interested in working out ...
  A: The Moon's pull does have an effect on the Earth -- it makes the Earth move around the center of ...
Matter and antimatter8/13/2010
  Q: After big bang , both matter and antimatter created in same ratio, than why we see only matter ...
  A: They were created in almost, but not exactly the same ratio. Estimates are that the ratio was off by ...
Light8/13/2010
  Q: You told in previous question that speed of every colour in vaccum is same but we know that ...
  A: All light waves (or if light is thought of as particles, all photons) move through empty space at ...
Light8/12/2010
  Q: Why light didn't disperse into seven colour in vaccum or air ? Why it also didn't disperse from ...
  A: Dispersion is caused by a difference in the speed of different colors of light, as the light passes ...
Light8/8/2010
  Q: Sir as you told previous that light bends due to the difference of refractive index but i think that ...
  A: As noted in my previous answer, the refractive index (the ratio of the speed of light in a vacuum to ...
Light8/6/2010
  Q: Sir, i read somewhere that bending of light is created due to difference in eletic and magnetic ...
  A: (In the last sentence of paragraph two of my original answer, I mis-stated the way that the light ...
Universe and its beginning8/5/2010
  Q: I can't manage to grasp something in what many cosmologists state: according to them, the Universe ...
  A: As far as our own space time is concerned, there is a beginning -- a time when there was only a ...
Space and big bang8/5/2010
  Q: Which of the following first created , big bang or space time?
  A: As noted in my previous answer, our Universe presumably formed as the result of a quantum ...
Before big bang8/5/2010
  Q: What was before big bang and how it created?
  A: According to quantum mechanics, any point in space can undergo random fluctuations in energy, as ...
black holes7/30/2010
  Q: Depictions of black holes are usually funnel-shaped, "down-the-drain" illustrations of warped ...
  A: Yes, it would be more spherical than not. There are a couple of complications that would affect the ...
gravity7/30/2010
  Q: It's generally understood that people weigh less in the morning than at night. I've read this is due ...
  A: I'm afraid that if it did, you couldn't tell, because the effect is so small; and it actually works ...
21 Aug 20177/24/2010
  Q: Seeing a total solar eclipse is in the top ten of my "bucket list," a list of things I'd love to do ...
  A: I presume that astronomy magazines (physical and online) will be full of weather predictions in the ...
solar/galactic plane7/21/2010
  Q: Courtney, Is there any relationship between solar ecliptics in galaxies (particularly in the Milky ...
  A: (Sorry about the delay. I thought I already answered this question (yesterday?), and presume I had a ...
Type 1a Supernovae7/20/2010
  Q: I have a couple of questions about the role of angular momentum in type 1a supernovae. First, does ...
  A: Type 1a supernovae result from the ignition of nuclear reactions in a white dwarf companion to a ...
Einstein's Cosmological Constant7/14/2010
  Q: I am reading Brian Greene's The Fabric of the Cosmos & managed to get confused about his description ...
  A: Einstein's original equations predict that if the mass-density of the Universe is greater than a ...
star/planet id7/8/2010
  Q: I was looking west from Wailea in Maui at 8:30 last night ( July 7, 2010) and there was an extremely ...
  A: That was Venus. It will be gradually getting lower and setting earlier over the summer, but will ...
Universe is eternal?7/7/2010
  Q: could the Universe have been beginless, or eternal (thus having actually INFINITE PAST)? some ...
  A: Standard cosmology (the study of the origin and evolution of the Universe) treats the Universe as ...
the primordial atom7/4/2010
  Q: I time, space and the primordial atom are frequent topics but I am unable to answer a specific ...
  A: I understand what you mean, but you are assuming that the Universe existed as a specific point-mass ...
Speed of Light7/1/2010
  Q: I usually hear that the speed of light is 186,000 miles per second vs the exact 186,282 miles per ...
  A: Whether the more exact measurement is used depends upon the accuracy needed. The rounded off value ...
seen from space6/28/2010
  Q: what type of lights or man made structures or for that matter any type of structures can be seen ...
  A: I presume you mean without optical aid... From low or near Earth orbit, city lights can be easily ...
Nostradamus6/19/2010
  Q: As a follow up question, a massive object ould pull the surface of the Earth like the moon does with ...
  A: I presume you mean, "could it also pull the atmosphere away from the planet". The answer to that ...
Black holes6/12/2010
  Q: If light has no rest mass, why is it attracted and captured by black holes?
  A: Light isn't "attracted" by black holes (or gravity). But according to Einstein's theory of general ...
Hubble Constant6/10/2010
  Q: Hey Courtney, I'm doing a culminating activity for my astronomy class and I'm having a little ...
  A: I'm not supposed to answer homework questions, but I can give you a couple of brief comments, and a ...
Galaxies6/7/2010
  Q: How long does it take for a galaxy to be formed? also, what are the steps to forming a solar system? ...
  A: For solar systems, refer to http://cseligman.com/text/ssevolve/ssorigin.htm (The Origin of the Solar ...
Ref question6/3/2010
  Q: Good day What cause the earth to spin? I am aware that the earth spins at a certain speed. How was ...
  A: The rotation rate of the Earth (approximately 23 hours 56 minutes 4 seconds) is determined by seeing ...
start escaping galasies5/24/2010
  Q: I just saw a show on astronomy, specializing in black holes. It stated that for a star to escape a ...
  A: To answer part B of your question first, yes, in a sufficiently crowded environment, single stars ...
designing a scarf with contellations5/19/2010
  Q: I am designing a scarf for a small charity called "Afghan Hands" to benefit tribal woman in ...
  A: I think that what you want can be found at http://www.astroviewer.com/ The site allows you to ...
big bang5/4/2010
  Q: What caused the big bang? I understand it to be a singularity that through gravity eventually ...
  A: In normal physics, empty space would be considered to have nothing in it -- neither mass, nor ...
Black Hole Singularity Formation4/1/2010
  Q: How exactly do black hole singularities form, when considering the effects of the zero gravity core ...
  A: (Sorry for the delay in answering. I had an appointment this morning, and just picked this up.) ...
Expanding universe4/1/2010
  Q: We see distant galaxies receeding away from us, doesn't this indicate that the universe is ...
  A: You have to make a distinction between what we can see, which is called the Observable Universe, and ...
Massive stars, supernovae, black holes.3/29/2010
  Q: Courtney, I hope you can help me understand a couple of empty area in my knowledge, about the death ...
  A: (1) Iron-core collapse: The maximum density of a star (or a region in a star) is usually determined ...
Follow up on asteroids and comets.3/13/2010
  Q: Christ!!! That wave from the asteroid that took out the dinosaurs to sweep over all the carribean ...
  A: (Note added March 16) I reread your question, and realized I forgot to answer the last part of it ...
Is there really a planet X?3/10/2010
  Q: I've been reading recently about theories for the end of the world in 2012, and apparently there is ...
  A: There is no planet X, in the sense discussed by those who believe in Nibiru. That is a fairy tale ...
Mt. Krakatoa3/8/2010
  Q: How many deaths/injuries happen during the 1883 MT. Krakatoa eruption in Indonesia?
  A: My internet connection failed as I was sending my reply, so I don't know if you received it. The ...
Hoag's Object3/5/2010
  Q: When you look at a picture of Hoag's Object you can clearly see the blue stars in the ring, but in ...
  A: The yellow object in the middle is the nucleus of the galaxy, and consists not of a single star, but ...
which way is up?3/2/2010
  Q: i have always wondered. is there an up down left and right when you reach outer space? let me ...
  A: You have pretty much answered your own question, but perhaps the following will help clarify things, ...
Moon2/27/2010
  Q: Why doesn't our 'Moon' spin on its axis ( rotates ) like the moon's of Jupiter and Saturn?
  A: Our Moon does rotate. It just rotates synchronously -- that is, it rotates once, each time it goes ...
space-time2/26/2010
  Q: since it has been proven that motion is relative to time and movement no mater how little actually ...
  A: Motion does not cause you to travel into the future. It simply changes how you age, as you do so. If ...
moons of Mars2/17/2010
  Q: /bows thank you for your time. I never see rover pics from Mars at night, but I love the sunrise ...
  A: I think they power down the rovers at night, to save on battery power, and because it gets so cold ...
The stars we see2/15/2010
  Q: How much closer to earth were the stars that we see at the time that the pyramids were being built!
  A: For all practical purposes, the stars were at the same distance then, as they are now. For one ...
Influence of Sun on sattelites2/12/2010
  Q: i'm 17 and curious about astronomy. I'm wondering: does the Sun an influence on planets' satellites ...
  A: Basically, the answer is no, for three reasons: (1) The Earth's gravity is the main cause of the ...
stars2/5/2010
  Q: Why each planet differs from each other in their content? Both are formed due to dust clouds. ...
  A: What different planets are like depends upon the temperature in the region where they formed. The ...
near light speed1/29/2010
  Q: How much mass would a photon effectively have at 99.995% the speed of light. How about an electon. ...
  A: Photons have no mass, and only move at the speed of light; so the first part of your question has no ...
universe age1/28/2010
  Q: As far as i am aware we have dated the universe to be approximately 14 billion years old,depending ...
  A: Until the last decade, the age was estimated by assuming a rate of expansion equal to the current ...
Planet orbital intersections1/24/2010
  Q: I can visualize how the planets corkscrew through space relative to the sun's moving path. My ...
  A: The Sun's motion through space is much faster than any of the planetary motions, and is at an angle ...
the sun1/14/2010
  Q: is it possible that the sun could have been a planet?
  A: it was considered a planet, when the term planet meant an object that moved around in the sky, ...
Sun & Moon Eclipses1/13/2010
  Q: Moslems are praying during Sun %26 Moon Eclipses! do you think there is a resone for their afraid? ...
  A: The gravitational effects of the Sun and Moon on the Earth are the same at every new moon, whether ...
White Dwarf stars1/13/2010
  Q: I was wondering if a white dwarf star was theoretically capable of supporting a living planet such ...
  A: The density of the star wouldn't make any difference -- just the size of the planet's orbit, and the ...
the iron1/6/2010
  Q: some people are saying the iron is not originally from earth! is it true??
  A: I've never heard of such a thing, so I don't know why they would say that. But there is no doubt ...
Light Years1/6/2010
  Q: I am a biology major but I've always been fascinated with astronomy, too. I've been following the ...
  A: For distances such as 13.2 billion light years, we use the assumption that the rate of expansion of ...
Sun Chasing speed1/3/2010
  Q: I am a sculptor and I have this idea for a kinetic sculpture that the top needs to be under the sun ...
  A: If I understand your idea, the lower part of your sculpture will rotate to the "east" as the Earth ...
black holes12/25/2009
  Q: We have 'calculated' expansion rates of the universe. We have observed black holes and learn so ...
  A: The idea that the Universe is or could become a black hole corresponds to the Closed Universe ...
Universe12/12/2009
  Q: Is the universe finite or infinite? If it is finite, how big is it?
  A: It is half-infinite, meaning that it presumably has some finite size now, but will be infinitely ...
Mayan Calendar12/3/2009
  Q: How is it possible that the Mayans could have accurately predicted the aligning of plants? What ...
  A: The planets will not be aligned at all in late 2012, but will be scattered all along the Ecliptic ...
Lunar rilles11/2/2009
  Q: i saw your answer about Lunar rilles but the other expert said "The surface of the moon is replete ...
  A: I read the answer I believe you're referring to, and the page you linked to. The latter makes a ...
expansion of the universe10/26/2009
  Q: !! Here is myGiven that the expansion rate of the universe is 74.2 ±3.6 ...
  A: The problem is your units (and a probable decimal place error). The "observable" Universe is 13.7 ...
Big Bang Theory10/12/2009
  Q: Was there at least one inward secondary shock-wave during the initial expansion from the big bang, a ...
  A: There wouldn't be any reflected wave, because that requires a surface to reflect off of, and no such ...
any scenario of seeing Neptune or maybe Jupiter's moons with the naked eye?10/5/2009
  Q: I know all about how Saturn is generally considered the last of the "naked eye" visible Planets but ...
  A: I've heard apocryphal statements about some people being able to see exceptionally faint objects in ...
THE SHAPE OF THE UNIVERSE9/24/2009
  Q: Where is cosmology now on the overall shape of the universe? Is it a big sphere, with the galaxies ...
  A: It is probably a blob of no particular shape, which is growing in all directions at an effectively ...
Moon Behavior.8/29/2009
  Q: For the sake of photography my question is what is the behavior of the moon relative to the sun? ...
  A: The position of the Moon changes relative to the Sun over the course of a lunation, or cycle of ...
orion's belt8/28/2009
  Q: Is orion's belt visible Aug. sky in Central Ontario Canada?
  A: Yes, but only late at night. In the early evening in mid-summer, the stars that are up are "summer ...
time and space8/24/2009
  Q: I saw a special on tv that theorized that time travel was possible either through the use of a ...
  A: You are correct in presuming that given our current understanding of the laws of physics, none of ...
astronomy question8/21/2009
  Q: I first heard Carl Sagan discuss a little known theory that the universe might be surrounded by ...
  A: I believe that the discussions you're referring to are a way of visualizing the multiple-universe ...
Supernova8/18/2009
  Q: Hai, I am Steffi Michael, When there is a supernova the energy given out by the star is more than it ...
  A: Supernovae can be several hundred billion times brighter than the Sun, but only for a short period ...
Appearance of Charon8/12/2009
  Q: How large would Charon appear in the sky of Pluto, or vice versa? Degrees of arc would be a clear ...
  A: Charon would appear to be a little over 3 degrees in diameter, as seen from the surface of Pluto. ...
orbit8/11/2009
  Q: Why everything revolves around everything in an elliptical pattern? And how it revolves with only ...
  A: It just has to do with how the force of gravity acts on the objects orbiting the Sun. As a planet ...
Dark Matter8/11/2009
  Q: What is dark matter and what it is doing in space? If there is no light in a room, will the dark ...
  A: We don't know what dark matter is. As far as dark matter in galaxies is concerned, it could be ...
Stellar Parallax8/10/2009
  Q: I am looking for a simple explanation of calculating the distance of a star using stellar parallax. ...
  A: For a simple diagram, you might use http://abyss.uoregon.edu/~js/glossary/parallax.html If you're ...
tools8/8/2009
  Q: If I were living during the days of Johannes Kepler (1571-1630) What tools and theorys would I have ...
  A: The mass of the Earth is determined by measuring the constant of gravity, G, and comparing its value ...
flares8/2/2009
  Q: Please forgive me if my questions sound arogant, I have little scientific knowledge, but I would ...
  A: A solar flare represents a phenomenon in which energy trapped in the Sun's magnetic field is ...
Big bang7/29/2009
  Q: is it true that before the Big Bang all matter was joined together as 1 unit. so the entire universe ...
  A: First, I apologize for the delay in answering your question. The electric company was working on our ...
Expansion of space/redshift7/9/2009
  Q: I was wandering what distance from our present position the present edge of the observable universe ...
  A: To answer the second part of your question first, I seriously doubt that you have any doppelgangers ...
Astronomy7/6/2009
  Q: This is a problem from my astronomy homework and I have been trying to figure it out but I keep ...
  A: You'll need to look up Io's mass in kilograms (it should be in your textbook, but if not, you can ...
moon going away6/28/2009
  Q: It's well known that the Moon is going away from the Earth at few centimers per years; but will it ...
  A: The Moon's retreat is caused by the tidal interaction of the Earth and Moon. This interaction ...
merging galaxies6/26/2009
  Q: seen one of those photographs from the hubble telescope of two galaxies on a collision course and ...
  A: The stars attract each other, but by and large, not as individual objects. Each star in each galaxy ...
ASTRONOMY6/24/2009
  Q: On March 21 is the length of daylight (sunrise to sunset) longer, shorter or the same on the Equator ...
  A: Ignoring its size, the Sun would be up exactly the same amount of time (half its 24-hour day-to-day ...
Lunar eclipse Saros series numbering6/21/2009
  Q: As far as I know, Saros series are numbered according to when the eclipse that has the least gamma ...
  A: The answer is simpler than I thought, so I was able to figure it out within a few spare minutes. ...
Astronomy6/20/2009
  Q: this is a question from my astronomy homework and I can't seem to figure out how to go about ...
  A: This is about Kepler's Third Law of Planetary Motion, which says that the mass of the bodies ...
Astronomy: Solstice and Equinox6/18/2009
  Q: I have a question about the ecliptic. What are the times or dates during the year when a place on ...
  A: At the Equator, there are 12 hours of sunlight and 12 hours of night every day, throughout the year. ...
planets6/15/2009
  Q: on bbc's the sky at night, they were looking at possible planet formations around other stars,they ...
  A: I don't know what the limiting size would be, but there are two factors which argue against very ...
Space6/15/2009
  Q: How are the weights and sizes of planets like uranus, mercury and earth and asteroids determined?
  A: Size Estimates: (1) TYPICALLY BEST -- observations of how large the object appears (in a telescope ...
black holes6/11/2009
  Q: the creation of black holes is formed in the aftermath of supernova so does this happen ...
  A: The formation of the black hole takes only a fraction of a second, but it wouldn't be 'visible' for ...
milky way6/9/2009
  Q: hypothetical question what if the only galaxy created from the big bang was our own, would life ...
  A: There are theories which propose that 'local' conditions can be affected by the nature of the ...
comets,astroids etc6/8/2009
  Q: often when mentioning these objects on tv,films etc the narrator will talk about the impact of a ...
  A: The only equivalence involved is the total amount of energy. If, say, an impact has a total energy ...
eclipses6/6/2009
  Q: solar eclipses are slightly more common than lunar eclipses. Why then is it more likely that your ...
  A: Lunar eclipses can be seen anywhere on the night side of the Earth, but solar eclipses can only be ...
black holes5/29/2009
  Q: what happens to all the matter that goes into black holes?
  A: It falls to the center, and becomes part of the singularity at the center. As it nears the ...
solar system5/26/2009
  Q: I agree the moon orbits the earth at aprox. 62,000 miles an hour,but Idon't agree that we orbit the ...
  A: The Moon only orbits the Earth at about 2,000 miles per hour. The Earth orbits the Sun at about ...
Solar & Planets5/24/2009
  Q: Has anyone ever done an estimate of the Mass of the Trojans- & why are they spaced at approx 60 ...
  A: I don't know if there is a specific estimate of their total mass, but there are only a few hundred ...
tidal locking5/23/2009
  Q: Can planets become tidally locked to stars in the same way that moons are tidally locked to their ...
  A: Yes, they can. In fact, until the 1960's, it was thought that Mercury was tidally locked to the Sun. ...
gravity & speed5/21/2009
  Q: mars has a gravity 38% that of earth...if earths gravity was the same how fast will a man be able to ...
  A: The difference in gravity wouldn't affect how fast things could go -- only how fast they fell. For ...
How close does something have to be to see it?5/19/2009
  Q: I'd like to know how far away I can see objects in the daytime sky, such as balloons. Assuming an ...
  A: There are two possible answers to this question, depending upon what you mean by 'seeing' it. One, ...
cat's eye nebula5/10/2009
  Q: could you tell me what the TRUE color of Cat's eye Nebula is? Is it that: ...
  A: I presume you mean, if you were looking at the nebula directly, without any optical aid or ...
GETTING UP CLOSE5/10/2009
  Q: I was wondering would you please tell me: why do we have daylight savings? Thanks
  A: The basic idea is that since most people are up in the evening and abed in the morning, they can do ...
particles5/5/2009
  Q: i saw a show on the science channel hosted by micho kaku and he was on top of a mountain talking ...
  A: He was probably talking about mu mesons produced by cosmic rays bombarding atoms in our upper ...
my grandma has a question for you5/3/2009
  Q: why do the planets orbit the earth on a parallel line like they all go across if you where to draw a ...
  A: This is explained by the Solar Nebula theory of the origin of the Solar System. The planets formed ...
Observing Our Early Universe4/30/2009
  Q: I just finished reading this article at ...
  A: When it is said that we see something 13 billion light years away, what it really means is that the ...
mars4/26/2009
  Q: I have recently recieved an email saying that mars will appear as big as the moon this year on ...
  A: This is a misconception or hoax that has cropped up every couple of years since 2003. Below, I've ...
about a detail on Earth's past4/23/2009
  Q: During Earth's past; more PRECISELY just before the formation of the first mountains-ranges (ie, 1 ...
  A: It is highly unlikely that general surface motions such as continental drift have ever exceeded a ...
Astronomy4/18/2009
  Q: At times, you can look into the sky and see the Sun and the moon at the same time. It's almost as ...
  A: The portion of the Moon that you see lit depends upon the difference between the angle you look at ...
where are we?4/16/2009
  Q: Goodmorning from Barcelona. After recently watching 2012 Nostradamus on the History Channel -which ...
  A: Although visible light cannot penetrate the clouds of gas and dust which lie in the plane of the ...
Angle of the solour system to the milky way disc.4/14/2009
  Q: what is the orientation of the plane of our solar system relative to the plane (disc) of our galaxy ...
  A: The galactic equator is tilted about 63 degrees relative to our equator. However, the answer to the ...
Straight lunar rilles3/30/2009
  Q: I'm posting this question again because I didn't write my E-mail adress correctly. i'm sorry for ...
  A: Here's a copy of the answer I posted a few hours ago. Lunar rilles were formed by lava tubes, ...
Lunar rilles3/30/2009
  Q: could you tell me if the idea of the cleavage of the Moon (allegedly occured thousands years ago) in ...
  A: Lunar rilles were formed by lava tubes, during the formation of the lunar maria, 3.2 to 3.9 billion ...
Space travel3/14/2009
  Q: Why can't the Cassini–Huygens space craft travel above or below the Asteroid Belt and the rings of ...
  A: There isn't any restriction on our ability to go above or below the plane of the Ecliptic. There are ...
Moon3/13/2009
  Q: Why does the moon look so much larger (in size) when it is rising in the East (when its a full moon) ...
  A: This effect (called the Moon illusion) is due to the apparent difference in distance between the ...
moon3/12/2009
  Q: 1.Which phase of the Moon rises in the east exactly when the Sun sets in the west? and why?
  A: That would be the full moon. East is opposite west, so if the moon is rising in the east when the ...
comets3/2/2009
  Q: I have got two questions concerning comets. I would like to know 1. how fast in general do comets ...
  A: 1. When very close to the Sun, comets may move more than 200 miles per second. At the orbit of the ...
planetary formation of water2/28/2009
  Q: Courtney: I have studied cosmology for about 30 years, and a question keeps popping up in my mind. ...
  A: A small portion of the water may have come from cometary bombardment, particularly in the period ...
The light horizon and cosmic microwave background2/26/2009
  Q: Does the Light Horizon include cosmic microwave background radiation? Or is one more distant then ...
  A: The light horizon is identical to the 'surface' defined by the microwave background radiation. When ...
Life cycle of a star2/17/2009
  Q: What is the complete life cycle of a normal star, starting with a protostar and ending with a black ...
  A: Apparently, whomever you sent this question to was on vacation, or declined to answer it, as I just ...
Planets2/9/2009
  Q: One story in Hindu Mythology was saying there was an astrological event of all the planets & Sun ...
  A: Sorry for the delay in answering, but I just got back from the hospital, and didn't receive your ...
Jupiter2/8/2009
  Q: Was Jupiter ever a star once?
  A: Apparently, whomever you sent this question to was either on vacation, or rejected it, as I just ...
Hubble discoveries2/5/2009
  Q: How come that we can see photographs of thousands of galaxies said to be > 10 billion years old made ...
  A: Observations always trump theories, so what those photographs show is that theories which predicted ...
Earth's axis1/25/2009
  Q: Does the earth's axis tilt away from the sun or towards it as it revolves around the sun? eg On the ...
  A: The axis always points in the same direction, so when it is on one side of the Sun, the northern ...
Moon and Sun1/20/2009
  Q: What is the term used to describe when the sun and moon are both visible? (I.e. the sun is setting ...
  A: I don't think there is a term to describe that, because it's almost an everyday occurrence (though ...
begining universe1/15/2009
  Q: Is it true that the Universe (rather, all the matter in the Universe) used to be, at its early ...
  A: I suppose it would depend upon what you mean by a cloud of smoke. Very early on, the 'matter' in the ...
the universe1/14/2009
  Q: 73 billion years old, Mars and earth about 4.5 billion years old, would it be fare to say that these ...
  A: As far as the 9 billion years, yes, that would be correct, presuming the 13.73by is correct (I know ...
Position of rising moon1/13/2009
  Q: I have noticed that the moon rises in at least two different places and would like to know if it ...
  A: Just as the Sun moves north and south during the year, the Moon moves north and south during the ...
geography1/12/2009
  Q: what caused the sun to become tilted at 7.25 degrees?
  A: The Sun's rotation is presumably the average of the net rotation of the material that fell into it ...
earth attraction1/10/2009
  Q: sir why earth attract the objects?
  A: I presume you mean, why do things fall toward the Earth, or orbit around it. Since your question ...
Flare activity on earth?1/8/2009
  Q: I have asked this question many times around about, but i thought i'd ask you. I have a pet theory ...
  A: As noted in my earlier (abortive) attempt to reply, this is outside my area of expertise, but I ...
constellations1/7/2009
  Q: Do you know of any websites that show a picture/animation of the milky way galaxy with the ...
  A: If you mean, where are the constellations in the sky, relative to the Milky Way, then there are many ...
not sure12/20/2008
  Q: Describe how a proton found in hydrogen gas cloud(nebula), subsequently found itself in an atom of ...
  A: (Sorry for the delay in answering; but I'm on vacation, and won't have regular access to the ...
Physics12/20/2008
  Q: Why does centrifugal force can keeps earth move and rotate on its axis?
  A: Centrifugal force has nothing to do with how the Earth moves in space. Its orbital motion is due to ...
astronomy12/15/2008
  Q: Mr.Seligman, I have been reading about the interrelation between moon-earth, many information about ...
  A: The slow increase in the Moon's distance will have no significant effect on the Earth, save for its ...
Considering the moon as a planet12/14/2008
  Q: I was trying to figure out what classifies the moon as a satellite, and not a planet. I found my ...
  A: (Please forgive the delay in answering this question. It was sent to the Question Pool, presumably ...
Astronomy12/12/2008
  Q: How many galaxy and nebulas are there in the constellation Draco? And can you name some?
  A: I have no idea, but there should be dozens of nebulae, and billions of galaxies, depending upon how ...
Questions12/9/2008
  Q: How can I become an astronomer? Is it easy? Is it fun?
  A: To be a professional astronomer, you need an undergraduate major in astronomy, physics, or ...
Astronomy12/9/2008
  Q: Seligman, I am a ninth grade student, and am doing a report and am required to interview an expert ...
  A: Most astronomers work as teachers, so their salaries are usually in the same range as other ...
Astronomical Sizes12/8/2008
  Q: I am trying to rank the relative sizes of the following: Jupiter, Sun, Spiral Galaxy, Neutron Star, ...
  A: In order of increasing size: Black holes have very little size (in fact none, if you mean the ...
Online planetariums12/4/2008
  Q: In a previous question on azimuth of celestial bodies, you mentioned that there are many free online ...
  A: Prior to answering your earlier question, I hadn't looked at online planetarium software in any ...
Theoretical question12/4/2008
  Q: What would happen to the planets in our solar system if the sun just vanished in the blink of an ...
  A: For one thing, they would get very cold as their surface heat was radiated into space, and not ...
astronomy12/1/2008
  Q: I am trying to write a programme to find location with the use of stars planets. Can you give me the ...
  A: Your question seems to be self-contradictory. The first sentence implies that you want to use the ...
planets mass bending star light12/1/2008
  Q: This problem has been bugging me since I saw the BBC drama about Eddington and Einstein. This is to ...
  A: I just noticed this question, in the Question Pool. (I presume the individual you sent it to either ...
Angle of setting sun - continued11/29/2008
  Q: Hope you had a good Thanksgiving, Professor Seligman. Thanks for helping me with the angle of ...
  A: You are correct. I inadvertently left out a step at the end of my answer. Although it correctly ...
Angle of setting sun11/26/2008
  Q: I found this site and your name while searching for information on my question. You answered a ...
  A: You are correct in saying that at the Equator, the Sun would rise vertically (at a 90 degree angle ...
solar system orbit-orbit resonance11/26/2008
  Q: In my current Msc course in Astronomical Climate Forcing we briefly touched the subject of ...
  A: It is true that each interaction between two planets will alter their orbits, but that doesn't alter ...
Cosmology and the Exansion of the Universe11/26/2008
  Q: Lets just say for instance that; 1). We have two Astronomers on two planets say 4ooo light years ...
  A: Because the expansion of space is very small unless the volume of space is very large, your example ...
Origin of life11/25/2008
  Q: Can we hypothesise that life was 'created' on earth more than once? If this was the case would the ...
  A: You might better ask this of a biologist, but I've read enough about the topic to give you a ...
Astronomia11/19/2008
  Q: What is supporting the solar system; I mean, what is down it? Quisiera saber si los demas planetas ...
  A: Nothing is "supporting" the Solar System, as there is no "down" in space, save for the direction to ...
interview questions11/18/2008
  Q: 1. How did you get involved in astronomy? Do you enjoy this profession? ...
  A: 1. I've been interested in astronomy all my life, but the critical event was my attendance at the ...
Measurement in space11/17/2008
  Q: What exactly is a parsec? I don't understand why astronomers would use this instead of light-years, ...
  A: When measuring the distance of a star, its apparent displacement in direction, due to our motion ...
About True Polar Wander theory11/14/2008
  Q: According to True Polar Wander theory, if a supersized volcano is on earth, then our planet would ...
  A: If it were possible for large structures (supersized volcanoes or continents) to not have deep ...
astronomy11/14/2008
  Q: Why does the moon appear to be "upside down or rotated to the right" in the morning or when it is ...
  A: (Please excuse the delay in answering this question. Apparently, whomever it was directed to ...
planet formation11/8/2008
  Q: Can you explain some of the processes involved in the alleged planet formation from dust particles ...
  A: You can find a detailed, albeit somewhat scattershot discussion of this topic on my website, ...
earth11/6/2008
  Q: What would happen if an object the size of Pluto rammed into earth at the speed of sound? Would ...
  A: When any object hits the Earth, it does so with at least the escape velocity of the Earth, which is ...
telescope10/27/2008
  Q: can you please help me with this question Explain how visual telescope shows the temperature and ...
  A: Hotter stars give off a greater percentage of blue and ultraviolet radiation than cooler stars, ...
sun exposure10/22/2008
  Q: what would happen if someone was never exposed to the sun?
  A: This is more a biology question than an astronomy question, but having some experience with the ...
mountains without roots10/21/2008
  Q: First of all, thank you for answering my first question. My new question is the following: if ...
  A: The only way I can imagine that happening would be for a massive volcanic eruption to blow the ...
Space10/20/2008
  Q: What direction are you traveling in a space vacuum?
  A: If you mean, how can you tell what direction you are moving in a completely empty Universe, the ...
Full Moon Duration10/16/2008
  Q: How many days is a full moon considered full (ie. 3, 5, ?) Thank you so much.
  A: (Please excuse the delay in answering this question. Apparently the person you asked to answer it ...
mountains and Earth's rotation10/16/2008
  Q: A friend of mine made the following analogy to prove me that mountains balance our planet while it's ...
  A: Your friend's analogy is wrong, on several counts. First, the purpose of wheel weights has nothing ...
astronomy10/15/2008
  Q: 1. Why does a cloud of interstellar gas heat up as it collapses?
  A: When you compress a gas, you have to exert a force to overcome the outward pressure that the gas ...
Sun's angle at sun rise and sun set10/14/2008
  Q: What is the angle created by the sun at sun rise and sun set on June 21 and what is it on December ...
  A: Your question was apparently rejected and placed in the general question pool, which is where I just ...
start or planet10/13/2008
  Q: I don t really know anything about astronomy. So my question may be a little vague. I see a light in ...
  A: Based on your description, the "light" is the planet Jupiter, which was in the southeast (in early ...
How to view the Dark Moon?10/5/2008
  Q: You can't see the Dark Moon (commonly often referred to as the New Moon) in the daytime sky using ...
  A: You can't see a new moon because it is so close to the Sun that the scattered sunlight that lights ...
astronomy10/2/2008
  Q: I am inquiring about sunspots. What is the size of a sunspot, in kilometres, given that it has an ...
  A: An object with an angular diameter of 1" is 206,265 times smaller than the distance between it and ...
The Universe9/29/2008
  Q: What is the size of the Universe (in light years)? Is the size of the Universe expanding, and, if ...
  A: There are some complications involved in answering your question, so a simple answer isn't ...
Asteroid9/26/2008
  Q: What would happen if an asteroid hit where the ice caps are melting? Thank you.
  A: I presume you mean an asteroid large enough that the impact and subsequent explosion would melt and ...
astronomy9/25/2008
  Q: how did early astronomers first know that the Solar System was not the center of the Galaxy? ...
  A: This was discovered by Harlow Shapley in 1918. Prior to that, it was thought that the Sun lay near ...
Radio Astronomy Data9/21/2008
  Q: Where could I download some raw data from a radio telescope for free? What file format is "standard" ...
  A: I am not an expert in the field of radio astronomy, but have looked at enough online material to ...
Star photos9/20/2008
  Q: Most photos of stars have four rays at 90 degrees to each other. Obviously they are an artifact of ...
  A: (Your question was sent to the Question Pool, because the expert you sent it to was on vacation, had ...
declination and right ascension9/16/2008
  Q: It's a long story...but I'm working on a historical novel, and have gone off on a tangent about ...
  A: If you only know the Moon's right ascension and declination at one date and time, you can't ...
mutual gravitation9/14/2008
  Q: what is the meaning of mutual gravitation?
  A: (Your question was apparently rejected by the system, or sent to an expert who was on vacation or ...
sight8/22/2008
  Q: what do we actually see? when we look at things are we seeing the Light or the objects
  A: Strictly speaking, we the light reflected by or given off by the object. Experience tells us that ...
blue shifted light8/22/2008
  Q: I have a Question to ask about light from distant galaxies. If We look at far off galaxies(12 bill. ...
  A: In any given part of the Universe, galaxies are moving more or less randomly relative to each other. ...
stellar parallax7/15/2008
  Q: what are the errors in star parallax measurement? thanks
  A: (Please forgive the lateness of this answer. I volunteered on August 5, and noticed that no one had ...

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Courtney Seligman

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Expertise

I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

Experience

I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Publications
Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

Education/Credentials
I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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