Expert: Tom Whiting - 1/19/2006

Question
Thanks for clearing up my misconception about the earth's wobble…now I understand why one full circle around the sun would necessarily bring you back to precisely the same angle relative to the sun (and same season), regardless of any change in earth's orbital speed. The factors that may affect that speed is also an interesting subject.

You also began to address my other question even though I hadn't asked you yet, which is how fast am I going? As you mentioned, the sun is moving around the Milky Way, which is also moving. I think that the list of my celestial movement is
Movement of galaxy away from the center of the universe
Orbit of our solar system around the center of the galaxy
Movement of our solar system away from the center of the galaxy
Orbit of the earth around the sun
Spin of the earth

Is that the complete list? Are these all the factors that determine my path through space? Is it possible to calculate my speed along this path? Of course that calculation should also consider my speed relative to the surface of the earth so let me clarify that in that sense I am not moving, as all this movement has made my head spin and I have had to lie down.

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Followup To
Question -
What is a year? Is it the time it takes the earth to complete an orbit around the sun or is it the time it takes the earth to wobble on its axis through the four seasons and return to the original angle? Or are these two time periods exactly the same…and if so, as the orbit of the earth continues to slow down, will they continue to be exactly the same?

Thanks for your time,
Kevin
Answer -
Hi Kevin,

The Earth's axis doesn't "wobble"....it maintains its
orientation in space precisely like a fast spinning gyroscope,
always pointing at a spot about 3/4 of a degree from
the North Star, Polaris, at least during our millenia.    
(In actuality though, it DOES have a slight wobble, called
precession, but the period for one precessional revolution is
25,800 years, so the bright star Vega was our "North star
in 13000 BC, and will be so again, in 13000 AD.  Polaris is
our North Star now, and will be again around 28000 AD.)

But I doubt that that was the wobble you were thinking of....the wobble you were thinking of, does not exist.
Our axis does not wobble on a seasonal basis.....
It's just our 23.5 degree axial tilt from the vertical  is what
causes the seasons, but the axial direction in space,
remains constant throughout the one year.  So for 6 months
the North Pole faces toward the sun, and for 6 months
away from the sun...but the axis orientation remains constant.

Now to your question:
What is a year?  The time it takes for the Earth (assuming
a stationary sun) to revolve once around the sun.
And the value is very close to 365.2422 days; or in layman's terms, about 365 1/4 days, and the length of a year  has nothing to do with axial tilt, seasons, daily spin, or anything else.

Of course, the sun is really moving around the Milky Way
Galaxy at high speed, so the Earth (and all the other bodies) are actually in a cork-screw spiral as we follow along with the sun.  So we aren't really back to the same spot in space that we were one year ago.....we are back "to the beginning" only
relative to the sun.  But in actuality, the Earth is always forging into new spacial territory as every second goes by,
because the Milky Way Galaxy is moving at high speed too,
but that is getting into another story.

Who says that the Earth's orbital speed around the sun
is slowing down?  I know the spin rate (one day) is slowing
down due to the gravitational effects of the moon.....about
3 millionths of a second per year  (65 million years ago, the
dinosaurs had a 20 hour day)....but I've never read where
our speed around the sun has changed over the millenia...
in fact, I would assume that as we spiral in closer to the
sun (very slowly) we would actually speed up, thus a slightly
shorter length of year.  But then again, the sun is continuously losing mass through the fusion process to produce energy, so that would allow the Earth (and all the planets) to drift outward slowly, thus lengthening the orbital (yearly) time period.
Perhaps the two effects offset each other, because I've
read nothing about the "year" growing  longer (or shorter) as the centuries and millenia go by.  But if it has, it certainly
is not measureable in our lifetimes, and it would probably
affect our calendar by only one day every few million years....
too minor to worry about, at least from a calendar standpoint.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA


Answer
Hi again Kevin,
and thanks very much for the kind comments on the Allexperts
website.....

How fast are you moving?  The only answer anyone can give
is.....relative to what?

All velocities are relative to some other object, because there
is no such thing as Absolute Rest....everything is moving,
relative to everything else.
The road speed limit sign on an Interstate Highway simply
says "65"....there are two built in errors there....perhaps it
would someday get you out of paying a speeding ticket.....

1. no units, like MPH  (miles per hour, Km per hour, etc)
2. AND.... relative to what?  The sun? A distant star?   No, the surface of the Earth.  (Guess they figure that is
understood,  without actually stating it).

You ask, "Is that the complete list?"  No, you missed a
couple on the very large scale of Universe structure.....
and you mistakenly listed two that don't exist....those that
use the phrase "away from"....why do you think that way?
Is it perhaps the general expansion of space?
Like the Hubble constant of 71 Km/sec/megaparsec?
That only applies on the very, very large scale, beyond
our Local Group  (the Milky Way and about 30 other
galaxies in our general region out to about 5 million lightyears.  A megaparsec is 3.2 million lightyears, so the
general expansion of the Universe hardly begins to affect
the Local Group....in fact, the Andromeda Galaxy (M-31),
another member of the Local Group,  (of which the Milky
Way Galaxy is a member), is actually approaching the Milky Way, it's local gravitational attraction easily over-riding the Hubble value.  In fact, the Milky Way and M-31 will
collide or merge in about 6 billion years from now, as they
are on a collision course.

Also, there is no "center of the Universe" any more than
ants crawling around on a balloon's surface know "their
center"......
(a 2-dimensional analogy to our 3 dimensional situation)
Where is "their center"?  It's undefined and the ants have
no conception of the "center of the balloon" as they know
only of the surface, and can't imagine a balloon center.
We are in the same boat, in a 3-dimensional sense.
That's because after the Big Bang some 13.7 billion years
ago, us, and everything we know and see...are INSIDE that
sphere of influence.  We can never see "something"
outside the Big Bang, nor can we ever be on the OUTSIDE
looking in.....therefore, we can not see or determine "a
center" or know the conditions outside of the Big Bang
(our Universe's) influence.....as far as we know today.
That's why, no matter which direction you look, you are
always looking backward in time  (Yes, a telescope is sort
of a time machine, always viewing stuff  (stars, galaxies, etc)
at an Earlier date in time....never a later date.  We are always
looking back at a time closer to the Big Bang, and eventually
we'll see that one too....actually, we already have....the light
of the Big Bang has been red-shifted down into the radio
portion of the electromagnetic spectrum, and we see the
remnant light of the Big Bang at 1.1 mm microwave radio wavelength----discovered in 1965.  It's called the CBR,
Cosmic Background Radiation from some 13 billion years
ago.

But to your question:
If you are around or near 40 degrees Latitude,
your speed relative to the center of the
Earth is about 700 MPH....at the Equator, it would be about
1000 mph, relative to the center of the Earth.
(At the poles, it's zero MPH relative to Earth's center).
The Earth is revolving around the sun at about 18.5 mps
(miles per second) relative to the sun.
The sun is revolving around the Milky Way at...I think,
around 200 mps, relative to the center of the Milky Way Galaxy, taking about 225 million years to make
one orbit around the Milky Way.
The Milky Way Galaxy is moving with, and around the
Local Group (of galaxies)....I don't know a velocity for that one.....and the Local Group, which is part of the Virgo
Supercluster, is moving at about 600 mps in the direction
of the constellation Leo, around the center of the Virgo
Supercluster, and I would presume that the Virgo Supercluster of Galaxies is also moving too, relative to
something else, but I don't think we know as yet, which way.
So notice that "our movement" thru space, is very, very complicated, and that we are always forging into new
territory, never returning to where we were originally.

Plus we haven't even taken into account that the moon
does not solely revolve around the Earth, but rather the two bodies actually orbit around a common center of gravity called the baricenter, buried deep within the earth....so, in part, the Earth also orbits the moon, as they actually orbit a common center of gravity, so in effect, the Earth actually orbits around this common center of gravity (and you and me along with it), once every 29.5 days.....if you really want to get technical.

So, in your question, eliminate the two "away froms" and add
motion of the Milky Way around the Local Group,
and the Local Group around the Virgo Supercluster of Galaxies, and motion of the Virgo Supercluster itself thru the Universe.

ALSO, does the whole known visible Universe....spin or rotate?
(as everything else does, right?)
At present, we just don't know.
AND, is our whole known visible Universe actually
moving thru the...Primordial Void...that which existed prior
to the Big Bang?  We just don't know.

There, that should keep you busy for a while, computing
our "actual velocity ---speed and direction thru space---
for a while.....Just remember, it has to always be relative
to something else.
If the Earth were the only body in the Universe, we could not
know which way, and at what speed we were moving...there
would be no reference point.  Given one other body besides
the Earth, then we could compute our speed and direction
(called velocity) ONLY to that one other body.  Fortunately,
we have a lot of bodies with which to state our velocity
thru space....the Sun, the center of the Milky Way, etc.
All this comes from the fact that there is no such thing
as Absolute Rest and everything is moving relative to
everything else.

As an example, picture Captain Kirk in the starship Enterprise
tooling along at close to light speed, with the starship
Constellation following the Enterprise far behind.  
And Captain Kirk says, "Let's stop here and wait for the
Constellation to catch up......"
What the hell does he mean by that?
Stop?  STOP,  relative to what?  That imaginary solar system off to the right? If so, then you are not stopped, relative to other bodies.
HERE?  Where is "here"?...here is defined as only
relative to some other body or bodies...there is no "here".....and there is no "stop" so his statement is bankrupt and meaningless.
It's only made for your enjoyment of the TV show.

They want you to assume that there exists this GRAND
Galactic coordinate system of up, down, left, right, latitude,
longitude, etc.  BUT it, a huge coordinate system,  does NOT exist out in space!  Everything is relative to everything
else.  Food for thought.

Hope all this helps,
and BTW, if you need any further help or clarification,
please contact me directly at
bwhiting@velocity.net
if you have any further questions....thus we can avoid
the "middle man"...
Clear Skies,
Tom Whiting
Erie, PA
Pres...Erie Co. Mobile Observers Group