Astronomy/Midterm Help

Advertisement

Expert: - 10/6/2009


Question
Hi my name's Chris and I'm in an Astronomy course this quarter and i wanted to know If the Earth-to-moon distance were greater than the  Earth-to-sun distance would an observer on Earth be able to see the Moon in it's first quarter phase? If the answer's yes i need to be able to show a diagram and if it's no i need to explain why a first quarter moon is impossible. thank you for your time.

Answer
Hi Chris,
Well, if the moon were greater than 93 million miles, then it couldn't be orbiting the Earth,
it would have to be orbiting the sun, like Mars at 141 million miles from the sun.
And we can never see Mars (or any outer planet) in a crescent or half moon (quarter) phase.
We only see Mars as a full phase, or at times when Mars is 90 degrees to our position
(called quadrature)...we see Mars as an 85% illuminated gibbous disk. That's the smallest
phase possible with an outer planet....So the answer is no. It's impossible because the body
would be outside our orbit, so the sunlit face is nearly always fully illuminated from our
vantage point.  Bodies have to be inside (closer to the sun) our orbit to see the phase change.

As a side note, any body inside our solar orbit, then you can see all phases, like with
Mercury and Venus.  Both inner planets go through the same phases as our current moon does....
in a telescope of course. Even the gibbous shaped Mars takes a telescope, as none of this
is visible to the naked eye like the moon's phases are...too far away and too small.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA

Astronomy

All Answers

Answers by Expert:

Ask Experts

Volunteer

Tom Whiting

Expertise

Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.

Experience

Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at:http://www.velocity.net/~bwhiting and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

Education/Credentials
BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

©2012 About.com, a part of The New York Times Company. All rights reserved.