Astronomy/planetary motion

Advertisement

Expert: - 4/6/2008


Question
I have been driven crazy trying to determine the difference between libration and nutation. I hope you can clarify this for a non-physics kind of astronomy lover. Thanks.

Answer
Hi Tom,
Libration is an equatorial oscillation (or east-west) of a rigid
body. Usually both of these terms involve our own moon, as it
displays both libration and nutation. As you probably know, the
moon displays synchronous rotation keeping only one side facing
the Earth at all time (called the near side), that is, one axial rotation for every one revolution. But with libration, the rotation rate either gets ahead of, or behind, the revolution rate a little, thus allowing Earth observers to, at times of maximum libration, see a little over the eastern and western limbs.
Nutation is the same idea, except it's nodding or oscillation in the north-south (up-down) direction with the lunar poles tipping slightly, thus allowing an Earth observer to look, at times, a little
over the north pole and over the south pole.
So overall, we can see 59% of the lunar surface, and 41% is forever
blocked from view, which we call the back side, or far side, of the moon. (Notice, there is no such thing as a "dark side"...it's the
back side or far side of the moon).
(Some people round these figures to 60/40%).
For more, see
http://burro.astr.cwru.edu/stu/me/photos/moon_phases.html

Astronomers aren't 100% certain what caused these oscillations
initially, but the prominent theory is large meteorite impacts,
in particular, the huge Orientale Basin impact may have triggered
these two processes on the moon.
Hope this all helps,
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.