You are here:

Astronomy/Moon sidereal period

Advertisement

Expert: - 1/19/2011


Question
dear expert,

An mere observer can know how long the Moon takes to do 1 synodic period by looking its phases,but
is it possible to measure Moon's sidereal period only by looking in the sky naked eye, as well?




                                    Sincerely

Answer
To measure the sidereal period, note exactly where the Moon is relative to the background stars on any evening (this is easiest when it is near bright stars, like Aldebaran in Taurus, or in between Castor and Pollus in Gemini and Betelgeuse in Orion). Each night the Moon will move to the east relative to the stars (the amount varies during the month, but is between 11 and 15 degrees per night). When the Moon gets back to where it started off, one sidereal period will have passed. What you should find is that after 27 days, the Moon is almost but not quite back to where it started off, and the next night, it is past where it started off; showing that the sidereal period is a little over 27 days.

Astronomy

All Answers

Answers by Expert:

Ask Experts

Volunteer

Courtney Seligman

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.

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)

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