We are taught that the moon is tidally locked to the Earth. Its orbital period equals its rotational period. We are taught that this results in one side of the moon always facing the Earth. This is true, but we actually see 59 percent of the moon. This is because of a wobble and is called libration. If you take high resolution pictures of the moon’s phases you would see the libration.

There are three parts to the wobble. A longitude wobble is caused by the moon’s elliptical orbit. A latitude wobble is caused by the moon’s orbital plane being inclined relative to the Earth’s. A third wobble is caused by the daily rotation of the Earth.

To see the moon and other wonders of the night sky, visit the Insperity Observatory on public night (except for cloudy nights), the first Friday of each month from sunset to 10 p.m.: humbleisd.net/observatory.


By Dr. Aaron B. Clevenson

Observatory Director, Insperity Observatory

Aaron Clevenson
Author: Aaron ClevensonEmail: This email address is being protected from spambots. You need JavaScript enabled to view it.
I am the observatory director at the Insperity Observatory in Humble ISD. I am also an adjunct astronomy professor at Lone Star College-Montgomery where I teach solar system astronomy and stars and galaxies astronomy. I am the author of the astronomy textbook, “Astronomy for Mere Mortals.” I am a past president of the North Houston Astronomy Club, and was the chair of Astronomy Day in Southeast Texas in 2015 and 2016. He is an observing program director with The Astronomical League, coordinates their Master Observer Progression Awards, and has authored six of their observing programs.

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