Why the earth is rising and setting seen from the moon, when the moon is tidally locked? Shouldn't the earth be always on the same spot because of the tidal lock, if observed from the moon?


1 Answer 1


You're quite right. The Earth is (nearly) stationary in the Moon's sky.

(I say "nearly" because the Moon is in a slightly elliptical orbit around the Earth, but rotates perfectly smoothly. This means that the Earth's motion through the Lunar sky is a bit faster when the Moon is at perigee and a bit slower when it's at apogee. Because the Moon is tide-locked, on average they match perfectly, but during the course of the sidereal month the Earth appears to move back and forth by a few degrees. The effect isn't huge, but it's easily observable with a telescope. See the Wikipedia article on "libration" for an OK discussion and a really great animated gif illustration of libration as viewed from Earth.)

Is it possible that you're reacting to the amazing movie from Apollo 8 showing Earthrise over the Lunar horizon? It's been int he news a lot given that this is the 50th anniversary of the flight. If so, what you saw there was not from the Moon's surface, but from orbit around the Moon and the Earthrise happened as Apollo 8 came around from behind the moon (where the Earth is not visible in the sky) and the Earth first became visible around the edge of the Moon.

  • $\begingroup$ I thought of it, when I was watching a documentary about the new horizon probe and Pluto. Pluto is tidal-locked with one of its moons. Both are tidal-locked so on the Pluto's surface its moon is always on the same spot and vice versa. Afterwards I entertained myself by watching these crazy flat earthers and their crazy conspiracy theories. And I challenged their believes by offering them a method to prove the curvature of earth. But then I remembered the images of the rising earth sen form moon. I said to myself that I need know the reasons before they think about that. $\endgroup$ Jan 13, 2019 at 23:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .