The moon orbits the earth. But only once every 27 days, 7 hours and 43 minutes. This moon looks like it isn't moving, but it is. Just at the same speed as the actual moon.
Thanks to @DoNotBeOnFire for fixing my moon facts
Maybe you have forgot "linear" in the animation property (it's "ease" by default)
@Thomas - I left it out because it was adding a visual artifact in Chrome. I don't know why, but I noticed that when I left the easing out, the artifact (a thin grey line) went away. ¯_(ツ)_/¯
Need to know how to enable it? Go here.