...a way to keep time measured by the stars (instead of the earth or sun). A sidereal day has passed when the stars can be seen in the same position again. We usually measure by solar days.
Again: Andromeda is the closest galaxy to ours, the Milky Way. It is moving towards "us" at about the speed of a passenger jet and will get "here" in about 4 billon years, the rough age of the earth. Given stellar distances, Andromeda will pass through "us" like a ghost. Galactic ships in the night.
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...a way to keep time measured by the stars (instead of the earth or sun). A sidereal day has passed when the stars can be seen in the same position again. We usually measure by solar days.