The night's companion light — cyclical, reflective, pulling tides; the sky's own emblem of phase and return.
Old English mona, from Proto-Germanic *menon, from the Proto-Indo-European root *me- (to measure); moon and month share one ancient root, because the moon's phases were the oldest measure of time.
The oldest clock in the sky — a light that never makes its own, only returns what it borrows, on a cycle nothing else in the house needs to be told twice.