From PIE *meh1- (to measure) — literally 'the measurer,' humanity's first calendar, root of 'month' and 'Monday.'
The natural satellite of the earth, visible by reflected light from the sun; also, a natural satellite of any planet.
From Old English 'mōna,' from Proto-Germanic *mēnōn, from PIE *meh₁n̥s- (moon, month), derived from the root *meh₁- (to measure). The moon was literally 'the measurer' — the celestial body by which early peoples tracked the passage of time. The same root produced Latin 'mēnsis' (month), Greek 'mḗn' (month, moon), and Sanskrit 'mās' (moon, month). The intimate connection between 'moon' and 'month' reflects the universal use of lunar
English 'moon,' 'month,' 'menstrual,' 'measure,' and 'semester' all descend from the same PIE root *meh₁- (to measure) — the moon was humanity's first clock, and the vocabulary of time-keeping still bears its imprint across dozens of languages.