Origins
The English word 'moon' encodes one of humanity's oldest intellectual achievements — the use of a celestial body to measure time. It descends from Old English 'mōna,' from Proto-Germanic *mēnōn, from PIE *meh₁n̥s- (moon, month), itself derived from the root *meh₁- meaning 'to measure.' The moon was, quite literally, 'the measurer' — the celestial timekeeper whose waxing and waning cycles provided the earliest reliable unit of time longer than a day.
This etymological connection between the moon and measurement is preserved across the entire Indo-European family. Latin 'mēnsis' (month) and 'mēnsūra' (a measuring, whence English 'measure') derive from the same root. Greek 'mḗn' (μήν) means both 'month' and 'moon,' and the related 'mḗniskos' (little moon, crescent shape) gave English 'meniscus.' Sanskrit 'mās' means 'moon' and 'month.' Lithuanian 'mėnuo' means 'month' and 'mėnulis' means 'moon.' The pattern is consistent: across Indo-European, the word for the moon and the word for a month are the same or transparently related, because the lunar cycle was the month.
The English word 'month' itself is a direct relative of 'moon.' Old English 'mōnaþ' (month) is formed from the same root with an abstract suffix, meaning literally 'a moon-period' or 'a lunar cycle.' The two words have diverged in pronunciation over the centuries — 'moon' retaining its long vowel while 'month' shortened its vowel before the consonant cluster — but their common origin is transparent.
Proto-Indo-European Roots
'Monday' is 'moon's day' — Old English 'Mōnandæg,' a calque of Latin 'diēs Lūnae' (day of the Moon), which was itself a translation of Greek 'hēméra Selḗnēs.' Like the other weekday names, Monday reflects the ancient system of naming days after celestial bodies. The Latin word for moon, 'lūna' (from an earlier *luksna, 'the shining one,' from PIE *lewk- 'light'), followed a completely different etymological path from 'moon' — it gave English 'lunar,' 'lunatic' (originally 'moon-struck,' reflecting the ancient belief that the full moon caused madness), and 'lune.'
The medical term 'menstrual' also belongs to the moon's word family. It derives from Latin 'mēnstruālis,' from 'mēnstruus' (monthly), from 'mēnsis' (month), from the same PIE *meh₁n̥s-. The connection between lunar cycles and the menstrual cycle was observed by ancient peoples across many cultures — both are approximately 29.5 days — though modern science considers the near-coincidence to be just that, a coincidence rather than a causal relationship.
In Proto-Germanic, the moon was grammatically masculine — *mēnōn was a masculine noun, and this persists in German ('der Mond'), where the moon is 'he.' This stands in contrast to the Romance languages, where the Latin feminine 'lūna' made the moon 'she' (French 'la lune,' Spanish 'la luna,' Italian 'la luna'). In Norse mythology, the moon god Máni (from the same Proto-Germanic *mēnōn) is male, while his sister Sól (the sun) is female — the exact reverse of the classical Mediterranean pattern where the sun is masculine (Helios, Sol) and the moon feminine (Selene, Luna). This inversion of gendered celestial bodies is one of the most striking cultural differences between the Germanic and Mediterranean branches of the Indo-European world.
Latin Roots
The word 'semester' — from Latin 'sēmestris' (six-monthly), from 'sex' (six) + 'mēnsis' (month) — provides yet another unexpected member of the moon's etymological family. Students attending university on a semester system are, etymologically speaking, organizing their academic lives by groups of six moons.
The Old English poetic tradition used several kennings for the moon, including 'nihtleoht' (night-light) and 'rodores candel' (sky's candle). The word 'mōna' itself appears frequently in the Anglo-Saxon Chronicle's entries recording lunar eclipses — events of great significance to a culture that still measured many things by the moon's cycles. The gradual replacement of the lunar calendar by the solar Julian and Gregorian calendars did not erase the moon's linguistic legacy; it remains embedded in our vocabulary of time, in the very word 'month,' and in the name of every Monday.