1The Moon — Earth's natural satellite visible in the night sky, with its familiar cycle of phases from new to full. The most basic meaning of 月, used in everyday expressions like 月がきれい (the moon is beautiful) or 月が出る (the moon is rising).
昨夜は満月で、月がとてもきれいだった。
It was a full moon last night — it looked beautiful.
雲の合間からきれいな月が顔を出した。
A beautiful moon peeked out from between the clouds.
秋になると、月がいっそう澄んで見える。
When autumn comes, the moon looks all the more clear.
今夜は月が出ていないので、星がよく見える。
There's no moon out tonight, so the stars are easy to see.
2A month, as a calendar time unit. The bare つき reading specifically denotes a month as a span of time — 月に一度 (once a month), ひと月 (one month), 月の半ば (the middle of the month). The reading shifts in compounds: 今月 (this month), 来月 (next month), 一か月 (one month) take げつ; month names like 一月 (January) take がつ.
月に一度、両親の家へ顔を出すようにしている。
I make a point of visiting my parents' place once a month.
ひと月もすれば、桜が咲き始めるだろう。
In about a month, the cherry blossoms will start to bloom.
月の初めに、必ず予定を見直すことにしている。
I make it a rule to review my schedule at the start of each month.
3Moonlight — though this is a literary/classical nuance; modern prose uses 月光 (げっこう) or 月明かり (つきあかり) instead. Survives mainly in set expressions in waka, haiku, and older literary writing — e.g., 月に照らされる (lit by the moon) or 月を浴びる (bathed in moonlight) — rather than as an independently used noun.
月に照らされて、湖面が銀色に輝いていた。
Lit by the moon, the surface of the lake shimmered silver.
古い庭は月を浴びて、まるで別世界のようだった。
Bathed in moonlight, the old garden looked like a world apart.
4In astronomy or popular science, a moon — any natural satellite orbiting a planet, not just Earth's Moon. Used in expressions like 木星の月 (Jupiter's moons) or 土星の月タイタン (Saturn's moon Titan). Appears in popular/explanatory writing; technical astronomical writing typically uses 衛星 (satellite) instead.
木星には八十個を超える月があると確認されている。
Jupiter is confirmed to have more than eighty moons.
土星最大の月タイタンには、厚い大気が存在する。
Titan, Saturn's largest moon, has a thick atmosphere.