Several parts combine into one character.

Compound ideograph built on top of 大 (big person from earlier batch): 大 with a horizontal line (一) above the head, indicating "what is above the head of a person" = sky / heavens. The encoding refuses to separate sky from human experience — the sky is defined relative to the person beneath. Identical across 繁體 / 新字体 / 简体.

天 is one of the most philosophically loaded characters in CJK culture. Beyond literal sky, it covers heaven, fate, divinity, and "the natural order": 天命 (Mandate of Heaven — the foundational concept of Chinese imperial legitimacy: rulers govern by divine right that can be withdrawn through misrule), 天子 (Son of Heaven — the emperor), 天皇 (tennō, Japanese emperor — literally "Heavenly Sovereign"), 天才 (genius — "heavenly talent"), 天然 (natural — "from heaven", as opposed to 人為 jin'i, "made by humans").

Mandarin: tiān, level 1st tone. Crucially, 天 in Mandarin doubles as a time unit meaning "day": 今天 (jīntiān, today), 明天 (míngtiān, tomorrow), 昨天 (zuótiān, yesterday), 三天 (sāntiān, three days), 整天 (zhěngtiān, whole day). This sky-as-day overlap is unique to Mandarin among the CJK languages — Japanese and Korean use 日 for "day". Other compounds: 天空 (tiānkōng, sky), 天气 (tiānqì, weather), 天才 (tiāncái, genius), 天堂 (tiāntáng, heaven/paradise).

Japanese: on-reading テン (ten) for compounds — 天気 (tenki, weather), 天才 (tensai, genius — used both as noun and admiringly toward people), 天国 (tengoku, heaven / paradise), 天皇 (Tennō, the Emperor of Japan). Kun-readings あめ (ame) and あま (ama) — note the homophone with 雨 (ame, rain) but distinguished by context. 天の川 (Amanogawa, "the heavenly river") is the Japanese name for the Milky Way, central to the Tanabata festival.

Memory aid: a horizontal line above a person — what is over your head when you look up.