天 is a person 大 with a stroke drawn above the head: the ceiling over humanity. Oracle forms show the head itself enlarged — the top of a person, then the top of everything. Sky, heaven, and 'day' all live under this one roof.
天 is Chinese cosmology in one character: 天子 the Son of Heaven ruled by 天命 Heaven's Mandate; lose the mandate, lose the throne. Daily life keeps it busy too — 今天 today, 明天 tomorrow, 天气 weather, and 天哪！'heavens!' for everything surprising.
The line above a person's outstretched arms — the SKY over everyone.
民以食为天 'the people take food as their heaven' — the oldest Chinese policy statement, and still the truest.