Ancient corals reveal Earth's day was about 22 hours 380 million years ago
Earth’s rotation has been slowing for billions of years due to tidal friction with the Moon, lengthening a day from roughly 22 hours in the Devonian (about 385–410 million years ago) to today’s 24 hours. Fossil corals show ~400 daily growth lines per year in the Middle Devonian, giving a day of ~21.9 hours, while Pennsylvanian corals imply ~22.4-hour days; the Moon’s recession (~3.8 cm/year) and internal mass redistribution also affect daily length in the short term. Proterozoic proxies suggest days as short as ~19 hours, highlighting how Earth’s spin has gradually slowed over deep time.