Earth's Day Used to Be Less Than 19 Hours Long

The Moon's gravity creates tides on Earth, and those tides act like a brake — slowly slowing down how fast our planet spins on its axis.

The effect is negligible on a human timescale (the day lengthens by fractions of a millisecond per century), but on a geological scale it makes a massive difference: hundreds of millions of years ago, a day on Earth lasted less than 19 hours.

Evidence comes partly from fossilized growth layers in corals and sediments, which preserved traces of daily and yearly cycles from hundreds of millions of years ago, letting scientists count how many days made up a year back then.

The flip side: as the Moon steals angular momentum from Earth, it's also drifting away from it, by about 3.8 centimeters a year.