Earth Once Had 370-Day Years

Ancient corals from the Devonian period serve as remarkable natural calendars, offering a window into Earth's distant past. Much like trees, these marine animals laid down new layers of their skeleton in daily and seasonal cycles. By examining well-preserved fossils, paleontologists in the 1960s were able to count the fine, daily growth lines between the larger, annual bands. They discovered that corals from 400 million years ago showed nearly 400 daily growth rings per year, revealing that a day was only about 22 hours long and a year was packed with more, shorter days.

The primary driver behind this gradual slowdown is the Moon's gravitational pull. The Moon creates tidal bulges in our oceans, and as Earth rotates, it tries to drag these bulges along with it. The Moon’s gravity, however, pulls back on these bulges, creating a constant, subtle braking force on our planet's spin. This phenomenon, known as tidal friction, transfers rotational energy from the Earth to the Moon, causing our planet's rotation to slow down and the Moon to slowly drift farther away from us.

This braking effect, while minuscule in human terms, has accumulated over geological time. The process adds about 2.3 milliseconds to the length of a day every century. While the time it takes for Earth to orbit the sun has remained relatively stable, the number of times it spins on its axis during that journey has steadily decreased. This cosmic tug-of-war has stretched our day to its current 24 hours and reduced our year from the frenetic pace of the Devonian to the 365 days we know today.