Our day is 24 hours long, but it is slowly getting longer. Every 100 years, a day gets about 1.8 milliseconds longer. This change is too small for people to notice.

The Moon is the reason. It pulls on the oceans and makes tides, and the tides work like a brake on the Earth. About 400 million years ago, one year had about 400 days!

A day on Earth is not always exactly the same length. It has been getting longer by about 1.8 milliseconds every 100 years. The main cause is the Moon. Its gravity pulls on the oceans and creates tides, and the friction between the moving water and the sea floor slowly brakes Earth's spin.

As Earth slows down, the Moon moves away by about 3.8 centimeters a year, which scientists measure by shining lasers at mirrors left on the Moon by astronauts. Fossil corals tell a similar story. In 1963, a scientist counted daily growth lines within yearly bands in 400-million-year-old corals and found about 400 days in a year.

Earth's rotation is gradually slowing, and ancient astronomers unknowingly recorded the evidence. In 2016, British researchers led by Stephenson analyzed records of eclipses from 720 BC to AD 1600, drawn from Babylonian, Chinese, Arab, and European sources, plus later records of the Moon passing in front of stars up to 2015.

By comparing where eclipses were actually seen with where they would have been visible if Earth's spin had never changed, they concluded that the day has lengthened by about 1.8 milliseconds per century. Tidal friction, mainly from the Moon, should produce about 2.3 milliseconds; the shortfall is attributed to land still rising after the last ice age, which changes Earth's shape and slightly speeds its spin.

Meanwhile, the Moon recedes by about 3.8 centimeters a year as angular momentum is transferred to its orbit. Over longer timescales, daily growth lines in 400-million-year-old corals suggest roughly 400 days per year. Other effects complicate things. Movements in the liquid core cause fluctuations over decades, and a 2024 study argued that melting polar ice, by shifting water toward the equator, is now lengthening the day as well. Coral counts also involve some subjective judgment.