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SCIENCE: Why do leap years exist?

Simple Science | Feb. 29, 2016

This year is a leap year, which means there is an extra day on the calendar. A leap day is added every four years, making the year 366 days long rather than 365.

The rotation of the Earth around its axis (a day) has essentially nothing to do with the length of its orbit around the Sun (a year). The length of a day is just a convenient way to measure periods of time.

The length of a year is about 365.2425 days long, meaning that with a 365-day-long calendar, you would be behind the seasonal cycle by one day every four years.

This lag would be horrible for farmers who rely on the seasons to plant crops, so a day is added every four years.

Large events, such as earthquakes, can speed up Earth’s rotation in the magnitude of microseconds. This adds up over time, and in order to keep with atomic times, a leap second is added when needed.

Tidal drag from the Moon is the primary cause of Earth's rotational slowdown. The tides, pulled up by the Moon’s gravity, are pushed ahead of the connecting line between the Earth and the Moon by Earth’s rotation. This creates a torque on the Earth, slowing down its rotation. 

Eventually it will become tidally locked, meaning the Earth's rotation will slow down enough that one face will always look at the Moon. The Moon is already tidally locked, meaning the same face has faced the Earth for as long as humans can remember.

Due to gravity, the Moon is also losing speed relative to the Earth. While it is currently moving a few centimeters further from the planet every year, this state will soon change.

When that happens, the Moon will continue to get closer to the Earth every year that passes.


Michael Makmur is a School of Arts and Sciences sophomore majoring in astrophysics. He is a staff writer for The Daily Targum. Follow him on Twitter @MikeMakmur for more.

Michael Makmur

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