Length of Year on Mercury

by Fraser Cain on December 17, 2009

Length of Year on Mercury

Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry


The length of year on Mercury is 87.969 days. In other words, it takes almost 88 Earth days for Mercury to complete one orbit around the Sun. Mercury completes just over 4 orbits for each year on Earth.

Mercury has the most eccentric of all the orbits of the planets. Its distance from the Sun varies between 46 million and 70 million kilometers. This means that the speed of its orbit varies dramatically depending on the point of its orbit. If you could stand on the surface of Mercury and watch the Sun, you would see the Sun rise in the morning go part way up in to the sky and then go backwards in the sky, and set again. And then it would rise again and this time it would go across the sky and set. Four days before the fastest point of its orbit around the Sun, Mercury’s orbital speed matches its rotational velocity so that the Sun appears to stop in the sky. Then it’s orbiting faster than it’s rotating for about 8 days and so the Sun appears to move backwards.

We’ve written several articles about the length of years for Universe Today. Here’s an article about the years of all the planets, and here’s an article about how long a year is on Mars.

If you’d like more info on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded several episodes of Astronomy Cast about Mercury. Listen here, Episode 49: Mercury.

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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