How Long is a Day on Mercury

Using radio waves to calculate Mercury's orbit. Image credit: NASA

Mercury has very very long days. How long is one day on Mercury? Each day on Mercury lasts as long as 58 days, 15 hours on Earth. This is quite a feat, considering a whole year on Mercury is only 88 days.

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It gets even stranger, though. Because the orbit of Mercury is very eccentric, it reaches a point in its orbit when the speed of its orbital velocity matches its angular rotational velocity. When this happens, the Sun will appear to go backwards in the sky before it resumes its regular direction.

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Astronomers used to think that Mercury was tidally locked to the Sun. In other words, its period of rotation matched its orbital period. In this situation, Mercury would always show the same side to the Sun. But in the 1960s, this was shown to be incorrect.

Cuánto tiempo es un día de Mercurio

Fraser Cain

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay. Here's a link to my Mastodon account.