# How Long is a Day on Mercury

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.

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.

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

Comments on this entry are closed.

• Jim Baerg August 20, 2008, 12:38 PM

You left out the really bizarre stuff about the difference between the sidereal day & the mean solar day.

The sidereal day is the time for the planet to rotate far enough that an observer on the surface sees the same star (not at a pole) due south or north again. While the mean solar day is the (average) time from noon to noon.

On earth the mean solar day is about 4 minutes longer than the sidereal day because the earth has moved in its orbit & the earth has to turn a bit farther before the sun is at noon again.

On mercury, since the rotation is only a little faster than the revolution, the solar day is much longer than sidereal day.

The sidereal day is 58 days & 15 hours, (2/3 of a mercury year) but the solar day is176 days (2 mercury years)

Ideally there should be some diagrams to show how this works.

• mitch September 21, 2008, 4:41 AM

Mercury has very very long days. 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.

wow is that real? amazing, but not sure does it mean that it has like 2000+ earth days, or only 88 earth days? so like there are only really 1.5 mercury days in 1 mercury year? please tell.