How Long is a Day on Mercury?

by Jason Major on December 28, 2012

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1/3 the distance from the Sun than Earth, it should be no surprise that a day on Mercury is a real scorcher with temperatures soaring over 400 ºC. But in addition to its solar proximity it also has an extremely slow rotation: a single day on Mercury is 58.6 Earth days long… and you thought your Mondays lasted forever!

A Day on MercuryTo be even more precise, for every 2 Mercury years, 3 Mercury days pass — a 3:2 spin-orbit resonance, caused by the planet’s varying elliptical orbit. (This also makes for some interesting motions of the Sun in Mercury’s sky.)

To illustrate this, UK’s The Open University has published a new video in their 60 Second Adventures in Astronomy series… check it out above (and see more of their excellent and amusing animations here.)

Video: The Open University. Narrated by David Mitchell.

About 

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

Kevin Frushour December 28, 2012 at 2:40 AM

Too much cute aliens, not enough explaining. The orbit would still make the sun rise or set before or after the spin of the planet made it happen.

And “Mercury is a scorcher” annoys me. It hardly has an atmosphere to be hot.

grapes December 28, 2012 at 4:00 AM

Rather than “1/3 closer to the Sun than Earth”, I think I’d’ve said “1/3 the distance to the Sun” instead

Jason Major December 28, 2012 at 4:37 AM

It should probably be “1/3 the distance from the Sun,” come to think of it.

Richard Kitchings December 28, 2012 at 5:24 AM

So how fast would you have to travel on Mercury to see the Sun in the same position in the ‘sky’? (I realize that no atmosphere means no sky.) Once around every 58.6×2 days? That’s only about 4700 miles, at 60 degrees latitude, in about 106 days which would be about 1.7 miles per hour. A slow walk. – My real question relates to maintaining a constant, comfortable temperature between, say, 0 and 100 C. Which would mean …

grapes December 28, 2012 at 3:05 PM

58.6×3. The rotation time is not the same as the length of day, of course. The earth spins 366 times per revolution, but “loses” a day by going around the sun. Mercury spins three times in two years, and “loses” two days–so the length of each day is two years long.

Skywise December 28, 2012 at 3:23 PM

As for realism, as close as Mercury is to the sun, I find it unlikely that the apparent size of the sun above Mercury in that animation is anywhere near accurate.

Jason Major December 28, 2012 at 4:55 PM

See this for an idea of the size of the Sun in Mercury’s sky: http://messenger-education.org/Interactives/ANIMATIONS/Places_In_Sun/places_in_sun.php

Tim Amato December 29, 2012 at 10:29 PM

If mercury has a day of 58.6 Earth days it would have over six days in one year.

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