Earth Sciences
Planetary Science

On which planet is a day longer than a year?

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2013-04-18 10:14:03

It depends on how you define "day". A "solar day" is roughly

from sunrise to the following sunrise, or more exactly noon to

noon. A "sidereal day" is one complete rotation of a planet

relative to the background stars. The orbit of the planet around

the Sun affects how the sidereal day is related to the length of

the solar day.

Mercury has a solar day longer than its year.

Venus has a sidereal day longer than its year.

Mercury takes 88 Earth days to go around the Sun. But it

spins very slowly on its axis, once every 58.6 Earth days relative

to background stars. Because it is turning as it orbits, the solar

day (sunrise to sunrise) on Mercury takes 176 Earth days, which is

twice the length of its year.

Venus takes 243 Earth days to spin on its axis relative

to the background stars (its "sidereal day"). It takes 224.7 Earth

days to orbit the Sun. Its rotation, moreover, is clockwise, unlike

the Earth and most other planets. Because of this reverse rotation,

and its slowness compared with the orbital period, a "solar day" is

only 116.75 Earth days.

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