On which planet is a day longer than a year?

Answer 1

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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