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Which Planet has the Longest Day?

Simply put, the planet with the longest day is Mercury with an average day lasting 175.94 Earth days or 4222.6 hours.

If you google this question then you might get a different answer since some websites have a strange idea of what a day is.

Why do some sites give a different answer?

There are different ways to measure the length of the day on a planet. For most people the length of a day is the time it takes for the Sun to reach the noon position on successive days. This is a planet's "Solar Day", and is what most people think of a "day". Earth's mean solar day is 24 hours long.

Another way to measure a day is to say it is the length of time it takes for the planet to spin once on it's axis (measured with respect to the stars. This is a planet's "Stellar Day". Earth's stellar day is about 4 minutes less than 24 hours.

So, if you think (as we do at the planets today - and also as wikipedia does) that a day should be how long it takes for the Sun to go around a planet (as observed from a fixed place on the planet surface) then Mercury has the longest day in the solar system.

However if you think a day should be how long it takes for the stars to go around a planet (as observed from a fixed place on the planet surface) then Venus has it.

Why is a planets rotational period (stellar day) not the same as it's solar day?

It is true to say that a planets Solar Day and Stellar Day are never exactly the same, and for some planets they are wildely different.

If we take the Earth as an example: as mentioned above, a day on Earth (noon to noon) on average takes 24 hours (called the Mean Solar Day). However the Earth rotates once on it's axis every 23 hours, 56 minutes and 4 seconds in relation to the stars (called the Stellar Day).

The reason for the difference is because, as well as rotating on it's axis, the Earth also orbits the Sun once every year.

The Earth’s spin makes the Sun appear to move westerly in the sky by 360 degrees every 23 hours and 56 minutes. The Earth’s orbital motion makes the Sun appear to move easterly in the sky by 1/365th of a rotation (e.g. slightly less than 1 degree) each day. Therefore it takes slightly longer for the sun to appear in the same place in the sky than the rotational period, hence 24 hours.

So every year, the Earth spins on it's axis 366 times, but only has 365 days because it's orbited the Sun once which has in effect cancelled out one revolution.

Here are some imaginary examples of planets with different spin and orbital periods to give practical examples of differences in solar and stellar day lengths:

Non-Rotating Planet Example

If a planet orbits a sun but does not rotate on its axis then a day would be the same as a year. E.g. the sun would only appear to move because of the planets motion in its orbit and so noon will only occur once a year. So each year the planet rotates zero times but the day is caused by the single rotation around the Sun.

Planet Rotating with Orbit Example

If a planet orbits a sun and rotates on its axis exactly once in every orbit (and rotates in the same direction as the orbit - e.g. both orbit and spin are anticlockwise) then one side of the planet will always face the Sun. The bright side will have an ever lasting day and the night side everlasting night. So each year the planet will rotate once and that rotation will be cancelled out by the orbit.

Planet Rotating Opposite to Orbit Example

If the same planet was rotating in the opposite direction to the orbit (e.g. planet spinning clockwise and orbit anticlockwise) then an observer on the surface will see two days a year. One day from the rotation and one day from the orbit.

Planet Mercury has the Longest Day

Graphics showing the orbit and spin of planet mercury and showing that it has the longest dy of any planet at 176 earth days

Mercury has the longest day of any planet. It orbits the Sun once every 88 days but rotates once every 59 days. This means that for every 2 revolutions it rotates exactly three times. The net result is that a solar day on Mercury takes 176 Earth days.
Breaking down the two components of the Suns apparent motion in Mercury's sky: In two orbits, the Sun has moved 2x360 degrees in one direction due to the planets orbital motion, and 3x360 degrees in the other direction due to the planets axial rotation, resulting in one single 360 rotation of the Sun.

How long is a day on each planet?

Planet Length of Day Description
in Earth Days in Hours
Mercury 176 4222.6

See above for a description.

Venus 117 2802.0 Venus orbits the Sun (in the same anticlockwise direction as all the other planets) once every 224.7 days but has a very slow rotation in a clockwise direction (which is opposite to most other planets) once every 243 days. Because the orbit and rotation are in opposite directions, it means that the movement of the Sun in the sky is faster than its rotation at 360 degrees in 116.7 days.
Mars 1.03 24.7 Mars has a day only slightly longer than Earth’s It rotates once every 24.6 hours and has an orbital period of 687 days. A martian year is therefore 667 martian days long.
Earth 1 24 ... as we all know.
Uranus 0.7 17.2

Like most of the gas giants, Uranus spins quite fast on its axis (once every 17.2 hours) but orbits very slowly (once every 84 years). This means that it's orbital motion has a very small effect on the length of the day, making it about 1 second longer than the rotation period.

Neptune 0.7 16.1 As with Uranus, Neptune's fast rotation (16.1 hours) is almost the same as its rotational period because of the long orbital period (165 years).
Saturn 0.45 10.7 Rotation is 10.7 days, orbit 29.45 years.
Jupiter 0.41 9.9 The planet with the shortest day is Jupiter at 9.9 hours. This is very surprising since it is Jupiter is, by far, the largest planet. It orbits every 11.7 years,