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.

 

Planet Mercury has the Longest Day

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.

 

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.

Strangley some websites which state Venus has the longest day, then go on to say that Venus' day is 243 Earth days long... which is only true if you define Earth's day as being its solar day (24 hours) and Venus' day as being a stellar day (5832.5 hours), If you define a day as being a stellar day on both planets then you should say that Venus's day is 244 Earth days long, not 243!

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 wildly 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 (about 1 degree every 4 minutes). The Earth’s orbital motion makes the Sun appear to move easterly in the sky by 360 degrees every year.... or (360 degrees/365 days per year =) 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 has 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.

 

How long is a day on each planet?

The following table shows the length of a solar day for each of the planets in our solar system.

Planet	Length of Day		Description
	in Earth Days	in Hours
Mercury	176	4222.6	See the "Planet Mercury has the Longest Day" section above for a full 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 axial rotation and orbital motion add up so that the movement of the Sun in the sky is faster than its axial rotation alone 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 earth 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	Again the fast rotation and slow orbit mean the solar and stellar days for Saturn are very similar. Rotation is 10.7 hours, 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,

Here are some answers to other popular questions about the solar system:

Which planet has the shortest day?

Jupiter, the largest planet in our solar system, has the shortest day at 9.9 hours.

 

Which planet has the longest day and the shortest year?

The answer to both questions is Mercury. It orbits the Sun every 88 days, and a Solar Day on Mercury lasts for two orbits which is 176 days.

 

What is the planet with the longest year?

Neptune has the longest year of 164.77 years. This is because it is the furthest planet from the Sun and so it moves slower and has further to go in its year than any other planet.

 

Which planet has the most moons?

Saturn has the most moons at 82. More Info.
New moons can be discovered at any time - 20 new Saturnian moons were announced in October 2019.

 

How many moons does each planet have?

Saturn has 82 moons.

Jupiter has 79 moons.

Uranus as 27 moons.

Neptune has 14 moons.

Mars has 2 moons,

Earth has 1 moon.

Venus and Mercury have no moons.

(Last updated April 2020)

 

Which planet has the least number of satellites?

Venus and Mercury have no satellites (moons).