What is the length of the day on each of the planets of the Solar System?

Answer 1

Using Earth hours and days as measurements for the 8 main planets:

Jupiter: 9 hrs 50mins

Saturn: 10hrs. 14mins

Neptune: 16hrs. 6mins

Uranus: 17hrs. 54mins

Earth: 23hrs. 56mins

Mars: 24hrs. 37mins

Mercury: 1408 hours (58 Earth days and 16 hours)

Venus: 5832 hours (243 Earth days)

Answer 2

We are accustomed to thinking of a "day" as a period of 24 hours. However, that definition is a narrow one that only applies to our own planet. A "sidereal day" is the length of time it takes for a planet to complete one rotation on its axis (360°). Since all of the planets rotate at different speeds, the length of a day on each one differs. Actually there are two main definitions of "day". Besides the rotation time there's the "solar day"

which takes into account a planet's orbit around the Sun.

Compared to Earth, Mercury has a very long day. A sidereal day on Mercury takes 58 days and 15 hours in Earth days. This "day" on Mercury is only a little shorter than a year on the planet (about 88 Earth days).

The "solar day" for Mercury is TWICE the length of its year.

Venus is the slowest rotating planet. In fact, it is so slow that while the other planets (except Mercury) have flattened out at the poles due to the speed at which they spin, Venus has not flattened. Venus has the longest sidereal day of any planet in our Solar System. A sidereal day on Venus is 243 Earth days. This day is actually longer than its year. It only takes 224.7 Earth days for the planet to orbit the Sun (its year).

We measure a "solar day" on Earth as a period of 24 hours, but it actually takes the planet 23 hours 56 minutes and 4 seconds to rotate on its axis. This measurement is actually called a sidereal day and is the length of time it takes the Earth to come back to the same position relative to the stars. While it seems that we would lose 4 minutes every day, that does not happen because the Earth is also orbiting the Sun.

A solar day on Mars takes 24 hours 39 minutes and 35 seconds in Earth time. The length of a day is similar to a day on Earth, which is one reason why scientists believed that plants and animals could survive on the Red Planet.

Jupiter is the largest planet, and it also has the shortest day. It takes Jupiter only about 9.92 Earth hours to complete a rotation. Because the planet spins so quickly, it is extremely flattened at the poles and bulges out at the equator. The diameter of the equator is 9,300 km bigger than the diameter of the planet's poles.

The length of a day on Saturn is extremely difficult to determine. It is always harder to determine the length of a day of a gas giant than it is to determine the length of a day of a terrestrial planet. Saturn's day was measured at 10 hours 39 minutes and 24 seconds in the '80s. Then it was measured at approximately 10 hours 45 minutes and 45 seconds. In 2006, astronomers used more advanced equipment to measure the planets ending up with a measurement of about 10 hours and 47 minutes. Some astronomers believe that we will never know how long a day on Saturn is.

Here are the full details, with slight approximations. Only one value is given for the "outer planets", because there is no significant difference between the two "days" for them :

Mercury: Sidereal day: 58.65 Earth days.

Solar day : 175.94 Earth days.

Venus: Sidereal day: 243.02 Earth days.

Solar day: 116.75 Earth days. Unusually, the Sidereal day is longer than the Solar day. This is because Venus has "retrograde" rotation.

Earth : Sidereal day: 23.934 hours.

Solar day: 24 hours exactly.

Mars: Sidereal day: 24.623 hours.

Solar day: 24.660 hours.

Jupiter: 9.92 hours.

Saturn: 10.66 hours.

Uranus: 17.24 hours.

Neptune: 16.11 hours.