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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 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.
Compared to Earth, Mercury has a very long day. A day on Mercury takes 58 days and 15 hours in Earth days. A day on Mercury is only a little shorter than a year on the planet.
Venus is the slowest moving planet. In fact, it is so slow that while the other planets have flattened out at the poles due to the speed at which they spin, Venus has not flattened. Venus has the longest day of any planet in our Solar System. A day on Venus is 243 Earth days. Venus’ day is actually longer than its year. It only takes 224.7 Earth days for the planet to orbit the Sun.
We measure a 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 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 9.9 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.
A day on Uranus only takes 17 hours 14 minutes and 24 seconds. Since the planet has such an extreme axial tilt (97.8°), it actually rotates on its side.
A day on Neptune lasts 16 hours 6 minutes and 36 seconds in Earth time. One odd thing about Neptune is that different parts of the planet rotate at different speed because the planet is mostly gas. The equator takes a couple more hours to rotate than the average while the poles rotate in about 12 hours.
If you are looking for more information, try how long a day on Saturn is and all about the planets.
Astronomy Cast has episodes on all the planets including Mercury.