Science, Tech, Math › Science How Long Is a Day on Other Planets? The Earth is the only planet with an approximately 24-hour day Share Flipboard Email Print Comfreak/Pixabay Science Astronomy Stars, Planets, and Galaxies An Introduction to Astronomy Important Astronomers Solar System Space Exploration Chemistry Biology Physics Geology Weather & Climate By Carolyn Collins Petersen Astronomy Expert M.S., Journalism and Mass Communications, University of Colorado - Boulder B.S., Education, University of Colorado Carolyn Collins Petersen is an astronomy expert and the author of seven books on space science. She previously worked on a Hubble Space Telescope instrument team. our editorial process Facebook Facebook Carolyn Collins Petersen Updated July 19, 2019 The definition of a day is the amount of time it takes an astronomical object to complete one full spin on its axis. On Earth, a day is 23 hours and 56 minutes, but other planets and bodies rotate at different rates. The Moon, for example, spins on its axis once every 29.5 days. That means future lunar inhabitants will have to get used to a sunlight "day" that lasts for about 14 Earth days and a "night" that lasts about the same time. Scientists typically measure days on other planets and astronomical objects in reference to Earth's day. This standard is applied across the solar system to avoid confusion when discussing events that occur on those worlds. However, each celestial body's day is a different length, whether it's a planet, moon, or asteroid. If it turns on its axis, it has a "day and night" cycle. The following table depicts the day lengths of the planets in the solar system. Planet Length of Day Mercury 58.6 Earth days Venus 243 Earth days Earth 23 hours, 56 minutes Mars 24 hours, 37 minutes Jupiter 9 hours, 55 minutes Saturn 10 hours, 33 minutes Uranus 17 hours, 14 minutes Neptune 15 hours, 57 minutes Pluto 6.4 Earth days Mercury NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Wikimedia Commons/Public Domain The planet Mercury takes 58.6 Earth days to spin once on its axis. That may seem long, but think about this: its year is only 88 Earth days long! That's because it orbits very close to the Sun. There's a twist, however. Mercury is gravitationally locked with the Sun in such a way that it rotates three times on its axis for every two times it goes around the Sun. If people could live on Mercury, they'd experience one full day (sunrise to sunrise) every two Mercurian years. Venus Kevin Gill/Flickr/CC BY 2.0 Planet Venus spins so slowly on its axis that one day on the planet lasts nearly 243 Earth days. Because it's closer to the Sun than Earth is, the planet has a 225-day year. So, the day is actually longer than a year, which means that Venus residents would only get to see two sunrises per year. One more fact to remember: Venus spins "backward" on its axis compared to Earth, which means those two yearly sunrises take place in the west and sunsets occur in the east. Mars ColiN00B/Pixabay At 24 hours and 37 minutes, the Mars day length is very similar to Earth's, which is one of the reasons that Mars is often thought of as something of a twin to Earth. Because Mars is farther than Earth from the Sun, however, its year is longer than Earth's at 687 Earth days. Jupiter Aurelien_L/Pixabay When it comes to gas giant worlds, "day length" is a more difficult thing to determine. The outer worlds don't have solid surfaces, although they do have solid cores covered with huge layers of clouds and layers of liquid metallic hydrogen and helium beneath the clouds. On the gas giant planet Jupiter, the equatorial region of the cloud belts rotates at a rate of nine hours and 56 minutes, while the poles rotate quite a bit faster, at nine hours and 50 minutes. The "canonical" (that is, commonly accepted) day length on Jupiter is determined by the rotation rate of its magnetic field, which is nine hours, 55 minutes long. Saturn NASA / JPL / Space Science Institute/Wikimedia Commons/Public Domain Based on measurements of various parts of gas giant Saturn (including its cloud layers and magnetic field) by the Cassini spacecraft, planetary scientists determined that the official length of Saturn's day is ten hours and 33 minutes. Uranus Orange-kun (old version user: Brian0918)/Wikimedia Commons/Public Domain Uranus is a weird world in many ways. The most unusual thing about Uranus is that it's tipped over on its side, and "rolls" around the Sun on its side. That means one axis or the other is pointed at the Sun during part of its 84-year orbit. The planet does rotate on its axis once every 17 hours and 14 minutes. The length of day and the length of the Uranian year and the weird axial tilt all combine to create a day that's as long as a season on this planet. Neptune Kevin Gill from Los Angeles, CA, United States/Wikimedia Commons/CC BY 2.0 The gas giant planet Neptune has a day length of approximately 15 hours. It took scientists a number of years to calculate the rotation rate of this gas giant. They accomplished the task by studying images of the planet as features rotated around in its atmosphere. No spacecraft has visited Neptune since Voyager 2 in 1989, so Neptune's day must be studied from the ground. Pluto NASA/JHUAPL/SwRI/Wikimedia Commons/Public Domain Dwarf planet Pluto has the longest year of all the known planets (so far), at 248 years. Its day is a lot shorter, but still longer than Earth's, at six Earth days and 9.5 hours. Pluto is tipped over on its side at an angle of 122 degrees with respect to the Sun. As a result, during part of its year, portions of Pluto's surface are either in continuous daylight or constant night-time. Key Takeaways The Earth is the only planet with an approximately 24-hour day.Jupiter has the shortest day of all the planets. A day on Jupiter lasts only nine hours and 55 minutes.Venus has the longest day of all the planets. A day on Venus lasts 243 Earth days. Uranus: More than Just a Distant Dot of Light Rotation and Revolution: Two Important Astronomy Terms Fascinating Astronomy Facts All About Planet Venus How to Use Your New Telescopes to View Planets The Imaginary Line That Marks the Earth's Circumference Why Do We Have Seasons? What Makes a Terrestrial Planet? Pluto and its Place in the Solar System Could Jupiter Become a Star? Meteorites from Other Planets How the Solar System Got Started Learn about the Worlds of the Outer Solar System Do You Know What Day the Earth is Closest to the Sun? How Fast Does the Earth Rotate on Its Axis and Orbit the Sun? What Is the Longest Day of the Year?