Days and years on other planets

Venus takes 243 Earth days to turn once, longer than its 225-day trip around the Sun.

Rotation and orbit

A planet spins on its axis (rotation) and travels around the Sun (orbit). One rotation sets the length of a day. One orbit sets the length of a year. The two have nothing to do with each other, which is why they come out so differently from planet to planet.

Read the table one row at a time. The rotation column is measured against the distant stars, and the word backwards means the planet spins the opposite way from the direction it travels around the Sun. The orbit column switches from days to years once the numbers get large: Jupiter’s 11.9 years is about 4,331 days.

Short days on the giants

The giant planets are the fastest spinners. Jupiter turns once in 9.9 hours even though it is 11 times wider than Earth, so a point on its equator moves at more than 12 km per second. Earth takes 23.9 hours to turn once against the stars. Our 24-hour day is a little longer, because Earth also moves along its orbit during that time and must turn a bit extra to bring the Sun back to the same place in the sky.

That difference is tiny for Earth but huge for slow spinners. Mercury rotates once in 58.6 days, yet the time from one sunrise to the next is about 176 Earth days. On Venus, sunrise to sunrise takes about 117 Earth days. NASA lists both numbers, rotation and length of day, for each planet.

Venus and Uranus spin backwards compared with most planets. Uranus is also tipped over by about 98°, so it rolls around the Sun on its side, and each pole gets decades of daylight followed by decades of night.

Tilt is also what gives a planet seasons. Earth’s axis leans 23.4° and Mars leans 25.2°, so Mars has seasons much like ours. Because a Mars year lasts 687 Earth days, about 1.9 Earth years, each Martian season lasts nearly twice as long as the same season at home.

Long years far out

Planets far from the Sun have longer years for two reasons. Their orbits are bigger, and they move more slowly: Mercury races along at 47.4 km per second, Earth at 29.8, and Neptune at only 5.4. Johannes Kepler found a simple rule that ties the two together. If you measure the year in Earth years and the distance in AU, the year squared equals the distance cubed. A planet at 4 AU would take √(4 × 4 × 4) = 8 years. Try it with Jupiter: 5.20 cubed is about 141, and the square root of 141 is about 11.9 years, just as the table says.