How Long Is a Day on Other Planets? — For Kids

Elementary: One Full Spin on Each Planet

NASA Space Place is a NASA site for kids. It tells how many hours one full spin takes on each planet. Here they are, in order from the Sun.

1. Mercury — first from the Sun. NASA Space Place says a day on Mercury lasts 1,408 hours. That is much longer than a whole Earth day. NASA's Mercury facts page says it in a second way: Mercury completes one rotation every 59 Earth days.
2. Venus — second from the Sun. NASA Space Place says a day on Venus lasts 5,832 hours. That is the longest spin on this whole list. NASA's Venus facts page says it in a second way: a day on Venus would be 243 Earth days long.
3. Earth — third from the Sun. This is our home. NASA Space Place says Earth takes 24 hours to complete one spin.
4. Mars — fourth from the Sun. NASA Space Place says Mars takes 25 hours to complete one spin. That is very close to Earth. One Mars spin is just a bit longer than one Earth spin.
5. Jupiter — fifth from the Sun. NASA Space Place says Jupiter takes just 10 hours to complete one rotation. That is less than half of one Earth day. Jupiter spins the fastest of all eight planets.
6. Saturn — sixth from the Sun. NASA Space Place says Saturn takes 11 hours to complete one spin. That is less than half of one Earth day too.
7. Uranus — seventh from the Sun. NASA Space Place says Uranus takes 17 hours. That is shorter than one Earth day, but longer than a day on Jupiter or Saturn.
8. Neptune — eighth from the Sun. NASA Space Place says Neptune takes 16 hours. That is shorter than one Earth day too.

Long spins and short spins

Mercury and Venus spin very, very slowly. One spin takes much longer than a whole Earth day. You would wait and wait for one spin to end.

Earth and Mars are in the middle. Their spins are close to each other: 24 hours for Earth and 25 hours for Mars, says NASA Space Place.

Jupiter, Saturn, Uranus, and Neptune all spin in less time than one Earth day. Jupiter is the fastest of all. Its spin takes just 10 hours.

Venus spins backward

NASA's Venus facts page says Venus spins slowly in the opposite direction from most planets.

NASA says the Sun would rise in the west and set in the east on Venus, because Venus spins backward compared to Earth.

Words to know — Elementary

Spin: To turn all the way around once. A planet spins like a top, but much, much slower.

Rotation: NASA's word for one full spin of a planet.

Solar day: One cycle of daytime to nighttime. NASA Space Place says a solar day on Earth is around 24 hours.

Sidereal day: The time it takes for a planet to completely spin around and make one full rotation. That is how NASA Space Place puts it.

Earth day: One day here on Earth. We use Earth days to talk about the long, slow spins of Mercury and Venus.

Middle: Rotation Periods, in Plain Words

A day on this page is one full spin. NASA's Planetary Fact Sheet calls that spin the Rotation Period.

NASA's Fact Sheet notes say the Rotation Period is the time it takes for the planet to complete one rotation relative to the fixed background stars, in hours.

Here are NASA's numbers, planet by planet, in order from the Sun. Each number says which NASA page it came from.

1. Mercury

NASA Planetary Fact Sheet: Rotation Period is 1,407.6 hours.

NASA's Mercury facts page says it in Earth days: Mercury spins slowly on its axis and completes one rotation every 59 Earth days.

Those are two NASA ways to give the same slow spin: one in hours, one in Earth days.

2. Venus

NASA Planetary Fact Sheet: Rotation Period is -5,832.5 hours. The minus sign matters. See the note on backward spin below.

NASA's Venus facts page says it in Earth days: your day on Venus would be 243 Earth days long.

That one spin takes longer than a whole Venus year. NASA's Venus facts page says a Venus year — one trip around the Sun — takes only 225 Earth days.

3. Earth

NASA Planetary Fact Sheet: Rotation Period is 23.9 hours.

NASA's Earth facts page agrees: as Earth orbits the Sun, it completes one rotation every 23.9 hours.

4. Mars

NASA Planetary Fact Sheet: Rotation Period is 24.6 hours.

NASA's Mars facts page agrees: as Mars orbits the Sun, it completes one rotation every 24.6 hours, which is very similar to one day on Earth (23.9 hours).

NASA's Mars facts page also says Martian days are called sols — short for solar day.

5. Jupiter

NASA Planetary Fact Sheet: Rotation Period is 9.9 hours.

NASA's Jupiter facts page agrees: one day on Jupiter takes 9.9 hours — the time it takes for Jupiter to rotate or spin around once. NASA calls it the shortest day in the solar system.

6. Saturn

NASA Planetary Fact Sheet: Rotation Period is 10.7 hours.

NASA's Saturn facts page agrees: one day on Saturn takes only 10.7 hours — the time it takes for Saturn to rotate or spin around once. NASA calls it the second-shortest day in the solar system.

7. Uranus

NASA Planetary Fact Sheet: Rotation Period is -17.2 hours. The minus sign matters. See the notes below — NASA gives Uranus in two slightly different forms, and this page keeps them apart.

NASA Space Place gives a rounded form for kids: Uranus takes 17 hours.

8. Neptune

NASA Planetary Fact Sheet: Rotation Period is 16.1 hours.

NASA's Neptune facts page gives a rounded form: one day on Neptune takes about 16 hours — the time it takes for Neptune to rotate or spin once.

Why Venus spins backward — what NASA states

NASA's Venus facts page states the direction plainly: Venus spins slowly in the opposite direction from most planets, and Venus spins backward compared to Earth.

Because of that backward spin, NASA says the Sun would rise in the west and set in the east on Venus.

NASA's Fact Sheet notes give the science word for this: a negative Rotation Period means retrograde rotation — backwards relative to the Earth. The notes also say Venus rotates in a retrograde direction, opposite the other planets.

The NASA pages read for this guide state that Venus spins backward. They do not state a cause for why it spins that way, so this page does not invent one.

One more Venus number, kept separate: NASA's Venus facts page says sunrise to sunset would take 117 Earth days, because of the planet's extremely slow rotation. That is a sunrise-to-sunset time. It is not the 243-Earth-day full spin above. This page does not merge the two numbers.

Uranus spins backward too — and on its side

The Fact Sheet prints Uranus with a minus sign, -17.2 hours, just like Venus. By NASA's own note, a negative number means retrograde — backwards relative to the Earth.

NASA's Fact Sheet notes also say Uranus rotates almost on its side relative to its orbit.

Words to know — Middle

Rotation period: The time it takes for a planet to complete one rotation relative to the fixed background stars. That is how NASA's Fact Sheet notes define it.

Sidereal day: The time it takes for a planet to completely spin around and make one full rotation. NASA Space Place says a sidereal day on Earth is almost exactly 23 hours and 56 minutes.

Solar day: One cycle of daytime to nighttime. NASA Space Place says a solar day on Earth is around 24 hours.

Retrograde: Backwards relative to the Earth. NASA's Fact Sheet notes use this word for a negative Rotation Period. Venus rotates in a retrograde direction, opposite the other planets.

Sol: A Martian day. NASA's Mars facts page says Martian days are called sols, short for solar day.

Length of Day (Fact Sheet meaning): The average time for the Sun to move from the noon position in the sky back to the same position. NASA's Fact Sheet notes define it this way. It is not the same measure as the Rotation Period.