How long does it take for Mars to rotate on its axis? What is the period of the Earth? The Moon?

A "period" usually means the amount of time it takes to do something. Since the planets and moons in our solar system all move in at least two different ways, a "period" could mean a couple of different things.

First, all the objects in our solar system spin on an axis. Just think of a top, or record, or wheel. All these things spin around a center point which we call an "axis". This type of motion is called "rotation", so the amount of time it takes an object to rotate is called a "rotational period". It takes the Earth 23 hours, 56 minutes to rotate on its axis. We call the rotational period of Earth a "day". Mars takes just a little longer to rotate, at 24 hours, 37 minutes. The Moon is much slower than either Earth or Mars. It takes over 655 hours for the Moon to turn just once on its axis--that's more than 27 days!

Second, all the planets and moons in our solar system also move around other things. This type of movement is called "orbit". Planets orbit the Sun while moons orbit around planets. The amount of time it takes to complete one orbit is called an "orbital period". On Earth, we call our orbital period a "year". It takes just over 365 days for the Earth to orbit the Sun. On Mars, it takes more than 687 days to orbit the Sun--that's almost twice as long to wait between birthdays!

The Moon's orbit centers around the Earth, rather than the Sun. It takes a little more than 27 days (about 656 hours) for the Moon to orbit the Earth. If that number looks familiar, it's because the orbital period of the Moon is exactly the same as its rotational period. This is the reason we can never see the far side of the Moon from Earth--it's always facing away from us.

How long does it take for Mars to rotate on its axis? What is the period of the Earth? The Moon?

One of the definitions Webster's dictionary gives for "period" is "the completion of a cycle". When we're thinking about the motions of planets, a period can be defined in a couple of different ways. The "rotational period" of an object in the solar system is the amount of time it takes for the object to complete one turn, or rotate on its axis. We call the rotational period of Earth a "day". Earth's day (or rotational period) is exactly 23.9345 hours (or, 23 hours, 56 minutes, 4.2 seconds). Mars takes just a little longer, at 24.6229 hours. The Moon is much slower than either Earth or Mars at 655.728 hours. In the time that it takes for the Moon to complete just one turn on its axis, the Earth will turn over 27 times!

In addition to rotational movement, the bodies in our solar system also move around, or orbit, other bodies. Planets orbit the Sun while moons orbit around planets. The amount of time required for a planet or moon to complete one orbit around another body is called an "orbital period". On Earth, we call this orbital period a "year". Our year is 365.256 days. On Mars it takes 686.98 days (Earth days, that is) to complete one orbit about the Sun--that's a long time to wait between birthdays!

Since the Moon's orbit centers on the Earth, rather than the Sun, it would be a little confusing to think of the Moon's orbit as a "year". Instead, this cycle of the Moon orbiting the Earth is much closer to our calendar system of "months". It takes 27.322 days (or 655.728 hours) for the Moon to orbit the Earth. If that number looks familiar, it's because the orbital period of the Moon is exactly the same as its rotational period. This is the reason we can never see the far side of the Moon from Earth--it's always facing away from us.

How long does it take for Mars to rotate on its axis? What is the period of the Earth? The Moon?

Webster's defines a "period" as "the completion of a cycle, a series of events, or a single action". With reference to the motion of planets and moons, a period can be defined in a couple of different ways. The "rotational period" of a celestial body is the amount of time required for it to complete one turn, or rotate on its axis. On Earth, we call this cycle a "day". Earth's day is exactly 23.9345 hours (23 hours, 56 minutes, 4.2 seconds). Mars' day is a little longer, at 24.6229 hours. The Moon is much slower, it takes 655.728 hours (27.322 days!) to complete one turn on its axis.

In addition to rotational movement, the bodies in our solar system also move around, or orbit, other bodies. Planets orbit the Sun while moons orbit their respective planets. The amount of time required for a body to complete one orbit around another body is therefore referred to as an "orbital period", also called a "revolutionary period". On Earth, we call this orbital period a "year". Our year is 365.256 days. On Mars you'll get a lot more time between birthdays since it takes 686.98 days (Earth days, that is) to complete one orbit about the Sun.

Since the Moon's orbit centers on the Earth, rather than the Sun, it would be misleading to think of the Moon's orbit as a "year". Instead, this cycle of the Moon orbiting the Earth more closely approximates our system of calendar "months". It takes 27.322 days for the Moon to orbit the Earth. If that number looks familiar, it's because the orbital period of the Moon is exactly the same as its rotational period. This is the reason we can never see the far side of the Moon from Earth--it's always facing away from us.

Last modified prior to September, 2000 by the Windows Team

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