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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.


Submitted by Elizabeth (West Virginia, USA)
(October 15, 1997)



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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA