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Learn about planets outside our solar system through Exoplanets and Alien Solar Systems by Tahir Yaqoob, Ph.D., a book in our online store book collection.
This image shows different objects that rotate. Rotational period is the time it takes for that object to rotate just once.

Rotational Period

Rotational period is the term describing the length of time necessary for a space object to make one complete rotation. This time varies from planet to planet, and on Earth it is 1 day.

This length of time is not always constant, and in fact the Earth’s rotational period is slowing (by 0.002 seconds every hundred years). This is because of the Moon’s influence on our rotation.

One planet that has already slowed down a lot is Venus. Scientists think this is because a long time ago something big hit Venus and slowed down its rotation. Now, Venus spins so slowly that it revolves around the Sun faster than it spins on its own axis. That means that a Venus day is longer than a Venus year!

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF