The period of rotation is the length of time it takes for an astronomical body to complete one full turn about its axis. As an example, the Earth's period of rotation is approximately 24 hours, which is the reason that a day on Earth is 24 hours long. Calculating the period of rotation for different bodies requires a small understanding of geometry. You can measure a rotation of a planet with respect to the position of the sun or the stars.

Determine how much the body has rotated in a given period of time. For example, assume that a planet or other spherical astronomical body has rotated 120 degrees in 15 hours with respect to the sun's position.

Divide the number of degrees in a complete circle  360 degrees  by the number of degrees the planet has rotated. In the example, dividing 360 degrees by 120 degrees yields 3. Call this result A.

Multiply result A by the time it took the planet to rotate the measured amount. Concluding the example, multiplying 3 by 15 hours gives 45 hours. This is the period of rotation of the planetary body.
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