Rotation and revolution are terms that describe the angular motion of objects, that is, motion about a real or imaginary axis. They are often confused not only for this reason, but also because they often apply in different ways to the same object at the same time (especially in astronomy) and to some extent because the words look somewhat alike in English.
The Earth on which you presently ride in various directions is an example of a body that undergoes both rotation and revolution. When you learn that any body does such a thing, the first question becomes "Around what does the body revolve?" You need not ask this about rotation, for reasons you will soon learn.
Revolving vs. Rotating
Before getting into the physics of rotating bodies, it is useful to dispense of the confusion between the terms rotation and revolution. The easiest way to remember the difference is that revolution is simply rotation around a distant (i.e., not physically connected) object. Thus, as hinted at in the above paragraph, revolution by definition involves two (or more) objects.
When describing motion in physics, "revolution" is usually an astronomy term, but the word is used loosely in the everyday world; for example the "RPM" on your car's tachometer stands for "revolutions per minute."
Rotation, or angular motion, is defined as the circular motion of an object around its center of mass. It is what is implied by the everyday term "spinning," although an object can rotate without completing one full "spin," or rotation.
Linear motion, or translation, is described in terms of displacement (x, y or z), time (t), velocity (v) and acceleration (a). Angular motion, or rotation, correspondingly employs the terms angular displacement (r and θ), time (t), angular velocity (ω) and angular acceleration (α ).
- The time it takes or would take for a rotating body to complete one rotation (or revolution) at constant average velocity is its period.
Rotation and Revolution in Astronomy
The Earth completes one rotation around its own axis every 24 hours, give or take a tiny amount. This is thus the Earth's period of rotation and is called a day. (The term "around its own axis" is redundant, as this describes all rotational motion, but it is good to reinforce concepts of motion.) This axis is not a physical one, as in the case of a movable globe, but an imaginary line drawn through the north and south poles – explaining exactly why they were chosen despite their inhospitable conditions!
The Earth also revolves around the sun, and does so once every 365.25 days or so. This period of revolution is known as year, and applies to other planets revolving around, or orbiting, the sun, the periods of which are usually given in terms of "Earth-years." Were the Earth connected to the sun by a long metal rod, it would be rotating rather than revolving, as the sun and Earth would then be one object, shaped like a very uneven dumbbell.
The Fun Case of the Moon
You may have noticed that the same side of the moon always faces the Earth. You might assume that, while the moon plainly revolves around the Earth, it must not be rotating at all.
In fact, this is not the case. Instead, the moon has a period of rotation that exactly matches that of its period of revolution about Earth – close to 28 days. As a result, its spinning keeps tempo with its circular path in space, and Earthlings therefore see only one half of their only natural satellite.
Extra study: What would the moon look like from Earth if it did not rotate at all? The best way to arrive at the answer is to move a labeled circle around another at a distance while keeping its labels facing the same direction. How would this affect the view from the same spot on Earth on successive days, when the moon has moved about 1/28 of its orbit around the Earth?
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.
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