# What are Rotation and Revolution?

## Astro-Language

The language of astronomy has many interesting terms such as light-year, planet, galaxy, nebula, black hole, supernova, planetary nebula, and others. These all describe objects in the universe. However, those are just objects in space. If we want to understand them better, we have to know something about their motions.

However, to understand them and their motions, astronomers use terminology from physics and mathematics to describe those motions and other characteristics. So, for example, we use "velocity" to talk about how fast an object moves. The term "acceleration", which comes from physics (as does velocity), refers to the rate of an object's motion over time. Think of it like starting up a car: the driver pushes on the accelerator, which causes the car to move slowly at first. The car eventually picks up speed (or accelerates) as long as the driver keeps pushing on the gas pedal.

Two other terms used in science are rotation and revolution. They do not mean the same thing, but they do describe motions that objects make. And, they are often used interchangeably. Rotation and revolution aren't terms exclusive to astronomy. Both are important facets of mathematics, especially geometry, where geometrical objects can be rotated and their motion described using mathematics. The terms are also used in physics and chemistry. So, knowing what they mean and the difference between the two is useful knowledge, particularly in astronomy.

## Rotation

The strict definition of rotation is "the circular movement of an object about a point in space." This is used in geometry as well as astronomy and physics. To help visualize it, imagine a point on a piece of paper. Rotate the piece of paper while it's lying flat on the table. What's happening is that essentially every point is rotating around the place on the paper where the point is drawn. Now, imagine a point in the middle of a spinning ball. All the other points in the ball rotate around the point. Draw a line through the center of the ball where the point lies, and that's its axis.

For the kinds of objects discussed in astronomy, rotation is used to describe an object rotating about an axis. Think of a merry-go-round. It rotates around the center pole, which is the axis. Earth rotates around on its axis in the same way. In fact, so do many astronomical objects: stars, moons, asteroids, and pulsars. When the axis of rotation passes through the object it is said to spin, like that top mentioned above, on the point of the axis.

## Revolution

It is not necessary for the axis of rotation to actually pass through the object in question. In some cases, the axis of rotation is outside of the object altogether. When that happens, the outer object is revolving around the axis of rotation. Examples of revolution would be a ball on the end of a string, or a planet going around a star. However, in the case of planets revolving around stars, the motion is also commonly referred to as an orbit.

## The Sun-Earth System

Now, since astronomy often deals with multiple objects in motion, things can get complex. In some systems, there are multiple axes of rotation. One classic astronomy example is the Earth-Sun system. Both the Sun and the Earth rotate individually, but the Earth also revolves, or more specifically orbits, around the Sun. An object can have more than one axis of rotation, such as some asteroids. To make things easier, just think of spin as something that objects do on their axes (plural of axis).

Orbit is the motion of one object around another. Earth orbits the Sun. The Moon orbits Earth. The Sun orbits the center of the Milky Way. It's likely that the Milky Way is orbiting something else within the Local Group, which is the grouping of galaxies where it exists. Galaxies can also orbit around a common point with other galaxies. In some cases, those orbits bring galaxies so close together that they collide.

Sometimes people will say that Earth revolves around the Sun. Orbit is more precise and is the motion that can be calculated using the masses, gravity, and the distance between the orbiting bodies.

Sometimes we hear someone refer to the time it takes for a planet to make one orbit around the Sun as "one revolution". That's rather more old-fashioned, but it's perfectly legitimate. The word "revolution" comes from the word "revolve" and so it makes sense to use the term, although it's not strictly a scientific definition.

The important thing to remember is that objects are in motion throughout the universe, whether they are orbiting each other, a common point of gravity, or spinning on one or more axes as they move.

## Fast Facts

• Rotation usually refers to something rotating on its axis.
• Revolution usually refers to something orbiting something else (like Earth around the Sun).
• Both terms have specific uses and meanings in science and mathematics.

Updated and edited by Carolyn Collins Petersen.

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