What are Rotation and Revolution?

star trails
This image shows a dark Chilean sky filled with spectacular star trails — caused by the Earth's rotation during the camera's long exposure time. Underneath these dramatic streaks lies the Paranal Residencia, an oasis to the staff and visitors to ESO's Very Large Telescope, located high on Cerro Paranal in the Chilean desert. John Colosimo/ESO public images. CC BY 4.0

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, 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: you move slowly at first, but you pick up speed (or accelerate) the longer you drive it. 

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. Actually, they aren't exclusive to astronomy. Both terms are important facets of mathematics, especially geometry, as well as physics and chemistry. So, knowing what they mean and the difference between the two is an important part of a person's education.

Rotation

The strict definition of rotation is the circular movement of an object about a point in space. If you've ever studied geometry, you probably know about this. But, imagine a point on a piece of paper. If you rotate the piece of paper while it's lying flat on the table, you're essentially rotating every point on that piece of paper around the central point.

If you drew the same point onto a ball and spun the ball around on the point, you'd be rotating the rest of the ball around the point. 

For purpose of the kinds of objects discussed in astronomy, rotation is applied to 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 many astronomical objects. When the axis of rotation pass through the object it is said to spin, like that top mentioned above. In astronomy, many objects spin on their axes — stars, planets, neutron stars, pulsars, and so on. 

Revolution

It is not necessary, however, 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 all together.

When that happens, we have an object 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 usually referred to as an orbit.

The Earth-Sun 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 rotates, or more specifically orbits, around the Sun. 

To make things easier, just think of spin as something that objects do on their axes (plural of axis). If you want, you can say it's rotating on its axis, too.

 

Orbit is the motion of one object around another. We see Earth orbiting 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 in the Local Group.

You will often hear someone say that Earth revolves around the Sun. That would be an accurate description of the same motion as "orbits the Sun".  Orbit is a bit more precise, and is the motion that can be calculated using the masses, gravity, and the distance between the orbiting bodies.

You will also often hear people 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. 

Updated and edited by Carolyn Collins Petersen.