Earth rotates on its axis once every day. At the same time, Earth revolves around the Sun once a year, but our movement doesn’t stop there – it is less well known, but the entire solar system orbits the center of the Milky Way Galaxy every 240 million or so years. What is Earth’s speed and acceleration for all of these motions?

## Speed, Velocity, and Acceleration Explained

People often use the terms velocity and speed as if they are the same thing. Physicists, however, make a distinction between velocity and speed. Velocity includes direction, but speed does not. For example, a person running might have a speed of 6 miles per hour, but the person’s velocity might be 6 miles per hour towards the north.

Most people also think of an acceleration as an increase in speed. Physicists define acceleration as any change in velocity, which includes direction. An acceleration can therefore be either an increase or a decrease in speed as well as a change in direction at a constant speed. A car accelerates when the driver presses down on either the gas or brake pedal, or when the driver turns the steering wheel at a constant speed.

Physicists calculate the average speed by dividing the total distance traveled by the time needed to travel the distance. In equation form, it looks like this:

Average speed = net distance/time.

*v* = *d*/*t*

The acceleration needed to keep an object moving at a constant speed in a circular path is the centripetal acceleration. Physicists calculate the centripetal acceleration, from the object’s speed and the radius of the circular path, using a formula that looks like this:

Centripetal acceleration = speed squared/radius.

*a* = *v*^{2}/*r*

Earth’s orbit around the Sun is elliptical, but it is close enough to circular that using the approximation of a circular orbit is close enough for most purposes.

**Click to Read Page Two: What is a Solar Day vs. a Sidereal Day?**