Description: All motion is relative to the observer.
Key Terms: Galileo, relative motion, Earth

Everything moves—even things that appear to be at rest. They move relative to the Sun and stars. As you’re reading this, you’re moving at about 107,000 kilometers per hour relative to the Sun, and you’re moving even faster relative to the center of our galaxy. When we discuss the motion of something, we describe the motion relative to something else. If you walk down the aisle of a moving bus, your speed relative to the floor of the bus is likely quite different from your speed relative to the road. When we say a racing car reaches a speed of 300 kilometers per hour, we mean relative to the track. Unless stated otherwise, when we discuss the speeds of things in our environment, we mean relative to the surface of Earth. Motion is relative.