Hubble's law

Hubble's law explains why galaxies (groups of stars, planets, and other things) appear to be moving away from us. It is based on measurements made by astronomer Edwin Hubble in the 1920s. He discovered that the farther away a galaxy is from us, the faster it is moving away. He also found that galaxies that are twice as far away are moving away at twice the speed. This is sometimes called the Hubble Constant. Hubble's law is important because it explains why the universe is expanding, which means that the galaxies in it are getting farther apart over time.