The galactic year, also known as a cosmic year, is the duration of time required for the Solar System to orbit once around the center of the Milky Way Galaxy. Estimates of the length of one orbit range from 225 to 250 million "terrestrial" years. According to Nasa, the Solar System is traveling at an average speed of 828,000 km/h (230 km/s) or 514,000 mph (143 mi/s), which is about one 1300th of the speed of light. If you could travel at that speed in a jet aircraft along the equator, you would go all the way around the world in approximately 2 minutes and 54 seconds. According to NASA, even at this incredible speed, it still takes the solar system 230 million years to orbit the center of the Milky Way Galaxy one time.

The galactic year provides a conveniently usable unit for depicting cosmic and geological time periods together. By contrast, a "billion-year" scale does not allow for useful discrimination between geologic events, and a "million-year" scale requires some rather large numbers.