An astronomical unit is one of the more common measurements used in astronomy. It is a distance equal to the average distance from the Earth to the Sun, or 92,960,116 miles (149,597,870 kilometers); it is often seen rounded off to 93,000,000 miles (149,598,770 kilometers) and used in reference to great astronomical distances. For example, the Earth is 1 AU from the Sun; the planet Venus is 0.7 AU; Mars is 1.5 AU; and Saturn is 9.5 AU from the Sun.

A light year is an even larger unit. As the name implies, it is the distance light travels in one year, or about 5.88 trillion miles (9.46 trillion kilometers). In most cases, light year measurement is reserved for deep space objects. (For more about measurement, see “Mathematics throughout History.”)