What is an AU (astronomical unit)?

An astronomical unit (AU) is defined as the average distance from the Earth to the Sun, which is approximately 93 million miles or about 150 million kilometers. This unit of measurement is primarily used in astronomy to provide a convenient way to express distances within our solar system. The AU is particularly useful because the distances between celestial bodies can be incredibly vast and complicated to communicate using standard units like kilometers or miles.

The AU serves as a benchmark for measuring other distances in the solar system, allowing astronomers to easily compare how far other planets, comets, and other celestial objects are from the Sun or from Earth. For instance, when we say Jupiter is about 5.2 AU from the Sun, it means that Jupiter is, on average, 5.2 times farther from the Sun than Earth is.

In contrast, the other options represent different measurements: the distance from Earth to the Moon is much smaller and is typically about 0.00257 AU; the distance light travels in one year is defined as a light-year, which is vastly larger than an AU; and the diameter of the Milky Way is on the order of tens of thousands of light-years, a scale far beyond the use of AU. By focusing on the relationship