What is an astronomical unit (AU) a measurement of?

An astronomical unit (AU) specifically measures the average distance between the Earth and the Sun. This unit provides a convenient way to express distances within our solar system, as comparing distances in kilometers or miles can become cumbersome due to the vast scales involved. The value of one AU is approximately 93 million miles or about 150 million kilometers. This measurement helps astronomers communicate and calculate distances between celestial bodies more easily, especially when considering the relatively short distances within our solar system compared to interstellar scales.

The diameter of the Milky Way is significantly larger than typical solar system distances, while the radius of a black hole varies depending on the size of the black hole itself, not fitting the context of AU. The speed of light, a fundamental constant in physics, is measured in units such as kilometers per second, not in AU, which further differentiates it from the context of distance measurement in astronomy.