What does an astronomical unit (AU) measure?

An astronomical unit (AU) is defined as the average distance between the Earth and the Sun, which is approximately 93 million miles or about 150 million kilometers. This unit of measurement is particularly useful in astronomy for expressing distances within our solar system, facilitating a clearer understanding of the vast distances between celestial bodies. For example, when saying that Mars is about 1.5 AU from the Sun, it indicates that Mars is, on average, 1.5 times as far from the Sun as Earth is. Using astronomical units allows astronomers to simplify calculations and comparisons of distances in the solar system.

Other options focus on different aspects of celestial objects that do not pertain to distance measurements. Understanding this foundational concept is crucial for grasping more complex topics in astronomy.