The astronomical unit, or AU, is the average distance between Earth and the Sun. It equals about 149.6 million kilometers. That number looks arbitrary, but it is tied to the geometry of Earth's orbit. The AU is the yardstick astronomers use to measure distances within the solar system.

Using kilometers for solar system distances gets unwieldy fast. Earth is 1 AU from the Sun. Jupiter is 5.2 AU. Neptune is 30 AU. The Kuiper Belt extends past 50 AU. The nearest star, Proxima Centauri, is about 268,000 AU away, which is why astronomers switch to light-years for interstellar distances.

The AU was originally defined by the length of Earth's orbit, but it is now fixed by international agreement as exactly 149,597,870.7 kilometers. That precision matters for spacecraft navigation. A small error in the AU becomes a large error over billions of kilometers.

The unit also connects to the definition of the parsec, which is the distance at which 1 AU subtends an angle of one arcsecond. That link makes the AU a foundational constant for all distance measurements in astronomy.