Astronomical Unit

The astronomical unit, commonly abbreviated as AU, is a unit of distance used in astronomy. It is approximately equal to the average distance between Earth and the Sun. Its agreed value is approximately 149,597,870,700 meters, or about 150 million kilometers.

The AU is commonly used to express distances within the solar system: the distances of planets, asteroids, and comets. Distances beyond the solar system are typically measured in light-years or parsecs, since the AU is too small for such comparisons.

Definition and value

The AU was originally defined according to Earth's orbit: the semi-major axis of the orbital path. However, Earth's orbit is elliptical, meaning the distance varies throughout the year. Therefore, in later years it was decided to define the AU as a fixed number of meters rather than as a measure dependent on Earth's motion. The AU is now a defined constant rather than a measured quantity.

How it was measured

In earlier centuries, astronomers attempted to determine the scale of the solar system using parallax—the apparent shift in an object's position when viewed from two different locations on Earth. Transits of Venus across the Sun were used as a well-known method. In modern times, radar bounced off Venus and radio signals from space probes provide highly accurate measurements, since the speed of light is known.

Use and examples

When using the AU, distances within the solar system become easier to express. For example, Mercury is approximately 0.39 AU from the Sun, Mars approximately 1.5 AU, while Jupiter is approximately 5.2 AU. Light travels one AU in approximately three and a half minutes, which is the time it takes for sunlight to reach Earth.