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How is an astronomical unit used to measure distances in the solar system?

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Illustration showing the eight planets of the solar system in order from the Sun-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune-depicted to scale by size, with labeled diameters in miles and kilometers. The Sun is partially visible on the left for comparison. — Solar system The planets (in comparative size) in order of distance from the Sun.

The astronomical unit (AU) is a unit of length defined as exactly 149,597,870.7 kilometers, which represents the average distance between Earth and the Sun.

In astronomy, the AU serves as a convenient standard for expressing and comparing distances between objects within the solar system. By using the Earth-Sun distance as a baseline, astronomers can easily relate the positions of various bodies. For example, stating that Jupiter is 5.2 AU from the Sun provides an immediate, intuitive sense of its distance relative to Earth's orbit. Similarly, noting that Pluto is nearly 40 AU away allows for a quick comparison of the scales of these orbits.1

While the AU was historically derived from complex orbital models and parallax measurements, it is now a fixed constant used to simplify calculations and provide a consistent scale for mapping the solar system.1