How does an astronomical unit help compare planetary distances in the solar system?

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Geocentric system — Celestial mechanics Ptolemaic diagram of a geocentric system, from the star atlas *Harmonia Macrocosmica* by the cartographer Andreas Cellarius, 1660.

The astronomical unit (AU) serves as a standardized unit of length defined as 149,597,870.7 kilometers, which is effectively the mean distance between Earth and the Sun.1

In the context of celestial mechanics, the AU provides a convenient, proportional scale to express and compare the distances of objects within the solar system. By using Earth's orbital distance as a baseline, astronomers can relate the positions of other bodies without relying on cumbersome raw measurements in kilometers or miles.1

For example, stating that Jupiter is 5.2 AU from the Sun immediately communicates that it is 5.2 times farther away than Earth, while Pluto at nearly 40 AU is 40 times that distance. This system simplifies astronomical calculations and allows for ready comparisons of the relative spacing of all bodies orbiting the Sun.1