Which unit is most often used to measure distances in the Solar System?

• A. Light-year
• B. Parsec
• C. Astronomical unit
• D. Kilometer

Answer: (C)

Distances within the Solar System are most usefully expressed with a unit tied to the scale of the planets’ orbits. The astronomical unit is defined as the average distance from the Earth to the Sun, about 149.6 million kilometers. This makes it easy to describe how far planets are from the Sun or from each other in a way that directly reflects the actual sizes of their orbits. For example, Mercury sits at around 0.39 AU from the Sun, Earth is 1 AU, and Jupiter is about 5.2 AU away.

Other units aren’t as practical for Solar System distances. A light-year or a parsec describes distances to stars and beyond, which are vastly larger than planetary orbits. Using kilometers is possible but unwieldy for the wide ranges involved in the Solar System. The astronomical unit is the standard because it corresponds to the scale of our planetary system and makes comparisons straightforward.