What units are used to measure distances in space?

• A. Kilometers for all space distances
• B. Light-years for solar system distances and parsecs for interstellar distances
• C. Parsecs for solar system distances
• D. Astronomical Units (AU) for solar system distances and light-years for interstellar distances

Answer: (D)

Distances inside the solar system are most naturally expressed in astronomical units because that unit is built from the Earth–Sun distance. One astronomical unit is about 149.6 million kilometers, so the locations of the planets fall into a comfortable, readable range (for example, Mercury is a bit under 1 AU from the Sun and Neptune is about 30 AU). Using kilometers would give numbers that are awkwardly large or tiny, and using a different unit would not line up with the scale of planetary orbits.

For distances to stars and farther, light-years are used because they relate distance to time—how long light takes to travel that far. This makes interstellar distances comprehensible: the nearest star is about 4.3 light-years away, and even faraway stars are measured in many light-years. (Parsecs are another common unit in professional astronomy, with 1 parsec about 3.26 light-years, but for this question the interstellar distances are described with light-years.)

So this combination—astronomical units for solar-system distances and light-years for interstellar distances—keeps the numbers reasonable and ties distance to a scale that’s easy to grasp.