Which unit is used to measure distances within the solar system?

• A. The astronomical unit.
• B. The light-year.
• C. The parsec.
• D. The kilometer.

Answer: (A)

The astronomical unit is the standard for measuring distances within the solar system. It’s defined as the average distance from the Earth to the Sun, about 149.6 million kilometers. This unit is perfectly suited to planetary orbits: Earth sits at 1 AU, Mars around 1.5 AU, Jupiter at about 5.2 AU, and so on. Using AU keeps numbers manageable and directly tied to the Sun–planet scale we’re dealing with.

Distances like a light-year or a parsec describe much larger scales—interstellar or galactic distances—so they’re not practical for solar system measurements. A light-year is the distance light travels in a year (about 9.46 trillion kilometers), and a parsec is about 3.26 light-years (roughly 206,000 AU). While kilometers are a real unit, they become unwieldy for planetary orbits due to the enormous distances involved. That’s why the astronomical unit is the best fit for this context.