Which of the following represents the commonly used unit for the Sun-Earth distance?

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

Answer: (B)

Distances within the solar system are typically expressed in astronomical units because it gives a natural, solar-system scale. An astronomical unit is defined as the average distance from the Sun to the Earth, about 149.6 million kilometers. This unit makes it easy to compare how far planets are from the Sun since their orbital sizes are measured relative to that same reference point.

Other options describe different scales or contexts: a light-second is how far light travels in one second, roughly 300,000 kilometers, which isn’t the standard way we express planetary distances even though light travel time to Earth is about 8 minutes. A light-year and a parsec are used for interstellar distances—much larger scales than the solar system. Therefore, the astronomical unit is the most practical and widely used unit for the Sun–Earth distance.