How is the Astronomical Unit defined?

• A. The diameter of the Sun
• B. The average distance between the Sun and Earth
• C. The distance from Earth to the Moon
• D. The distance light travels in a year

Answer: (B)

The Astronomical Unit is a unit of length used for distances inside the solar system, defined as the average distance from the Earth to the Sun. This natural scale—because Earth’s orbit sets the size of the solar system—lets us talk about planetary distances without writing huge numbers. It’s about 149.6 million kilometers, and in modern terms is fixed as an exact value in meters. This is not the diameter of the Sun, nor the Earth–Moon distance, and it’s not the distance light travels in a year (that would be a light-year, used for much larger, interstellar distances).