What is an Astronomical Unit (AU)?

• A. A unit of time equal to one day
• B. A unit of distance equal to the distance from Earth to the Moon
• C. The distance between Earth and the Sun, about 150,000,000 km
• D. The distance light travels in one year

Answer: (C)

An Astronomical Unit is a distance unit used to describe how far objects are within our solar system. It is defined as the average distance between Earth and the Sun, about 150 million kilometers (roughly 93 million miles). This single number helps compare planetary distances easily: Earth is 1 AU from the Sun, Mars is about 1.5 AU, Jupiter around 5 AU, and so on. It’s not a unit of time, and it’s not the distance to the Moon, which is much smaller, nor is it the distance light travels in a year—the latter is a light-year and is far larger. The AU is a practical scale for the solar system because it captures the overall spacing of the planets in a simple reference.