What is an astronomical unit (AU)?

• A. A unit of measurement equal to the average distance from the Earth to the Sun, approximately 93 million miles or 150 million kilometers.
• B. A measurement of light speed.
• C. A unit describing the brightness of a planet.
• D. A time interval used in space missions

Answer: (A)

An astronomical unit is a unit of length used to describe distances within the Solar System, anchored to the Earth–Sun distance. One AU is the average distance between Earth and the Sun, about 93 million miles or 150 million kilometers. This handy unit lets us compare planetary positions without writing huge numbers—for example, it’s common to say Mars is a few AU from the Sun. It’s not a measure of speed, brightness, or time: those concepts are described by other units (speed like light speed, brightness by luminosity or magnitude, and time by durations). The precise modern value is 149,597,870,700 meters, but the approximate figure is widely used for everyday calculations.