An astronomical unit (AU) is defined as which distance?

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

Answer: (D)

An astronomical unit is a length based on the Sun–Earth geometry: it’s defined as the average distance from the Sun to the Earth. This unit is handy for expressing distances within our solar system without dealing with huge numbers in kilometers. It’s not a light-year (that’s the distance light travels in a year), and it’s not the Earth–Moon distance (which is only about 0.0026 AU). It also isn’t a fixed distance to Neptune, which sits around about 30 AU from the Sun. The current exact definition fixes 1 AU at 149,597,870,700 meters, making the Sun–Earth distance the standard reference.