What is the astronomical unit (AU) defined as?

• A. the maximum distance between the Earth and the Sun over a year.
• B. average distance between the Earth and the Sun.
• C. the distance from the Earth to the Moon.
• D. the distance from the Sun to Pluto.

Answer: (B)

The astronomical unit is a length unit defined as the average distance between the Earth and the Sun, effectively the semi-major axis of Earth’s orbit. Because Earth travels in an ellipse, its distance from the Sun changes over the year, but the average distance sets the unit. This works out to about 149.6 million kilometers (exactly 149,597,870,700 meters). It’s used to express distances inside the solar system in a convenient scale.

The other notions don’t define the unit: the farthest distance would be the aphelion, which is about 1.017 AU; the Earth–Moon distance is only about 0.0026 AU; Pluto’s distance varies widely but is measured in AU relative to the Sun, not as the definition of the AU itself.