Which statement correctly defines an Astronomical Unit (AU)?

Which statement correctly defines an Astronomical Unit (AU)?

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

Answer: (D)

An Astronomical Unit is a unit of length used to describe distances within our solar system. It is defined as the average distance from the Sun to the Earth, about 149.6 million kilometers. The Earth’s orbit is slightly elliptical, so this distance isn’t constant; using the average provides a convenient reference point for comparing how far other planets are from the Sun. This definition distinguishes it from the other ideas: it is not a unit of time, so it doesn’t measure duration. It isn’t the distance between the Earth and the Moon, which is much smaller and not a standard solar-system scale. And it isn’t the distance light travels in a year, which is a light-year and is far larger than an AU. The AU is a practical, solar-system distance unit, with 1 AU about 1.5 × 10^8 kilometers, and it helps relate planetary distances to one familiar reference point.

An Astronomical Unit is a unit of length used to describe distances within our solar system. It is defined as the average distance from the Sun to the Earth, about 149.6 million kilometers. The Earth’s orbit is slightly elliptical, so this distance isn’t constant; using the average provides a convenient reference point for comparing how far other planets are from the Sun.

This definition distinguishes it from the other ideas: it is not a unit of time, so it doesn’t measure duration. It isn’t the distance between the Earth and the Moon, which is much smaller and not a standard solar-system scale. And it isn’t the distance light travels in a year, which is a light-year and is far larger than an AU. The AU is a practical, solar-system distance unit, with 1 AU about 1.5 × 10^8 kilometers, and it helps relate planetary distances to one familiar reference point.