Which term is defined as the average distance between Earth and the Sun?

Which term is defined as the average distance between Earth and the Sun?

• A. Light-year
• B. Parsec
• C. Astronomical unit
• D. Kilometer

Answer: (C)

Distances within the solar system are measured in astronomical units because this unit is tied directly to the Sun–Earth distance. An astronomical unit is the average distance from Earth to the Sun, about 149.6 million kilometers. Using this measure makes it easy to compare how far other planets are from the Sun relative to Earth; for example, Venus is closer and Mars is farther than Earth, so those distances are understood in terms of that same baseline. The Earth–Sun distance is not fixed because Earth travels in an ellipse, so the actual distance varies over the year, but the mean distance—the average over time—is what defines the unit. Light-year and parsec describe much larger cosmic distances between stars and galaxies, not the scale of our solar system. A kilometer is far too small to express planetary distances.

Distances within the solar system are measured in astronomical units because this unit is tied directly to the Sun–Earth distance. An astronomical unit is the average distance from Earth to the Sun, about 149.6 million kilometers. Using this measure makes it easy to compare how far other planets are from the Sun relative to Earth; for example, Venus is closer and Mars is farther than Earth, so those distances are understood in terms of that same baseline. The Earth–Sun distance is not fixed because Earth travels in an ellipse, so the actual distance varies over the year, but the mean distance—the average over time—is what defines the unit. Light-year and parsec describe much larger cosmic distances between stars and galaxies, not the scale of our solar system. A kilometer is far too small to express planetary distances.