What is the average distance between Earth and the Sun, about 150 million kilometers?

• A. Light year
• B. Absolute brightness
• C. Apparent magnitude
• D. Astronomical Unit (AU)

Answer: (D)

Astronomical Unit is the standard measure for distances within our solar system. It represents the mean distance from the Sun to the Earth, which is about 149.6 million kilometers. Because Earth’s orbit is an ellipse, the distance varies over the year, but the average distance—often approximated as about 150 million kilometers—is what the AU conveys. The other terms describe different ideas: a light year is the distance light travels in a year (much larger, used for stars and galaxies); absolute brightness measures intrinsic luminosity; apparent magnitude is how bright an object looks from Earth, depending on both distance and intrinsic brightness.