What is an astronomical unit (AU)?

An astronomical unit (AU) is defined as the average distance between the Earth and the Sun, which is approximately 93 million miles or about 150 million kilometers. This unit is crucial in astronomy for measuring distances within our solar system, as it provides a convenient way to express distances that would otherwise be unwieldy due to their vastness.

By using the AU, astronomers can easily compare distances between various celestial objects, such as planets, moons, and other bodies in our solar system. For example, when stating that Jupiter is about 5.2 AU from the Sun, it helps illustrate that it is significantly farther than Earth, which is at 1 AU.

The other options do not accurately define an astronomical unit. Time measurements, star brightness, and weight units do not relate to the definition or purpose of an AU in the context of distance measurement in astronomy. Thus, defining an astronomical unit as the distance from the Earth to the Sun positions it effectively as both a standard measurement and a practical reference point in astronomical calculations.