Final answer:
The Astronomical Unit (AU) is based on the average distance between Earth and the Sun, approximately 150 million kilometers, and provides a more constant value for cosmic measurements than the actual variable distance.
Step-by-step explanation:
The Astronomical Unit (AU) is the unit of length defined as the average distance between Earth and the Sun, which is approximately 150 million kilometers. This measurement is used instead of kilometers because it provides a convenient scale for the vast distances within the solar system. The term 'average distance' is important because Earth's orbit around the Sun is not a perfect circle; it's slightly elliptical. Therefore, the distance between Earth and the Sun varies throughout the year. When Earth is closest to the Sun (perihelion), it is about 147.1 million kilometers away, and when it is farthest (aphelion), the distance is about 152.1 million kilometers. The average of these two distances is the AU, making it a more constant value for measurements.