Final answer:
An Astronomical Unit (AU) is the average distance from the Earth to the Sun, while a light year measures the distance light travels in one year. A light year is much longer than an AU, and is used to express much greater distances in the universe.
Step-by-step explanation:
The difference between an Astronomical Unit (AU) and a light year is primarily in the distance that each represents.
An Astronomical Unit is defined as the average distance between the Earth and the Sun, which is approximately 150 million kilometers (93 million miles). On the other hand, a light year is the distance that light travels in one year, which is about 9.461 trillion kilometers (5.879 trillion miles).
The use of these units makes it easier to express and understand the vast distances in the universe. To illustrate, the distance between the Earth and Alpha Centauri, which is the star system closest to our solar system, is about 4.367 light years, which is immensely larger than an Astronomical Unit. Measuring such cosmic distances in light years allows astronomers to communicate these immense scales effectively.
Despite the name, a light year is a measure of distance, not time, just as when we describe a location by how long it takes to reach it, even though we're talking about distance.