SECTION #4: MEASUREMENT IN ASTRONOMY
By now, you should have some idea that space is so huge, it's kind of hard to wrap your head around just how big it is. Let's consider an object that is very close to the Earth: the Sun. On average, the Sun is about 152,000,000 kilometers or 94,500,000 miles away from Earth.
That's a pretty big number, but what about something that is farther away, like the next closest star to Earth after the Sun, Proxima Centauri? It's about 24,800,000,000,000 miles away from us. That's a much bigger number.
Now, about how far away is our "next door neighbor" galaxy, Andromeda? It is... approximately 151,200,000,000,000,000,000 miles away. Yikes! That number is so big that it is too hard to understand that amount or to even say the number out loud. This is why studying astronomy requires units of measurement that are bigger than miles and kilometers.
When we study individual planets, moons, stars, asteroids, comets, and other small space objects, we do use kilometers (km) to measure length or distance. For example, the average distance between the Earth and the Moon is 369,510 km or 229,605 mi. That is a large number but not so big that it can't be understood or difficult to communicate.
When we study greater distances within our solar system, we use Astronomical Units (AU). 1 AU is the distance between the Earth and the Sun, which we have already seen is 152,000,000 kilometers or 94,500,000 miles. So, 1 AU = 152,000,000 km. Everyone can agree it's a lot easier to say and understand 1 AU than 152,000,000 km.
If we use Astronomical Units to measure the distance between the Sun and each planet in our solar system, you can see how using this unit makes things simpler. By this unit of measurement, Mercury and Venus are less than 1 AU away from the Sun because they are closer to the Sun than Earth. Objects farther away from the Sun than the Earth have a measurement greater than 1 AU.
If you look at the second picture below, you can see its a lot easier to compare the distances of the planets using Astronomical Units than it would be to compare several distances that are millions or billions of kilometers.