Angular diameter distance

Angular diameter distance is a big phrase that means "how much things look like they're moving when they're really not." Imagine you're standing in a big open field and you can see a tree far away. From where you're standing, the tree looks small and far away. But if you were to walk closer to the tree, it would start to look bigger and closer.

Now let's talk about galaxies. Galaxies are like big groups of stars that are really far away from us. When we look up at the night sky, we can see lots of galaxies, but they all look pretty small because they're so far away. Scientists use something called the angular diameter distance to help them figure out how far away each galaxy is.

The angular diameter distance is like a way of measuring how big something looks in the sky compared to how big it actually is. It's a bit like holding up your fingers and using them to measure how big something is from far away. When scientists look at a galaxy, they can measure how big it looks in the sky and use that to figure out how far away it is.

So, just like how the tree looked small when you were far away from it, galaxies also look small when they're far away from us. But using the angular diameter distance, scientists can figure out exactly how far away those galaxies are, even though they're billions of miles away from us. And that's why the angular diameter distance is such an important tool for studying the universe!