The night sky plays tricks. Objects that appear close together in the sky can actually be far apart. As an example, look at the Moon and the star Antares before dawn tomorrow. There’s almost no separation between them. In fact, for some parts of the eastern U.S., there won’t be any separation at all — the Moon will cover the star for a while, hiding it from view.
That makes it look like the Moon and Antares are close together in space. But that’s not the case. The Moon is our closest astronomical neighbor — about a quarter of a million miles away. Antares is much farther. Much, much farther. There’s a little uncertainty in the exact distance, but it’s about 550 light-years. That’s 13 billion times farther than the Moon.
Astronomers determine a star’s distance by measuring its parallax. They pinpoint its location when Earth is on opposite sides of the Sun. The star appears to shift back and forth by a tiny bit against the background of more-distant objects. The size of that shift reveals the star’s parallax. From that, astronomers calculate the distance.
Stars that are farther are harder to measure because the back-and-forth shift is tinier. It’s a special problem for stars like Antares, which puffs in and out. That means the star’s position on the sky is hard to pinpoint. So it takes a lot of work to tell us how misleading the night sky can be — revealing the huge gap between the Moon and the stars.