From candles to ladders: how astronomers measure very large distances in the universe How do we know the universe is expanding? Your high school trigonometry is involved in the process. Ever wondered how scientists know the distance to planets, stars, galaxies? Or even better, how to measure the universe’s size and its expansion? Well, they certainly don’t have a giant ruler, but they have other tools — including “candles” and “ladders”. The Parallax and 1 parsec = 3.26 light-years. Credits: astronomy.com The easiest way to measure things far from Earth is called parallax and was discovered by ancient scientists — not just the Greeks (who are usually responsible for discovering things), but several other ancient societies with knowledge of astronomy.