Science & TechHow it works
How astronomers measure star distances with a 300-million-kilometre ruler.
Hold a thumb up and blink each eye in turn: it jumps against the background. Stars do the same when Earth sits on the far side of the Sun six months later, a baseline of 300 million kilometres. The tiny angle gives the distance. Friedrich Bessel was the first to publish a stellar parallax, on 61 Cygni in 1838. Gaia now measures millionths of an arcsecond.
Useful: A parsec is exactly the distance at which this jump is one arcsecond: 3.26 light-years.
Source: en.wikipedia.org