Try this before reading further
Hold a finger up in front of your face. Close one eye, then switch to the other. Your finger seems to “jump” against the background — even though it never actually moved. That apparent shift is called parallax, and it’s the same trick astronomers use to measure distances to nearby stars.
The original claim, in plain language
Las Cumbres Observatory’s guide explains that Earth orbiting the Sun gives astronomers two different “eyes” to look from — one position in January, the opposite position six months later in July. A nearby star will appear to shift very slightly against the much more distant background stars between those two observations, exactly like your finger did.
Turning an angle into a distance
By carefully measuring how big that apparent shift is (an extremely tiny angle, since even “nearby” stars are trillions of miles away), astronomers can use basic geometry to calculate the actual distance. The bigger the shift, the closer the star; the smaller the shift, the farther away it is.
Where this method runs out
Parallax only works for relatively close stars — telescopes on Earth can reliably use it out to about 100 parsecs (roughly 326 light-years), and space telescopes can stretch that further. Beyond that, the apparent shift becomes too tiny to measure accurately, even with the best instruments, and astronomers have to switch to other methods (like comparing a star’s true brightness to how bright it appears from Earth).
Why this matters
Parallax is the foundation that almost every other distance measurement in astronomy is built on and checked against. Before we can trust a method for measuring distances to galaxies billions of light-years away, it first has to agree with parallax measurements of nearby stars — making this simple, centuries-old trick still one of the most important tools in modern astrophysics.