Here's why:
* Parallax is the apparent shift in an object's position when viewed from two different locations.
* Astronomers use the Earth's orbit around the Sun as their two "locations".
* They measure the apparent shift in a star's position against the background of distant stars (which appear essentially fixed) over the course of six months.
* The greater the parallax angle, the closer the star.
This method works well for relatively nearby stars, but the parallax angle becomes too small to measure accurately for stars that are farther away.
