Parallax

Stars are so far away that astronomers cannot measure their distances directly. Astronomers have developed various methods of determining the distances to stars. Different methods are used for stars at different distances.

To understand how astronomers can measure distances to nearby stars, hold your thumb up at arm's length in front of you, as the student is doing in Figure 9. Close your left eye and look at your thumb with just your right eye open. Then cover your right eye and look with just your left eye open. Even though you didn't move your thumb, it appeared to move relative to the background because you looked at it from slightly different angles. The apparent change in position of an object with respect to a distant background is called parallax.

As Earth moves in its orbit, astronomers are able to observe stars from two different positions. Imagine looking at the stars in winter and then six months later in summer. During this time, Earth has moved from one side of its orbit to the other—a distance of about 300 million kilometers. Because people on Earth are looking from a different angle, the nearby star appears to move against the more-distant background stars.

Before the invention of the telescope, astronomers couldn't measure a star's position very accurately. They couldn't detect the apparent movement of even a single nearby star as Earth moved around the sun.

With the invention of the telescope, astronomers could measure the positions of stars with much greater accuracy. Astronomers measure the parallax of nearby stars to determine their distance from Earth. The closer a star is to Earth, the greater is its parallax.