The two different sightlines, one at each end of Earth’s orbit, create a triangle the parallax angle is defined as half the angle at the triangle’s apex. If you draw a simple diagram, you’ll see that the distance the star appears to move is related to the angle at which it is viewed. Translated to the stars in the sky, two photographs of the same nearby star taken six months apart will show it appearing to move against the background of more distant stars because Earth has moved to the other side of the Sun in its orbit. Your finger will appear to shift because each eye views it from a slightly different angle. Next, open your left eye and close your right. Close just your left eye and observe where your finger appears against the background. One of the simplest ways to see for yourself how this works is to hold your hand at arm’s length in front of your face and raise one finger. This is because as our planet moves, our viewpoint changes. Over the course of several months, nearby stars appear to move with respect to more distant objects - an effect called parallax.

Earth circles the Sun, making one complete orbit per year. Question: Why is a parsec 3.26 light-years and not some other number?Īnswer: A parsec, or “parallax second,” is defined as 3.26 light-years because of how it is measured.

10 parsec