Parallax is one of the most important distance measurement methods used by astronomers. It can only be used for nearby stars, but it is very accurate.
The method works by measuring how nearby object appears to move against the background of more distant objects. You can try this yourself by looking at a nearby lamppost against a building in the background. When you move position the distance between the lamppost and the background building changes.
You can also see this effect by stretching out your arm whilst holding a pencil. If you close one eye and move your head from side to side. See how the pencil appears to move against wall behind it.
The same effect can be used as the Earth orbits around the Sun. We look at the position of nearby objects when we are at one side of our orbit. We then wait 6 months and look again when we are at the other side of the orbit. Nearby planets and stars appear to move against the background. We can measure the movement to work out the distance, because we know how our position has changed.
Parallax is measured in maths using the distance to the object and the angle in the sky it seems to have moved. We measure the distance in parsecs, and the angle in arcseconds. Arcseconds are used because the changes in angles are so small. There are 3,600 arcseconds in one degree. One parsec is the distance to an object with a parallax angle of 1 arcsecond. So 1 parsec is equal to 3.26 light-years, or 206,000 AU, or 31,000 billion km
One parsec is the distance (d) to an object which has a parallax angle (p) of 1 arcsecond. This is basically using trigonometry. We know that the distance the Earth has moved over 6 months is equal to the diameter of our orbit. There is a special distance used in astronomy for the Earth's average distance from the Earth to the Sun. It is called the astronomical unit.