Parallax
When an object is close to me, you can use a ruler to measure the distance. But, it isn't easy to measure when the object is away from me. In this case, how can you measure the distance?
Humans have two eyes. The world with the left eye and the right eye is slightly different. If the object is very close to me, the world seen with the left eye and the world witnessed with the right eye look very different. Parallax is an angle that varies depending on where you are observing. Parallax increases as the distance of the object get closer.
Stellar Parallax
Stars in the night sky do not seem to have changed their position. However, if we look at the stars' position close to Earth, they change every six months. This does not mean that the stars move around every six months. It is a phenomenon that appears because the earth revolves around the sun.
Stellar parallax is quite small and is expressed in seconds(˝). The distance to a star with a parallax of 1˝ is called 1pc(parsec). Therefore, the relationship between stellar parallax p(˝) and the distance d(pc) to the stars is as follows.
\[ d(pc)=\frac{1}{p( \prime \prime )} \]
A star's distance can be represented by light year(LY) and parsec(pc). 1LY is the distance that the light travels in a year because the light travels about 300,000 km per second, so 1LY is a very long distance. The distance of 1pc is about 3.26LY.
You can use the parallax to get the star's distance, but this method can be applied to relatively close stars. Very distant stars have a quite small parallax. In this case, it isn't easy to measure the distance by using the parallax method. In recent years, we measure the distance to the stars more precisely by satellite. There are various ways to obtain the distance to the stars, such as using the brightness of stars.