A large astronomical unit
I recently wrote a text about the light year. Now let me clarify Parsec. It is another astronomical unit of measure by which we can find out the distance of objects in space. However, because of the size of the universe and in order to make it easier to understand the distances of the stars – astronomers have adopted a new unit of measure – parsec. One parsec is approximately 31 trillion kilometers or 19 trillion miles. And one parsec is about 3.26 light-years. The question is why 3.26 light years? Before answering, you should know what parallax is. This is so easy to answer. Place your middle finger in front of your face, and look at it with your right eye only. Now look at the same finger (not moving) with your left eye only. You will notice that the finger moves from left to right, relative to the background. This apparent shift is called parallax. If you measure the angle at which your finger moves, you can understand how far your finger is from your face. In the same way astronomy measures the distance of a star.
How to calculate parsec?
Because of long distances, astronomers use the motion of the Earth around the sun for calculations. As the Earth revolves around the sun, every six months it is on the opposite side from where it was six months ago. At some point, astronomers register the position of a star in the sky. They do the same after six months when Earth is on the other side of the sun. If the star is relatively close, then the view from one side or the other of Earth’s orbit will make the star move slightly. Because astronomers know that the distance between these two positions of Earth is 2 AU (two Earth distances from the Sun) and by measuring the angle – they get the distance of the star. What is a parsec? It is the distance to a star whose angle of parallax (explained above) – fixed one arc second. Logically, the parallax angles are very small in these cases. That is why they are usually measured in arc seconds. One degree has 3600 arcsec. From Earth (and counted with this technique) – one parsec is about 3.26 light-years.
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