Since astronomers deal with objects that have such vast scales, it is important to have a good feel for the kinds of units used for distances and time. One such astronomical constant is the astronomical unit, abbreviated A.U., equal to the distance from the Earth to the Sun. One A.U. is equal to 1.49597870 ´ 108 kilometers or 95 million miles. To give meaning to this unit, Venus is about 0.7 A.U. from the Sun. Another unit, the light-year (ly), is defined as the distance light travels in one year. One light-year is equal to 63,000 A.U. or about 1013 kilometers. This incredible distance covered by light is possible because light travels at the Universe’s maximum speed limit: 2.99792 ´ 108 meters per second. The nearest star to Earth is Proxima Centauri and it lies 4.2 light years away. What this means, of course, is that when we view Proxima Centauri through our telescopes, we are viewing it as it was 4.2 years ago. And if we are observing a star that is 20 million light years away from us, we are seeing the light that left that star 20 million years ago. This fact that we are seeing “ancient light” when we view the night sky is what allows astronomers to see back into time to events that happened billions of years ago.
Another common unit is the parsec (pc) – a word created by combing the words parallax and second of an arc. One parsec is equal to 206,265 A.U. and is defined as the distance to a star with a parallax of one second of an arc. What is “a parallax of one second of an arc”? Parallax is a phenomenon that can be simply envisioned as the amount of apparent “wandering” that a star does in the sky due to the position of the Earth around the Sun. You can quickly demonstrate parallax to yourself by placing one finger in front of you and keeping it in that position. Close your right eye and make a mental note of your finger’s position against the background. Now close your left eye and view your finger again – note how the position against the background has changed! This change in position is measured by an angle called the parallax.
Parallax: Since the Earth moves around the Sun, a star will appear to change position in the sky. The angle it moves, symbolized by the Greek letter theta, is called the parallax.