Parallax
Written by Tega Jessa
So what is parallax? The best explanation is that it is the illusion of an object moving when you look at it from two different points of view. A perfect example is to hold up your thumb and place it in front of other objects for a frame of reference. Now try to looking at your thumb with only your left eye. Now do the same thing with your right eye. You will notice that it looked like your thumb moved in reference to the other objects behind it. Yet the entire time you know that you did not move your thumb.
This visual principal also works in astronomy. If you look at certain stars they seem to have different positions in the night sky in relation to other stars depending on the time of year. This is a stellar parallax. For the longest time astronomers did not have the tools to physically measure the exact distance of stars. So how did they find out? They did so using geometry and you can thank the ancient philosopher Pythagoras for coming up with the concept of using geometry to determine large distances. If you remember from high school and middle school math, Pythagoras came up with a theorem that you could use any two sides of a triangle or one of its sides and one of its angles to determine the exact length of the other sides. This is how Pythagoras calculated the circumference of the Earth.
The same principal was used to find the distance of a star. First off it had to be close enough to Earth to see an actual parallax. The further an object is the less it seems to move when you change your point of view. By using double the radius of the Earth's orbit as one side of a triangle and the angle of the chosen star's parallax, you could find out the exact distance to the star. This is because both unknown sides of the cosmic triangle would be the same, since the true distance did not change, only the perspective. The formula boils down to distance to the star equals the radius of Earth's orbit divided by the tangent of half the angle of a stars parallax.
This method works will for stars under 100 light years away. This is due to the reason stated before that the parallax of an object diminishes as it is placed further away. The other reason is that the angle of parallax for stars is often very small.
If you liked this article about Parallax there are others on Universe Today you might enjoy. First there is a great article about the parsec since this astronomical measurement depends on parallax. There is also an article about parallax being used to find new stars near our solar system.
There are also great articles on the web. NASA.gov has a great article that clearly explains how stellar parallax works. The Cornell University Astronomy website has an interesting animation that shows stellar parallax in action.
You will also enjoy listening to Astronomy Cast. The December 14, 2008 Questions episode deals with the distances in space.
Filed under: Astronomy
