|Hold out a finger or a thumb and close your right eye and then your left. Your finger will appear to move. This is very much how distances are calculated in space!|
|If you focus on a distant object - such as a house or a car - the object will appear to move relative to your finger. (I made this image for my course this semester.)|
If you hold up a finger and look at a distant object, closing first your left and then your right eye, the object will appear to move. Objects further away will appear to move less than objects that are close to you.
For example, your finger appears to move a lot if it's very close to your nose, but if you extend your arm it only moves a little bit.
This is due to the fact that your eyes are a small distance apart from one another and so they see the object at slightly different angles.
But for objects very far away - like stars - our eyes aren't far enough apart to detect this angle. If you try it with the moon, the moon will not appear to move.
In fact, in most cases the distances across the Earth aren't even far enough. We have to use something else.
What we use is the same idea as your eyes, but at a large, large scale - the scale of the solar system!
|From the NASA educational website http://www2.jpl.nasa.gov/teachers/attachments/parallax.html|
We measure the angle of the star at one time of year, such as the summer, and then the angle when the Earth is on the other side of the sun, in the winter. By measuring the angle we can use trigonometry to measure how far away the star is.