However, the same method will not work for distant objects, because, due to the small distance between your eyes, the angle P will be too small to see any change in direction. To get around this problem, the baseline between measurements must be larger. For example, the military use a rangefinder, which is essentially a pair of binoculars with the lenses about 1 metre apart! For stars, the distance between the two vieing points needs to be even larger, as they are so far away. The best we can do is to use the opposite sides of the Earth's orbit. A photo of an area of sky is compared with one taken six months previously.
* Click here for the history of stellar measurements.
* Click here for how to work out the distance of a star from its parallax.
* Click here for information on modern developments.