Large distances such as the distance of a planet or a star from the earth cannot be measured directly with a metre scale. An important method in such cases is the parallax method. For the objects nearer than about 20 metres, we have the ability to distinguish their relative distances. The brain does this by using the fact that each of our two eyes sees objects from a slightly different point of view. Close, or cover, your eyes alternately and observe this difference. The position of nearby objects relative to more distant objects is different for each eye. This difference, due to different view points is called parallax.
The brain process these two images, translating the parallax differences into a sensation of depth, solidity, and relative location of objects in space. This process is called stereoscopic vision. While stereoscopic parallax gives very good relative location of objects in space. Our brain is easily fooled when good relative distance information, we look at an object of unknown size against a featureless background.
We also use parallax in another way. When we move about, our eyes see our surroundings from a continually changing point of view. This provides the brain with additional depth information, and is particularly striking when we observe a scene from a moving vehicle.
The amount of shift is proportional to the ratio between the separation of the observation stations and the distance to object. There is formula of ta trigonometric parallax distance.
There are number of methods to measure the large distances. they are:
1. Echo Method
2. SONAR
3. RADAR
4. Determination of height of inaccessible object
