Imagine you are looking at a body, and then you keep moving behind from it. How does it appear? The body appears to be decreasing in height. So we normally use trigonometry to find the height, but it is always the third person and if we want to calculate the height of the body in the frame of reference of the viewer, hence we cannot use trigonometric ratios. So we need to derive a new expression; the relative height is dependent on; the distance between the bodies, the angle of sight and the original height. Also, we can apply this on heavenly bodies like any star that looks small from earth but is very large. So we can use the derived expression to find the radius of the star and the distance.