3.2 The Rangefinder

This experiment enables you to understand the basic method of measuring distances in astronomy.

Here the instrument, known as rangefinder, has a 2-metre base. The left sighting tube is attached at right angles to the horizontal bar, whilst at a distance of 2 metres, the right sighting tube swivels allowing to angle to be set on the dia 1. With the help of trigonometry, the distance of an abject can be calculated. 

To test this effect, point the fixed tube at the first landmark situated 2 metres away. Then without moving the horizontal bar, point the adjustable sighting tube at it and note down the angle. Repeat the exercise on other landmarks that are further away. 

What do you notice when you aim at the mountains in the distance? The measure becomes less and less precise. lt is necessary to lengthen the base. This is what astronomers do, when they measure the relative positions of celestial bodies al six monthly intervals.