This nautical sextant is part of our Science Department collection.
A sextant is an instrument that measures the angle between two objects that are visible. Primarily, it is used to measure the angle between a celestial body and the horizon. The angle measured and the time at which it was measured is then used to identify the location of the user on the grid map of the world.
The principle of the instrument was invented by Sir Isaac Newton during his life time but the actual tool was developed later on by John Hadley and Thomas Godfrey in 1730.
The sextant consists of a telescope, a horizontal mirror which the telescope "looks" through, and a moving arm on which the index mirror is fixed. By manipulating this arm a star or other celestial body can be made to appear on the horizon. Accurate adjustments are made by means of a micrometer knob. The angle can then be read off the arc and micrometer. The shades are used when the object being looked at is bright - such as the sun.