Introducing of Traditional and Current Sextant
Sextant, It is an instrument for measuring the altitude angle between the horizon and a celestial body such as the Sun, the Moon, or a Star, used in celestial navigation to determine the ships latitude and longitude during voyage.
Early marine sextants were hand held and had a fixed telescope leveled on the horizon. A radial arm is moved against an arc scaled in degrees. Adjust the radial arm until a known star's image reflects from the index mirror and then off the horizon mirror until down the telescope until it lines up with the horizon. The radial arm's position on the scale gives the star's elevation.
In modern sextants, which use arcs of greater or lesser size than the original type, the light ray from the celestial body is reflected in two mirrors (in series), one of which is adjustable and the other of which is half-silvered. By rotating one mirror and its attached index bar, the image of the body is brought down to the horizon. The rotation measures the altitude on the limb.