In earlier times, marine sextants had a fixed telescope leveled on the horizon and a radial arm is moved against an arc scaled in degrees. The radial arm is adjusted to get the reflection, of a known star, from index mirror and then off the horizon mirror down the telescope until it lines up with the horizon. The position of the radial arm on the scale gives the stars elevation. The nautical sextant range includes: slow motion nautical sextant, octants, slow motion nautical sextants and round dubble telescope nautical.

In modern sextants, the light ray from the celestial body is reflected in two mirrors (in series) one of which is adjustable and the other 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.