A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight and it is an essential part of celestial navigation. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at solar noon and to measure the elevation(altitude) angle or at night to measure the elevation angle from the horizon plane to polaris to find one's latitude. Since the sextant can be used to measure the angle between any two objects, it can be held horizontally to measure the angle between any two landmarks which will allow for calculation of a position on a chart. A sextant can also be used to measure the Lunar distance between the moon and another celestial object (e.g., star, planet) in order to determine Greenwich time which is important because it can then be used to determine the longitude.

The scale of a sextant has a length of 1/6 of a full circle (60°); hence the sextant's name. An octant is a similar device with a shorter scale (1/8 of a circle, or 45°), whereas a quintant (1/5, or 72°) and a quadrant (1/4, or 90°) have longer scales.

Sextants for astronomical observations were used primarily for measuring the positions of stars. They are little used today, having been replaced over time by transit telescopes, astrometry techniques, and satellites such as Hipparcos.

There are two types of astronomical sextants, mural instruments and frame-based instruments.

Mural sextants are a special case of a mural instrument. Many were made that were quadrants rather than sextants. They were a kind of speciality of medieval Muslim astronomers to whom the credit of building the first mural sextants is attributed.

A sextant based on a large metal frame had an advantage over a mural instrument in that it could be used at any orientation. This allows the measure of angular distances between astronomical bodies.

These large sextants are made primarily of wood, brass or a combination of both materials. The frame is heavy enough to be stiff and provide reliable measures without flexural changes in the instrument compromising the quality of the observation. The frame is mounted on a support structure that holds it in position while in use. In some cases, the position of the sextant can be adjusted to allow measurements to be made with any instrument orientation. Owing to the size and weight of the instrument, attention was paid to balancing it so that it could be moved with ease.

These instruments were used in much the same way as smaller instruments, with effort possibly scaled due to the size. Some of the instruments might have needed more than one person to operate.

If the sextant is permanently fixed in position, only the position of the alidade or similar index need be determined. In that case, the observer moved the alidade until the object of interest is centred in the sights and then reads the graduations marked on the arc.

For instruments that could be moved, the process was more complex. It was necessary to sight the object with two lines. The edge of the instrument would typically be supplied with sights and the instrument was aligned with one of the two objects of interest. The alidade was then aligned with the second object as well. Once each object was centred in one set of sights, the reading could be taken. This could be a challenge for a moving star observed with a very large instrument as a single person might not be able to confirm both sights with ease; an assistant was a great benefit. The illustration of the Hevelius instrument to the right shows how two persons would use such a sextant. In the image, Elisabetha is aligning the instrument while Johannes sets the alidade.

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