Sextants measure the angular distance between two different objects — usually distant stars, although on Earth the Sun and Moon can be used as navigational aids as well. Information derived from sextants can be used to identify one’s position on a map or chart and is vitally important when no land is in sight. Sextants were widely adopted after their introduction in the 1700s, as they could be used day or night and operated even aboard a shifting or unstable platform. Sextants have been used for centuries on Earth, usually aboard ships at sea, but they function aboard aircraft as well (early versions of the venerable Boeing 747 even came equipped with a sextant port for making optical sightings). From a practical point of view, sextants require no power and work independently of other navigational systems, and as such can be employed as a failsafe if electrical power and/or communications fail. The Apollo sextant played just this role, working with the AGC and often functioning as a navigational aid. Ground-based personnel compared their computed results to those obtained via the sextant as a further backup.

The Apollo sextant combined two separate optical devices that worked in conjunction as a functioning sextant: a 1x wide-field telescope (which was used to identify a target constellation or a single star) and a 28x telescope (which was used to make the actual angular measurement). Based on this measurement, which was extremely precise, the AGC could then compute the position of the Command Module based on previously stored data. Over the course of an entire mission, constellations, stars, Earth, and the Moon itself were all targets of the sextant.

A multipurpose tool

The Apollo sextant was used in Earth and lunar orbit, as well as while en route between Earth and the Moon. It played different roles in each of those contexts: in orbit around Earth or the Moon, the sextant could be used to compute the spacecraft’s altitude and position; whereas in transit between Earth and the Moon, it could be used to compute the spacecraft’s attitude (orientation), position, and velocity. A proper attitude during the flight to and from the Moon was critical for accurate course corrections and burns to reach the Moon and correctly insert the spacecraft into the desired lunar orbit.