Early astronomical instruments
type: Concept
narrower: Astronomical sextants

Astronomical sextants
type: Concept
contributor: AAS Frey.Katie Admin
modified: 2020-11-05T15:23:17.746Z
modified: 2020-11-05T15:22:59.988Z
alternative label: Mural sextants
alternative label: Framed sextants
alternative label: Astronomer's sextants
broader: Early astronomical instruments
definition: An instrument used mainly in nautical astronomy to determine the angular distances of celestial bodies above the horizon. The sextant consists of a 60° graduated arc, or limb, a small telescope, and two mirrors. Only half of one of the mirrors, the horizon glass, is silvered and, like the telescope, it is fastened to the frame supporting the limb. The other mirror, the index mirror, moves with an index arm pivoted at the center of the arc. The index arm is equipped with a vernier which moves along the limb. An object in direction S can be observed in the telescope through the un-silvered portion of the horizon glass. By moving the index arm, the second object S' is made to coincide with S in the telescope. According to the law of reflection, the angle between S and S' is double the angle between the mirrors. The angular distance between the objects can therefore be obtained from the measurement of angles between the planes of the mirrors.