An ancient instrument for solving problems relating to time and the position of the Sun and stars in the sky. It had many uses, including telling time during the day or night, finding the time of sunrise and sunset and, thus, the length of the day, and locating celestial objects in the sky. It was widely used until replaced by the → sextant. The → planispheric astrolabe, which is the most common type of the instrument, is typically made up of a graduated disk hanging vertically, which is rotated so that it can be directed to the star chosen. The local time can thus be read from the face of the astrolabe, and different tables, at various latitudes, can be used. More specifically, a planispheric astrolabe is made up of the following main components: → mater, → tympanum, → rete, → alidade, → throne, → limb, → pin, → horse, → front, and → back. The astrolabe was invented by Greeks, and some historians have attributed it to Hipparchus (c190-c120 BC). Modern, sophisticated versions (such as → prismatic astrolabe and → Danjon astrolabe), are used for high precision measurements of star positions. See also → particular astrolabe, → spherical astrolabe, and → universal astrolabe.
M.E., from O.Fr. astrelabe, from M.L. astrolabium, from Gk. astrolabos (organon) "star taking (instrument)," from astron "star," → astro- + lambanein "to take."
Ostorlâb, from Ar. usturlab, from Gk. astrolabos, as above.
Fr.: astrolabe catholique
Same as → universal astrolabe.
Catholic, M.E., from Fr. catholique, from Church Latin catholicus "universal, general," from Gk. katholikos, from phrase kath' holou "on the whole, in general," from kata "about," → cata-, + genitive of holos "whole," → holo-; → astrolabe.
Fr.: astrolabe de Danjon
A modern unportable astrolabe which is used for high precision measuring of stellar and geographical coordinates. The instrument uses the simultaneous observations of two images of the same star, one of the images formed directly by the lower face of a prism and the other by the light rays reflected first from a mercury bath and then by the upper face of the prism. The images coincide when the zenithal distance of the star attains a prefixed value (Gauss method of equal altitudes, → almucantar). Apart from astrometry, the Danjon astrolabe was used for studying the Earth's rotation and is currently used for solar radius measurements.
After André Danjon (1890-1967), French astronomer, who developed the instrument at the Strasbourg Observatory before the Second World War and at the Paris Observatory in 1948. The concept of prism astrolabe was initially invented by the French Auguste Claude (1858-1938) around 1900 and was later modified in collaboration with Ludovic Driencourt (1861-1940); → astrolabe.
Fr.: astrolabe linéaire
A version of → planispheric astrolabe in which the → celestial sphere and the various circles of altitude and declination are projected on to a line represented by a staff. The staff is equivalent to the meridian line and contains markings to indicate the centers of these circles and their intersections with the meridian. By attaching three ropes to the appropriate points on the staff to act as radii, the circles and their intersections can be reconstructed. One of the ropes was attached to a plumb line. A scale giving chord lengths in the meridian circle extended the linear astrolabe's range of applications. It was invented by the Iranian mathematician and astronomer Sharafeddin Tusi (c1135-1213), but no early example has survived. Same as → Sharafeddin's staff and Tusi's staff.
Fr.: astrolabe particulier
ostorlâb-e taxt-sepehri, ~ kore-yi
Fr.: astrolabe planisphérique
The most common form of the → astrolabe in which both the → celestial sphere and the observer's horizon are projected on to one or more plane surfaces by means of the stereographic projection. See also → universal astrolabe and → particular astrolabe.
Fr.: astrolabe à prisme
An instrument used to determine the precise timing of a star's passage across a vertical circle. It is used for making precise determinations of the positions of stars and planets, and can be used inversely to determine the latitude and longitude of the observer, assuming the star positions are accurately known. It consists of an accurate prism, a small pool of mercury to serve as an artificial horizon. The most notable example of this type of instrument is that of → Danjon astrolabe.
ostorlâb-e sepehri, ~ kore-yi
Fr.: astrolabe sphérique
A type of → astrolabe in which the observer's horizon is drawn on the surface of a globe, mounted with a freely rotating spherical lattice work or 'spider' representing the celestial sphere. The earliest description of the spherical astrolabe dates back to the Iranian astronomer Nayrizi (865-922).
Fr.: astrolabe universel