ASTROLABE

ASTROLABE, Astrolabium, was originally used for a System or Assemblage of the several Circles of the Sphere, in their proper Order and Situation with respect to each other. See CIRCLE and SPHERE. The ancient Astrolabes were the same with our Armillary Spheres. See ARMILLARY.

The first and most celebrated of this Kind was that of Hipparchus, which he made at Alexandria, the Capital of Egypt, and lodged in a secure Place, where it served for diverse Astronomical Operations.—Ptolemy made the same use of it, but as the Instrument had several Inconveniences, he bethought himself to change its Figure, though perfectly natural and agreeable to the Doctrine of the Sphere; and to reduce the whole Astrolate upon a Plane Surface, to which he gave the Denomination of Planisphere. See PLANISPHERE.—Hence, Astrolabe, is used among the Moderns for a Planisphere; or a Stereographic Projection of the Sphere upon the Plane of some Circle thereof. See PROJECTION and STEREOGRAPHIC.



The usual Planes of Projection are that of the Equinoctial, the Eye being supposed in the Pole of the World; and that of the Plane of the Meridian, the Eye being supposed in the Point of Intersection of the Equinoctial and Horizon.

Stofler, Gemma Frisius, and Clavius, have treated at large of the Astrolabe.—For a further Account of the Nature and Kinds thereof, see the Article PLANISPHERE.

ASTROLABE, or Sea Astrolabe, is an Instrument chiefly used for taking the Altitude of the Pole, the Sun, or Stars, at Sea. See ALTITUDE.

The Astrolabe represented Tab. Navigation, Fig. 22. consists of a large Brass Ring, about 15 Inches in Diameter, whose Limb, or a convenient Part thereof, is divided into Degrees and Minutes; fitted with a movable Index, or Label, which turns upon the Centre and carries two Sights.—At the Zenith is a Ring, to hang it by in time of Observation.

To use the Astrolabe, turn it so to the Sun, as that the Rays may pass freely through both the Sights F and G; in which Case, the Edge of the Label cuts the Altitude in the divided Limb.
The Astrolabe, though now grown into disuse, is at least equal to any of the other Instruments used for taking the Altitude at Sea; especially between the Tropics, when the Sun comes near the Zenith—There are a great many other uses of the Astrolabe; whereof Clavius, Henrion, &c. have wrote entire Volumes.

The Word is formed from the Greek ἄστρον, Star, and λαμβάνω, I take —The Arabs call it, in their Tongue, Asturlab; a Word formed by Corruption from the common Greek Name: Though some of 'em have endeavored to give it an Arabic Original.
But the learned are generally satisfied that the Arabs borrowed both the Name and the Use of the Instrument from the Greeks.—Nasir al-Din al-Tusi has a Treatise in the Persian Language, entitled, Bar Bahjat Al-Asturlab, wherein he teaches the Structure and Application of the Astrolabe.

ASTROLABE