ASTROLABE
, from ashr, star, and lam<*>anw, I take; alluding to its use in taking, or observing, the stars. The Arabians call it in their tongue astharlab; a word formed by corruption from the common Greek name.
This name 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. And the ancient instruments were much the same as our armillary spheres. |
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 divers astronomical operations. Ptolemy made the same use of it: but as the instrument had several inconveniences, he contrived to change its figure, though perfectly natural, and agreeable to the doctrine of the sphere; and to reduce the whole Astrolabe to a plane surface, to which he gave the name of the Planisphere. Hence,
Atsrolabe is used among the moderns for a Planisphere, or a stereographic projection of the circles of the sphere upon the plane of one of the great circles; which is usually either the plane of the equinoctial, the eye being then placed in the pole of the world; or that of the meridian, the eye being supposed in the point of intersection of the equinoctial and horizon; or on that of the horizon.
The Astrolabe has been treated at large by Stoffler, Gemma Frisius, Clavius, &c. And for a farther account of the nature and kinds of it, see the article Planisphere.
Astrolabe, or Sea Astrolabe, more particularly denotes an instrument chiefly used for taking altitudes at sea; as the altitude of the pole, the sun, or the stars.
The common Astrolabe, represented Plate II, fig. 7, consists of a large brass ring, about 15 inches in diameter, whose limb, or a convenient part of it, is divided into degrees and minutes. It is fitted with a moveable label or index, which turns upon the centre, and carries two sights; and having a small ring, at A, to hang it by in time of observation.
To make use of the Astrolabe in taking altitudes; suspend it by the ring A, and turn it to the sun, &c, so as that the rays may pass freely through both the sights F and G; then will the label cut or point out the altitude on the divided limb. There are many other uses of the Astrolabe; of which Clavius, Henrion, and others have written very largely.
The Astrolabe, though now grown into disuse, is by many esteemed equal to any other instrument for taking the altitude at sea; especially between the tropics, where the sun comes near the zenith.
