HISTORY OF ASTRONOMY A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site astrolabe An Arabic and medieval European sighting instrument on an altazimuth mounting, used to show the appearance of the celestial sphere at a given moment and to determine the elevation above the horizon of celestial objects. It comprises two or more flat, metal, calibrated disks, one with a star map and one or more others with a scale of angles around its rim, attached so that both or all can rotate independently; the modern equivalent is the planisphere. For early navigators and astronomers it served as star chart, compass, clock, and calendar. As a navigational device it was eventually replaced by the sextant. The Danjon astrolabe is a type of portable solstitial armillary, modified for stellar observation. Suspended by a small hook or eye, the instrument consisted initially of a single ring that hung in a vertical plane. Pivoted at the center of the ring was a rod equal in length to the ring diameter, carrying sights at either end. When aligned on a star or planet, an angular scale inscribed on the armillary ring showed the object's altitude. Related category    • HISTORY OF ASTRONOMY Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP