The following sections of this BookRags Literature Study Guide is offprint from Gale's For Students Series: Presenting Analysis, Context, and Criticism on Commonly Studied Works: Introduction, Author Biography, Plot Summary, Characters, Themes, Style, Historical Context, Critical Overview, Criticism and Critical Essays, Media Adaptations, Topics for Further Study, Compare & Contrast, What Do I Read Next?, For Further Study, and Sources.
(c)1998-2002; (c)2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc. Gale and Design and Thomson Learning are trademarks used herein under license.
The following sections, if they exist, are offprint from Beacham's Encyclopedia of Popular Fiction: "Social Concerns", "Thematic Overview", "Techniques", "Literary Precedents", "Key Questions", "Related Titles", "Adaptations", "Related Web Sites". (c)1994-2005, by Walton Beacham.
The following sections, if they exist, are offprint from Beacham's Guide to Literature for Young Adults: "About the Author", "Overview", "Setting", "Literary Qualities", "Social Sensitivity", "Topics for Discussion", "Ideas for Reports and Papers". (c)1994-2005, by Walton Beacham.
All other sections in this Literature Study Guide are owned and copyrighted by BookRags, Inc.
The astrolabe is an astronomical instrument used to observe the positions of the stars. With modifications it has also been used for timekeeping, navigation, and surveying. The most common type, the planispheric astrolabe, consisted of a star map (the rete) engraved on a round sheet of metal. Only the angular relationship of the stars was accurate, since this was all that was needed. A metal ring could be moved across the map to represent the position of the local horizon. An outer ring could be adjusted to allow for the apparent rotation of the stars around the North Star, using prominent stars as reference points.
Astrolabes were forerunners of mechanical clocks, and looked somewhat like watches. With a set of tables, the observer could determine the day and hour for a fixed location by the position of the stars. With the addition of a sighting-rule, called an alidade, an astrolabe could be used as a surveying instrument. The rule could be moved across a scale to measure elevation.
It is thought that the Greeks had astrolabes, but it is known that the Arabs at least perfected them and made regular use of them. With the clear desert sky at their constant disposal, the Arab people excelled in astronomy and used the stars to navigate across the seas of sand. Regular use of astrolabes continued into the 1800s. The newer prismatic astrolabe continues to be used for precision surveying.