1911 Encyclopædia Britannica/Astrolabe

From Wikisource
Jump to: navigation, search

ASTROLABE (from Gr. ἄστρον, star, and λαβῖν, to take), an instrument used not only for stellar, but for solar and lunar altitude-taking. The principle of the astrolabe is explained in fig. 2. There were two kinds,—spherical and planispheric. The earliest forms were “armillae” and spherical. Gradually, from Eratosthenes to Tycho, Hipparchus playing the most important part among ancient astronomers, the complex astrolabe was evolved, large specimens being among the chief observatory instruments of the 15th, 16th and even 17th centuries; while small ones were in use among travellers and learned men, not only for astronomical, but for astrological and topographical purposes. Nearly every one of the modern instruments used for the observations of physical astronomy is a part of the perfected astrolabe. A collection of circles such as is the armillary sphere, if each circle were fitted with a view-tube, might be considered a complete astrolabe. Tycho’s armillae were astrolabes. In fact the modern equatorial, and the altitude and azimuth circle are astrolabes in the strictest and oldest meaning of the term; and Tycho in one of his astrolabes came so near the modern equatorial that it may be taken as the first of the kind.

Britannica Astrolabe - Persian Astrolabe c.1712.jpg
Fig. 1. — PERSIAN ASTROLABE (c.1712) INSCRIBED IN ARABIC.
Front, showing the Rete or Spider, a network of star pointers. Beneath the Rete, in a hollow, are four thin brass discs, called Tables or Climates, engraved with projections of the sphere for different latitudes. Back, showing graduations, parallelogram for measuring heights, and other tables, together with the Rule with sights (A) held by a moveable pin (B), known as the Horse or Wedge.

Britannica Astrolabe - Principle.jpg

Fig. 2. — Principle of the Astrolabe. If a solid circle be fixed in any one position and a tube be pivoted on its centre so as to move; and if the line C D be drawn upon the circle pointing towards any object Q in the heavens which lies in the plane of the circle, by turning the tube A B towards any other object P in the plane of the circle, the angle B O D will be the angle subtended by the two objects P and Q at the eye.


Britannica Astrolabe - Mariner's Astrolabe.png
From Exercises, by T. Blundeville.
Fig. 3. — Mariner's Astrolabe, A.D. 1594. Made of brass,
or of heavy wood; it varied in size from a few inches to
1 ft. in diameter.

The two forms of the planispheric astrolabe most widely known and used in the 15th, 16th and even 17th centuries were: (1) the portable astrolabe shown in fig. 1 (Plate). This originated in the East, and was in early use in India, Persia and Arabia, and was introduced into Europe by the Arabs, who had perfected it—perhaps as early as A.D. 700. It combines the planisphere and armillae of Hipparchus and others, and the theodolite of Theon, and was usually of brass, varying in diameter from a couple of inches to a foot or more. It was used for taking the altitudes of sun, moon and stars; for calculating latitude; for determining the points of the compass, and time; for ascertaining heights of mountains, &c.; and for construction of horoscopes. The instrument was a marvel of convenience and ingenuity, and was called “the mathematical jewel.” Nevertheless it passed out of use, because incapable of any great precision.

(2) The mariner’s astrolabe, fig. 3, was adapted from that of astronomers by Martin Behaim, c. 1480. This was the instrument used by Columbus. With the tables of the sun’s declination then available, he could calculate his latitude by meridian altitudes of the sun taken with his astrolabe. The mariner’s astrolabe was superseded by John Hadley’s quadrant of 1731.

Authorities.—Chaucer, Treatise on the Astrolabe (Skeat’s edition of Chaucer); J. J. Stöffler, Elucidatio Fabrice ususque Astrolabii, &c.; Thomas Blundeville, His Exercises (1594); F. Ritter, Astrolabium; W. H. Morley, Description of Astrolabe of Shah Husain; M. L. Huggins, “The Astrolabe” (Astrophysical Journal, 1894); Penny Cyclopaedia, article “Astrolabe;” R. Grant, History of Physical Astronomy. (M. L. H.)