quadrant


Portable astronomical quadrant
by John Bird, London, c. 1760 
(ASTRONOMY) An old instrument,
based on a quarter of a circle and designed to measure the altitude
above the horizon of astronomical bodies. As it was originally used,
the plane of the quadrant was adjusted to lie in the plane of the meridian.
Vertical alignment was indicated by a plumbbob suspended from the quadrant's
center. Pivoted from this center was one end of a movable rod approximately
equal in length to the radius of the quadrant. Sights mounted on the
rod enabled observations to be made of stars and planets as they crossed
the observer's meridian, and an angular scale inscribed on the periphery
of the quadrant gave their meridian altitudes. It isn't certain whether
Ptolemy actually constructed such an
instrument or not. The Arabians, however, subsequently adopted the idea
of the quadrant and greatly improved upon its design: in particular,
quadrants were developed that could rotate about a vertical axis. They
culminated in an enormous masonry device, 55 meters high, erected in
the 15th century by Ulugh Beg at Samarkand. See also Tycho Brahe.
• HISTORY
OF ASTRONOMY
 (MATH) Any one of the four portions
of the plane into which the plane is divided by the Cartesian
coordinate axes. In spherical geometry, a quadrant is a spherical
distance π/2. A quadrant of a circle
is a sector with a central angle of π/2.
• ANALYTICAL
GEOMETRY
