The Quadrant and Sextant

An astronomical quadrant is essentially a graduated quarter of a circle, set up to measure the altitude of celestial objects above the horizon. The graduations from 0 - 90° are on the circumference, or limb of the instrument, over which usually a sight or index arm moves.

Quadrants came in two forms: mural quadrants which were fixed to a meridian wall and used to measure meridian altitudes, and altazimuth quadrants which could be rotated to any bearing, measuring altitude and azimuth simultaneously. Mural quadrants were considered the most accurate as movement, wear and flexure were kept to a minimum, although other mountings were devised. Observations were restricted to the meridian, but this meant that calculation of declination from the observations was relatively easy. By the late seventeenth century, telescopic sights had become standard attachments on astronomical quadrants. In the eighteenth century many portable quadrants were built, by both French and English makers.

One of the earliest examples of a quadrant is the large 'plinth' described in Ptolemy's Almagest and used for measuring the meridian altitude of the Sun. While Islamic astronomers made extensive use of the quadrant, the most celebrated quadrant of the European Renaissance was the mural quadrant of 2-metre radius built by Tycho Brahe in 1582. The quadrant was for a long time the principal measuring instrument of astronomy and continued to be used in all the foremost observatories of the seventeenth and eighteenth centuries. The quadrant was the key instrument with which accurate measurements of a stars declination were made.

Very similar to the quadrant in design and purpose was the sextant. While the quadrant was a quarter of a circle, the sextant was a sixth of a circle (60°) and its smaller arc meant that it was often more portable than a quadrant. While Islamic astronomers often used mural sextants, in the west the sextant was often an adjustable instrument, being used to measure the angular separation of two stars in any plane.