Quadrant (instrument)

Lore & Background

During Rigvedic times in ancient India, quadrants called 'Tureeyam's were used to measure the extent of a great solar eclipse. The use of a Tureeyam for observing a solar eclipse by Rishi Atri is described in the fifth mandala of the Rigveda, most likely between c. 1500 and 1000 BC. Early accounts of a quadrant also come from Ptolemy's Almagest around AD 150. He described a 'plinth' that could measure the altitude of the noon sun by projecting the shadow of a peg on a graduated arc of 90 degrees. This quadrant was unlike later versions of the instrument; it was larger and consisted of several moving parts.

Islamic astronomers in the Middle Ages improved upon these ideas and constructed quadrants throughout the Middle East, in observatories such as Marageh, Rey and Samarkand. At first these quadrants were usually very large and stationary, and could be rotated to any bearing to give both the altitude and azimuth for any celestial body. As Islamic astronomers made advancements in astronomical theory and observational accuracy they are credited with developing four different types of quadrants during the Middle Ages and beyond. The first of these, the sine quadrant, was invented by Muhammad ibn Musa al-Khwarizmi in the 9th century at the House of Wisdom in Baghdad.

During the Middle Ages the knowledge of these instruments spread to Europe. In the 13th century Jewish astronomer Jacob ben Machir ibn Tibbon was crucial in further developing the quadrant. He invented the novus quadrans, or new quadrant, which was revolutionary because it was the first quadrant to be built that did not involve several moving parts and thus could be much smaller and more portable. Tibbon's work influenced Copernicus, Christopher Clavius and Erasmus Reinhold; and his manuscript was referenced in Dante's Divine Comedy.

Reader's Guide

The quadrant's significance lies in its evolution from a large, stationary astronomical instrument to a portable device essential for navigation and timekeeping. Ptolemy's initial concept as an improved astrolabe led to medieval Islamic innovations, including the sine quadrant by al-Khwarizmi and three other types. The 13th-century novus quadrans of Jacob ben Machir ibn Tibbon made the instrument smaller and more portable, enabling its use at sea. The first documented use of the quadrant to navigate at sea is in 1461, by Diogo Gomes. Sailors began by measuring the height of Polaris to ascertain their latitude, a practice attributed to Arab sailors. In 1618, Edmund Gunter's quadrant added projections of the tropics, equator, horizon, and ecliptic, allowing users to find time, date, day length, and meridian, though its scales limited its use at sea. Mural quadrants became important in 18th-century European observatories for positional astronomy. The quadrant's legacy includes its role in advancing celestial measurement, navigation, and timekeeping across cultures and centuries.

Did You Know?

• The term 'quadrant' refers to the fact that early versions were derived from astrolabes, condensing the astrolabe into one fourth the size.
• The oldest horary quadrant was found during an excavation in 2013 in the Hanseatic town of Zutphen (Netherlands) and is dated ca. 1300.
• The sine quadrant was invented by Muhammad ibn Musa al-Khwarizmi in the 9th century at the House of Wisdom in Baghdad.
• The first documented use of the quadrant to navigate at sea is in 1461, by Diogo Gomes.