The Astrolabe Quadrant

The earliest known description of an astrolabe reduced to a quadrant with no moving parts was in 1288, by Jacob ben Mahir ibn Tibbon (1236-1304), more widely known by his Latin name of Prophatius Judaeus or Profeit Tibbon. Tibbon's treatise was quickly improved by Peter Nightingale whose account received wide distribution. The instrument was quickly named the quadrans novus (new quadrant) to differentiate it from the traditional quadrant or quadrans vetus (old quadrant).

The basic idea behind the idea of the quadrans novus is that the stereographic projection that defines the components of a planispheric astrolabe is just as valid if the astrolabe parts are folded into a single quadrant. The result is an instrument that can perform many of the functions of a standard astrolabe at much lower cost, but without the intuitive representation of the sky provided by the rotating rete.

[Astrolabe Quadrant]

It is not clear how popular the astrolabe quadrant became as few examples survive. There were, however, a number of treatises on the instrument so there is reason to believe that many were made, perhaps of cardboard or wood. Further, it is possible that brass quadrants were not as highly prized as true astrolabes due to their simplicity and were recycled into other instruments. The astrolabe quadrant was more popular in the Ottoman Empire from the 17th century until the early 20th century.

The astrolabe quadrant in the picture is not a specific instrument but, contains the arcs and scales that can be considered typical on a 14th or 15th century quadrans novus. The arcs reproduce the ecliptic and horizons of a regular round astrolabe folded over the East-West line and then folded again on the meridian. Additional arcs are provided to determine the unequal hours and to find the sines and cosines of angles. The astrolabe quadrant is equipped with a thread (usually of silk) with one end attached at the north pole and a weight on the other end. A bead or pearl can be slid up and down the thread to mark positions on the face of the instrument. The thread can be moved to any position on the face of the quadrant to simulate the rotation of the astrolabe rete.

The scales around the quadrant's margin show the Sun's position in the zodiac, degrees, time and right ascension. The scales across the top are for solving trigonometry problems and a declination scale is along the meridian.

Many astrolabe functions are performed in the same way on the astrolabe quadrant as they are on a noramal astrolabe. However, finding the time from the Sun's altitude requires the solution of an equation. All of the steps required to solve the equation can be performed on the astrolabe quadrant, but the procedure is somewhat involved.