© Museum of the History of Science, Oxford

A quadrant is quite a common type of mathematical instrument. It is also one of the oldest types of mathematical instrument. Quadrant literally means 'quarter of a circle'.
Quadrants are most often used to measure the height of the sun or a star above the horizon. This measurement is called the 'altitude' of the sun or the star in question. Another measurement that quadrants can be used for is called the 'zenith distance'. This is the angular distance of the sun or star from a point directly above the head of the person making the measurement.
The word 'zenith' refers to the point in space directly above the head of an observer. Zenith comes from an Arabic word. The fact that it is a term used in modern astronomy shows just how much of modern astronomy originally came from Arabic astronomers. The opposite of zenith is 'nadir'. Nadir also comes from an Arabic word. Nadir means the point in space directly below the observer.
Quadrants are usually used to measure angles, or to take a measurement that is mathematically dependant on an angle in some way. Both altitude and zenith distances are measured in angles. The altitude of the sun or a star and the zenith distance of the sun or a star are mathematically related to each other. They are complementary angles. This means that one of them is ninety degrees minus the other one.
Although quadrants are common mathematical instruments, this quadrant is very unusual. Only one other quadrant like it exists. It has numbers on it that are to do with the lengths of shadows cast by the sun, as well as scales that show the signs of the zodiac. This probably means that it was used by someone who made sundials to help them draw hour lines correctly.
All the numbers and letters on the quadrant are written in a type of Sanskrit. Sanskrit is the ancient Hindu language of India. Words are engraved on the quadrant that say it was made in Surat in the state of Gujarat in western India by someone called 'Sonimoraji' in the early 18th century.
In ancient times India was an important place for the study of maths. Famous Indian mathematicians are Aryabhata, who wrote a summary of Hindu mathematics in AD 499. which was all written in poetry. Another famous Indian mathematician was Bhaskara. Bhaskara was head of an observatory in Ujjain. One thousand years ago, Ujjain was the most important place for the study of mathematics in India. It is in Madhya Pradesh state, very close to Surat where this quadrant was made.
