The Gunter Quadrant and Practical Knowledge

The quadrant is an instrument used to simplify astronomical calculations and to make observations. It also carries an image of the heavens, and can be understood as a means of gaining practical knowledge of geometry and astronomy. The Whipple has 14 such instruments, one typical example of which is this boxwood 'Gunter-type' quadrant, made by Isaac Carver in 1706.

The Gunter-type quadrant

Edmund Gunter'sDe Sectore et Radio (1623) describes this type of quadrant, a form that was extremely popular throughout the 17th century. The quadrant could be used to observe and measure astronomical phenomena, to perform the basic tasks of surveying, and to carry out astronomical calculations. Gunter's book lists some of these uses:

"to finde the day of the moneth
to finde the houre of the day.
to find the beginning of day-breake, and end of twi-light
to find the houre of the night by the stares
to find the houre of the rising and setting of the Sun, and thereby the length of the day and night"

This particular quadrant departs from Gunter's description in some small details: the table of star names includes more information than is usual, allowing further calculations to be made. It also has a 'line of shadows', a versatile tool for measurement, which could be used to find, among other things, "Heights and Distances, accessable or inaccessable."

Instruments and knowledge

The quadrant was a very practical, useful and portable instrument. It also has a projection (a 2-dimensional representation) of the heavens. Understanding the use and construction of the instrument was a way of learning geometry and astronomy. These subjects converged under the general heading of 'dialling', an area of study in its own right that dealt with sundial construction.

A great number of books on dialling were published during the 17th century, when the quadrant was at its most popular. These books primarily deal with the description of the heavens and how celestial co-ordinates are represented on instruments. They also explain the solution of geometrical and trigonometrical problems, which would now be called theoretical, but in the 17th century they were explained and understood practically. For those interesested in dialling knowledge of astronomy and mathematics was never divorced from an understanding of the construction and use of instruments.

Read more: images of projections

Boris Jardine, 'A Gunter quadrant and practical knowledge', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2008

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