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Inventory No. 40847 & 35515 - Exhibition Label Text 'The Double Horizontal Dial: Then and Now' 13 January to 10 March 2009

A 'horizontal instrument' and a 'double horizontal dial', both made by Elias Allen, London, in the early 1630s

In the ‘horizontal instrument’ imaginary lines on the celestial sphere (lines of right ascension and declination for the sun in its apparent daily motion, and the ecliptic line representing its apparent annual motion) are projected on to the plane of the horizon.  This allows a range of astronomical problems involving the sun’s position and the time to be solved for a single latitude, with the help of a pivoted rule.  To measure the sun’s current position, the instrument can be suspended vertically, when the shadow of a pin fitted at right angles to the plate at its centre, will register the sun’s altitude on the outer scale.  As is usually the case with these instruments, the other side of the plate has Oughtred’s ‘circles of proportion’, the earliest form of logarithmic slide rule. This example was given by George Barkham to St. John's College in 1635 and is on loan from the College.


The same projection is used for the ‘double horizontal dial’ (note its permanent style or gnomon for casting the shadow), which combines the projected lines with a normal horizontal dial (hence a ‘double’ dial).  The gnomon is shaped to work with both dials – its inclined edge for the horizontal dial and its vertical edge registering the solar azimuth on the horizontal instrument.


Elias Allen was a friend and associate of the inventor of these instruments, the mathematician William Oughtred, and he made the earliest examples.


Inventory nos 40847, 35515



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