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1The projecting piece on a sundial that shows the time by the position of its shadow.
‘To ensure that the sundial registered roughly the correct time all the year round the gnomon had to be set at exactly the correct angle.’
‘He also probably introduced the gnomon (a perpendicular sun-dial) into Greece and erected one in Sparta.’
‘He placed a pole perfectly upright in the ground to make a simple sundial, or gnomon.’
‘The problem is that the gardeners have been caught on the gnomon of their own sundial.’
1.1Astronomy A structure, especially a column, used in observing the sun's meridian altitude.
‘The text measures the positions of the heavenly bodies using shadow gauges which are also called gnomons.’
‘The simplest astronomical instruments was the gnomon, nothing other than a stick which was erected and the length of its shadow measured.’
‘He is also said to have constructed a armillary sphere, a water clock, and a bronze gnomon, a pointer whose shadow gives the time of mid-day.’
‘Pergolas, open steel stairs, lattices and wooden blinds all act as shadow-casters and gnomons.’
‘It is an astronomy text, showing how to measure the positions of the heavenly bodies using shadow gauges which are also called gnomons, but it contains important sections on mathematics.’
2Geometry The part of a parallelogram left when a similar parallelogram has been taken from its corner.
‘When the gnomon is turned up, it can measure height; when it is turned over, it can measure depth and when it lies horizontally it can measure distance.’
‘Revolve the gnomon about its vertex and it can draw a circle; combine two gnomons and they form a square.’
Origin
Mid 16th century: via Latin from Greek gnōmōn ‘indicator, carpenter's square’ (related to gignōskein ‘know’).