A gnomon, (literally: “one that knows or examines”) is the part of a sundial that casts a shadow.
The term has been used for many different purposes in mathematics, astronomy and other fields. In distant past, this vertical stick, was called ‘shadow stick’ but today, it is known as ‘gnomon’.
It was probably the first device for indicating the time of day dating from about 3500 BC. The length of the shadow this simple instrument cast, it gave an indication of the time of day.
Gnomon was known in Inca’s Peru, megalithic Europe. In ancient China, the gnomon was useful instrument for astronomical observations through the centuries of Chinese civilization.
The instrument is mentioned in the 2nd-century ‘Nine Chapters on the Mathematical Art’ as being used much earlier by the Duke of Zhou (11th century BC).
Pre-Socratic Greek philosopher, Anaximander (610-546 BC) introduced the instrument to the Greeks. Greek mathematician (geometer) and astronomer, Oenopides (lived ca 450 BC) used the phrase drawn gnomon-wise to describe a line drawn vertically to another.
As defined by Hero of Alexandria, the gnomon was a figure, which when added to another figure, formed a similar figure to the original.
By the 8th century BC more precise devices were in use.
At midday, the shadow of a stick is shortest, and this alignment was used by the civilizations of Mesopotamia, Egypt, and China as the North – South direction.
According to Egypt’s hieroglyphic texts , an instrument was an L-shaped object with a short vertical arm and a long graduated horizontal arm and called ‘setchat or merkhet’ (instrument of knowledge’.
Today, a three-dimensional gnomon is commonly used in CAD and computer graphics as an aid to positioning objects in the virtual world.
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