What is a gnomon?

what is a gnomon
In pale imitation of,
And in patient thrall to, 
The blistering sun,
Lies a face, etched
Numerically and differentially,
Orderly and circumferentially,
The entire point of which
(See what I did there)
Rears up splendid and slender
Before bending tenderly 
To gaze upon its own shadow,
A dark Narcissus
Caressing the pallid face
That each day is kissed
By the sun

Gnomon: the projecting piece on a sundial that shows the time by the position of its shadow

Have a go at writing a ‘what is’ poem and share it in the comments!