gnomon
nounUK /ˈnəʊmɒn/
US /ˈnoʊmɑːn/
A gnomon is the part of a sundial that stands up and makes a shadow to show the time.
The shadow of the gnomon moved across the sundial as the morning passed.
OTHER MEANINGS
2. In geometry, the part of a parallelogram that remains after a similar parallelogram is removed from one corner.
The student shaded the gnomon formed by cutting a smaller parallelogram from the corner.
3. A person who knows or judges; an expert or interpreter.
In the old guild, the gnomon settled disputes about weights and measures.
4. The raised central spike on an astrolabe or similar instrument used for sighting.
He aligned the gnomon of the astrolabe with the horizon to find the star's altitude.