Gnomon is a needle, usually made of stone, erected on the ground whose shadow can be used to read the time. 

It is one of the most basic ways to measure the time with the help of the sun. It is the ancestor of the sundial. Obelisk in Paris can be considered as a gnomon.

A gnomon displays the true solar time. If we observe the moment when its shadow is the shortest and compare it with the mean solar time, we can calculate the equation of time.