A gnomon
A stick standing upright in the ground tells you the time of day, the season and which way is north. The instrument is really its shadow. Herodotus says the Greeks learned the gnomon from Babylon, along with dividing the day into twelve parts.
How a stick tells you things
The shortest shadow of the day falls at noon, and it points north (anywhere north of the tropics). The length of that noon shadow changes through the year: shortest at midsummer, longest at midwinter. Mark the tip of the shadow day after day and you have a calendar drawn on the ground. The glowing dots in the picture mark the tip at the end of each hour through one midwinter day, the way later Greek sundials counted hours.
The famous use, and its catch
The best-known thing anyone did with a gnomon is measure the size of the Earth. Eratosthenes compared the noon shadow at Alexandria with Syene, far to the south, where at midsummer noon the sun stood straight overhead. But his own book is lost. What we have is a summary written by Cleomedes, centuries later, so the exact method and figures are that later writer's account, not his.
What Herodotus actually says
Only that the Greeks learned the gnomon, and the twelve parts of the day, from the Babylonians. What he meant by twelve parts is still argued; probably not hours on a dial. Whether he is right about where the idea travelled from is a softer question than whether he said it, so this card says "Herodotus says".
Why there is nothing to show you
A gnomon is any upright stick, and a stick in the ground rots or gets pulled out for firewood. Nobody could tell an ancient one from any other bit of wood. The proper sundials that came later survive because someone carved them in stone.
- Herodotus 2.109: the polos, the gnomon and the twelve parts of the day, learned from the Babylonians
- Cleomedes, On the Heavens 1.7, summarising Eratosthenes' measurement of the Earth
- Aristophanes, Assemblywomen (Ecclesiazusae) 651-652: go to dinner when the shadow is ten feet
- Remijsen, 'Living by the Clock: the introduction of clock time in the Greek world', Klio 103 (2021): when the Greeks began telling the time in hours
- Picture: rendered from our own 3D model, the one on the workbench: a reconstruction, not a record of how it looked.