[Mittelwaechter]

The circumference of the planet is about 40 000 km at the equator. Any location on the surface moves around the planets axis in 24h.
It moves about 20 000 km in 12 h when using the am/pm system.

Let's scale it down to your watch:

a 4 cm diameter x 3.1415 results in about 12.5 cm circumference. So roughly every centimeter a mark will space your marks into 12 hours.

We construct the gear to move your watches short hand those 12.5 cm in half a day (12h)

Now, what you do when kooking at your watch is scaling down your position on the planet in relation to its rotation, represented by the short hand of your watch.

You don't read the time, but you check your location in relation to the sun.

Edit: I guess mankind started with a constantly repeating phenomenon to be observed by everyone: the sun rising, travelling over the sky and disappearing under the horizon. The geocentric sundial positions a shadow in relation to the position of the sun.
The heliocentric system moves the surface of the planet in relation to the fixed sun, so the shadow represents the movement of the planets surface, the location of the sundial itself.