Vitruvius's Analemma

Epoch

The sky is tilted to the latitude of the place: the equinoctial ray leaves the gnomon tip A at altitude 90−φ. The menaeus supplies the noon ray for each sign; extended to the ground, it lays the date scale on the meridian line of the dial. The declination calculator itself is explained on the page Menaeus.

sin δ = sin ε · sin λ  ·  umbra = g · tan(φ − δ)

Drag the pointer on the menaeus to choose the solar longitude λ.

Layer 1 = noon date scale from the menaeus · Layer 2 = hours by rotation of the day circle (equal or temporal).