Menaeus: analog solar declination calculator

The month circle (menaeus) is drawn on the chord between the two solstitial points and divided uniformly into the twelve signs. Projecting the pointer perpendicularly onto the chord gives the parallel of declination: where it meets the meridian circle, the radius from O marks the angle δ.

sin δ = sin ε · sin λ

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