Analematisch

Analemmatic dials
"are not very common. They are unusual because the gnomon is vertical, and the hours are marked not by lines but by points falling on the circumference of an ellipse. The gnomon has to be moved depending on the time of year, so that the shadow falls on the correct point. Analemmatic dials are particularly suitable for sundials laid out on lawns, where a person can act as a gnomon; the position where the person should stand at any given month of the year is marked out along the north-south axis which crosses the mid-point between the foci of the ellipse. "