This type of horizontal sundial makes it possible to know the time by observing the shadow cast by a person on the ground. This type of analemmatic sundial is composed of an ellipse drawn or constructed on the ground on which the hours of the day are marked. The shadow intersects the ellipse at a point which corresponds to the solar time. The position of the person on the minor axis of the ellipse depends on the date.
There is a great variety of analemmatic sundials. There are about forty of this type in France. The nearest are in Nice, Avignon, Montpellier and the College of Cadenet.
It is our shadow that gives the solar time. This sundial is about 25 minutes ahead of Universal Time (U.T.), because Thoard is located at 6°25' East longitude. On the other hand, it is 60 minutes behind (official winter time). Its constant/delay is therefore 35 minutes. In summer time, this delay must be increased by one hour. To be even more precise, the equation of time must be taken into account (see graph).
Example on June 29th. You place yourself on the small axis of the dial according to this date. Your shadow indicates that it is 11 o'clock in the sun. We are in summer time, so we have to add 1 hour 35 minutes to obtain approximately the standard time. To be more precise, the graph in the equation of time tells us that we need to add 4 minutes on this day. It is therefore 12:39pm on our watch.