Analemmatic

Analemmatic is an adjective used in the context of sundials to describe a method and dial layout known as the analemmatic sundial. The defining feature is that the hour marks lie on an ellipse and the base of the gnomon is moved along a central north–south line through the year so that the shadow indicates the correct local solar time.

Principle and design: The dial plate is laid out so that the hour lines form an ellipse, with the major axis running east–west and the minor axis north–south. A vertical gnomon stands on the dial, but its foot is not fixed. Instead, for each date, the gnomon’s foot is placed at a specific point along the central north–south axis according to the Sun’s declination. The shadow of the gnomon then falls on the corresponding hour line on the ellipse.

Construction and use: To build an analemmatic sundial, select the local latitude and draw an ellipse with appropriate size and orientation. On the central axis, mark a sequence of date positions that correspond to the Sun’s declination over the year. The gnomon is set at the date mark for the current day; its shadow projects onto the ellipse to indicate the time. The method produces readings from sunrise to sunset and is often attractive for gardens and educational settings.

Etymology and context: The term analemmatic derives from analemma, the figure-eight path of the Sun observed over the year, reflecting the interaction of solar declination and time in this design. Analemmatic sundials are discussed in sundial literature and by makers of educational time-keeping devices.