ἀνάλημμα

Análēmma in Greek means a support, a pedestal, or the base of a sundial. Vitruvius used the term in his De Architectura for a geometric projection used in sundial design. The modern astronomical meaning — the figure-eight pattern traced by the sun's position at the same time of day over the course of a year — was applied later, borrowing the sundial-design term for the pattern the sundial measures.

The figure-eight shape results from two factors: the tilt of the Earth's axis (23.4 degrees) and the eccentricity of the Earth's orbit (slightly elliptical rather than perfectly circular). The axial tilt causes the sun to move north and south through the seasons. The orbital eccentricity causes the sun to move slightly faster at perihelion (January) and slightly slower at aphelion (July). These two motions combine to produce the asymmetric figure-eight.

Photographing an analemma requires extraordinary patience. The photographer must expose the same frame (or stand in the same spot with the same camera angle) at exactly the same time of day, every week or so, for an entire year. The resulting composite image shows 30 to 50 dots of sunlight tracing the figure-eight against the background sky. Dennis di Cicco published the first successful analemma photograph in 1978-1979 in Sky & Telescope magazine.

The analemma appears on some globes and maps as a figure-eight diagram, usually located in the Pacific Ocean where there is space for it. It encodes the equation of time — the difference between solar time (measured by a sundial) and mean time (measured by a clock). The analemma is the shape of the mismatch between the clock and the sun. The support that the Greeks named is still holding up the math.