ArticleA little-known map projection casts light on the relationship between time and maps
Have you ever wondered why map projections are called “projections?” In gnomonic projections, the surface points of a sphere are projected from the center of the sphere onto a plane tangent to the surface. Imagine a transparent globe on a piece of paper, with a light in the center of the globe. Rays of light from the center pass through points on the globe and then onto the paper.
This projection was used as early as 6th century BCE, mainly for star maps. These maps were originally called horologia (Latin for clock or sundial) or sundial maps because the angles around the central point at a particular latitude correspond to the face of a sundial at that latitude; 15° around the central point (1/24th of a full 360°) corresponds to one hour on a sundial. These maps came to be called gnomonic projections in the 19th century, named after the gnomon—the part of a sundial that casts the shadow.
Shown here is Franz Ritter’s 1607 sundial map, centered on the North Pole. To use as a sundial, the map would be flipped to have north at the bottom, and a gnomon would be placed rising up from the North Pole. The shadow-tip would point at the meridians numbered around the edge of the map. The map was published on a sheet with 10 other horological diagrams in Ritter’s “Speculum Solis”. The multi-volume work on time and clocks included intriguing illustrations of theoretical clocks.