Sundials use the shadow of the sun to point out times on a face similar to a modern clock face. As the sun moves across the sky, different numbers are touched by the shadow of the "gnomon" (pointer). This gives the "local" time, or "solar" time, and cities at different longitudes will record different times on their sundials.
Sundials must be carefully built and aligned. A sundial created for use at one latitude will give inexact times at other latitudes and times of the year. Many sundials even have two or more "hour lines" carved into them to indicate times in, for example, the summer and winter.
Sunlight Recorders; Armillary Spheres are a type of sundial.
Similar devices were used since at 1500 BCE. They were still in common use in navigation until the 18th century. Sundials are primarily decorative today.
- How a sundial works from Yale Scientific.
- A more detailed description of how sundials work from the British Sundial Society. Page also available in Czech, Polish, Russian, and Brazilian Portugese.
- Wikipedia's pages on Sundials.
- An unusual timber sundial at the Sydney Observatory.
- The Equation of Time, used to reconcile the actual reading on a sundial with the "mean solar time" which assumes that every two noontimes are exactly 24 hours apart.