Description: 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.
Usage Dates: 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.
A pair of ivory sundials that combine multiple instruments into one. Their functions are described in greater detail in the video below. Shown at the Harvard Museum of Historical Scientific Instruments.
Close-up on the larger antique ivory sundial. Visible elements include hour marks, constellations, days for lunar phases, and copious decoration.
Backs of the antique ivory sundials. Visible elements include hour marks, associated cities world-wide, and intricate etchings.
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.
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