William Andrewes' Longitude Dial tells time—assuming the Sun is shining—but it also does something no other dial can do: it tells place. As the daylight hours pass, the telltale shadow cast by the wire, or gnomon, moves across a laser-etched map; wherever that longitudinal shadow falls, it's noon. Part of what makes this feat possible is that the dial is custom-built for its location, with that very spot serving as the center of a computer-generated map on the dial face. In this dial, customized for a client in New York State, the gnomon's shadow indicates it's 11:45 a.m. at the dial's home base. Wherever the gnomon's shadow falls on the map, it's noon, and where it crosses, the degree scale marks the longitude of those places. The spherical shadow in South America, cast by the round bead on the gnomon, indicates where the Sun is precisely overhead.
CALENDAR CIRCLE
This ring is encircled with the number of minutes added or subtracted to convert solar time (as shown on a sundial) to so-called mean time (as kept by clocks and watches).
SUMMER SOLSTICE
The shadow of the gnomon's bead traces the Tropic of Cancer on this day, June 21. The time of the day's sunrise and sunset are also indicated on the ring encircling the hours and minutes.
ROMAN NUMERALS
Indicate the hour, when the gnomon's shadow falls on them. Minutes are marked by Arabic numerals.
(Description from The Smithsonian Magazine)
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