The Longitude Dial reveals the time in the shadow of the wire, or "gnomon" (after a Greek word meaning "one who knows"), that runs from the North Pole on the map to the top of the post. At installation, the Dial is oriented so that the gnomon points to the North Star, Polaris. The Dial can be designed to tell either Daylight Saving Time or Standard Time.
In the photograph, the gnomon's shadow indicates the time (10:54 a.m.) on the hour scale. On the adjacent degree scale and on the map, it shows that it is noon everywhere 43° West of Greenwich. The bead's shadow marks the place where the Sun is directly overhead (i.e., Brazil).