The gold circumpolar map in the background of this dial has lines running from major world cities to the periphery. The 24-hour World Time Disc, which rotates in front of the circumpolar map, is read with reference to any of the above lines. Note that the numerals representing the hours of daylight are shown in blue on gold. The midday sun (shown encircling the numeral 12) advances around the world at its zenith position indicating High Noon. The hours of darkness appear as gold numerals on a dark background.

The horizontal line, marked Greenwich, on the right side of the circumpolar map, represents meridian 00. This is an imaginary line of longitude drawn through Greenwich, England and it is used as the prime basis for computing standard time throughout the world. Greenwich Mean Time (G.M.T.) or Universal Time (U.T.) as it is also known, is computed on a 24-hour basis and is the mean solar time at 00 longitude. Persons all over the world use G.M.T. to synchronize their operations.

Because the earth rotates as it does, the sun appears to travel from East to West. In one hour the sun travels through 15 degrees of longitude. For instance, when it is 12 noon on meridian 00, one hour later it will be 12 noon on meridian 150. The four standard times of the United States are respectively 5, 6, 7 and 8 hours earlier" than the instant time at Greenwich.

This disc must be set for solar time at your location in order to be correct for Greenwich Mean Time. For this disc only, it is the equivalent of setting it for standard time at your nearest standard
time meridian.

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