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Fig. 2.1 Longitude and latitude lines on the Earth

Longitude lines have an interesting relationship to time. Because the Earth
rotates 15° every hour (15° x 24 hours = 360°) local time changes by one hour for

every 15° of longitude traveled. For example, when it is noon in Greenwich,
England, it is 2:00 PM in Moscow, 30° to the east and 7:00 AM in New York, 75°
to the west.
Using longitude and latitude, any point on the surface of the Earth can be

assigned a unique coordinate. For example, New York City is located
approximately 41 degrees north of the equator and 74 degrees west of the Prime
Meridian. This would be written:

New York City: 41°N, 74°W
However, we can be much more accurate than this using degree subdivisions
of minutes and seconds. For example, the American Museum of Natural History
in New York City is located at these coordinates:

A.M.N.H.: 40° 47 min. 00 sec. N, 73° 57 min. 50 sec. W

MAP GRID SYSTEMS

Map grid systems allow the map user to locate- or report on a specific point
on the map. For example, longitude and latitude lines on a Mercator-projection
map form a rectangular grid system that can be used to identify locations. In the
United States, four kinds of grid system are common found on maps: (a) map

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