To locate places precisely, cartographers use an imaginary grid of lines based on latitude and longitude. The equator is the 0° latitude line that divides Earth into the Northern and Southern Hemispheres. The prime meridian, the 0° longitude line, runs through Greenwich, England. Together, these lines let every point on Earth be described by a unique pair of coordinates. Near the 180° meridian lies the International Date Line, an imaginary boundary where the calendar date shifts by one day as it is crossed.
Other key parallels mark the limits of the sun's vertical reach and the polar day-and-night cycle. The Tropic of Cancer, at 23.5° N, and the Tropic of Capricorn, at 23.5° S, are the northernmost and southernmost latitudes where the sun can appear directly overhead. The Arctic Circle at 66.5° N and the Antarctic Circle at 66.5° S are the latitudes beyond which the sun does not set on at least one day in summer or rise on at least one day in winter, producing the famous midnight sun and polar night phenomena.
Modern geography depends on tools that go well beyond paper maps. A Geographic Information System, or GIS, is a digital framework for capturing, storing, analyzing, and displaying spatial data. GPS, the Global Positioning System, is a satellite-based navigation system that allows receivers anywhere on Earth to determine their precise location. Cartography, the science and art of making maps, has evolved from hand-drawn charts into a highly technical discipline. On a topographic map, contour lines connect points of equal elevation, allowing the shape of the land, including hills, valleys, and slopes, to be read directly from the map.