Latitude, Longitude

Measuring and plotting your position on the Earth’s surface
Latitude is distance north or south of the equator, and is measured in degrees, minutes, and seconds of arc. Each degree equals 60 nautical miles, and each minute approximately 1 mile.Actually, because the Earth is not exactly round—being flattened at the poles and bulging at the equator—there is a small difference in the length of a degree at various latitudes. At the poles, for example, a degree equals 60.27 nautical miles; at latitude 45°, it is 59.96 miles; and at the equator, it is 59.66 miles. This is, however, of little consequence to the amateur navigator.Latitude is shown on charts by lines running east and west. Scales at the left and right (west and east) edges of the chart show degrees, minutes, and (depending on the chart’s scale) seconds. Because 1 minute equals 1 nautical mile, you can measure a distance on the chart with a pair of dividers, comparing it with the side scale shown at the same latitude.Longitude is angular distance east or west of the prime meridian, an imaginary line running from pole to pole through Greenwich, England. The prime meridian is numbered 0; from it, longitude is reckoned 180° eastward and 180° westward. On the opposite side of the world from Greenwich, the 180th meridian is known as the International Date Line. If you cross the line going west, you must drop a day in your calendar reckoning; if you cross the line going east, you repeat the day.Meridians, or lines of equal longitude, are marked on charts as lines running north and south, and their scales are given at the top (north) and bottom (south) of charts. Because lines of longitude converge at the poles, the length of a degree of longitude varies according to latitude. At the equator, it is approximately 60 miles; at the poles, it is zero.Always measure distances from the latitude scale at the east and west edges of the chart, never from the longitude scales at the north and south edges.See also Miles