Marine Glossary

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Longitude

Latitude and longitude are an imaginary grid used to locate positions on the earth's surface. Longitude is the side of the grid that tells how far a place is east or west of the Greenwich meridian. The longitude of a place is the angular distance "around" the earth from Greenwich to the place, when the earth is viewed from the top, looking down at the north pole. Lines of constant longitude are called meridians, and since they converge at the poles the number of miles between them decreases with increasing latitude.

At the equator, and to a good approximation throughout the Tropics, 1° of longitude is 60 nautical miles (the same as 1° of latitude), but at higher latitudes this distance must be read from a chart or plotting sheet. Though there is rarely any need to do so, the length of a longitude degree at some arbitrary latitude can be figured with a calculator from: 1° of Lon = 60 nmi x Cos(Lat). At 48° North, there are 40 miles to a degree of longitude. Traveling west from Greenwich, longitude is labeled west and increases until you reach 180°, and then it decreases and is labeled east. The meridian that lies 300° west of Greenwich, for example, is called 60° East.

For most applications in marine navigation we express lat and lon in units of degrees and minutes with tenths, denoted dd mm.mm, such as Lat 38º 23.72' or Lon 125º 45.65' W. In some applications of navigation and weather we might want to express Lat Lon in decimal degrees, denoted dd.ddd. To make the conversion note 38º 23.72' = 38º + (23.72/60)º = 38.395º. Likewise, if we had a Lon expressed as -125.7608º, we can convert it to dd mm.mmm as 125º + (0.7608 x 60') = 125º 45.65' W. When using decimal degrees, west lon and south lat are negative values.

Periodically we see deg min and seconds, but that is rare, and often when those are used they also give decimal degrees. In marine weather decimal degrees is used frequently.

Abbreviation: Lon