Longitude

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Overview

Longitude is used in navigation and on maps to measure east-west distances. Lines of longitude all pass from the north pole to the south pole. When drawn on maps, longitude is shown as curved lines running from north (up) to south (down). Unlike latitude, there is no "center" point on the planet, so the 0-degree longitude line was arbitrarily chosen as being the line which passes through Greenwich, England, and is called the prime meridian.

See also: latitude - used to measure north-south distances.

How is Longitude Measured

Longitude is measured by distance from the prime meridian, which passes through Greenwich, England. Traveling either direction from the prime meridian you count up until you hit the exact opposite side of the Earth, which is exactly 180 degrees. Since the Earth is curved, the distance between lines of longitude varies, depending on how far you are from the equator.

At the equator, a degree of longitude is approximately:

• 60 nautical miles. Nautical miles are the miles commonly used for ocean navigation and are also commonly used by NASA for spacecraft navigation. They were originally defied to be exactly 1/60 of a degree, but increased accuracy in measurements have changed the value slightly.
• 69 statute miles. Statute miles are the miles normally used for land-based navigation, such as driving.
• 111 kilometers.

At the north and south poles, a degree of longitude is approximately 0.

Each degree of longitude can be broken down into 60 minutes of longitude. Each minute of longitude can be further broken down into 60 seconds of longitude. Seconds can be further broken down by using fractions of seconds. One second of longitude at the equator is approximately 300 meters or 1000 feet. Hand-held GPS units of the type typically carried by hikers can typically measure to an accuracy of 5 meters, or approximately .02 seconds of longitude.

How to Express Longitude

Longitude is normally expressed in spoken form as, "110 degrees, 49 minutes, 14.95 seconds West Longitude" or "110 degrees, 49 minutes, 14.95 seconds West. Latitude is always given before longitude. Often, this degree of accuracy is not required, so it's common to round to the nearest whole second.

Longitude is expressed in written form as 110°49'14.95"N. Note that the single quote means minutes and the double quote means seconds while used in latitude and longitude, not feet and inches.

Full coordinates are given as "44 degrees, 27 minutes, 39.71 seconds North, 110 degrees, 49 minutes, 14.95 seconds West. Elevation is rarely stated for location information. If used, it can be added as "Elevation 7379 feet" or "Elevation 2249 meters".