Longitude is a geographic coordinate that specifies the east-west position of a point on the surface of the Earth or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ) . Longitude is measured by imaginary lines that run around the Earth vertically, called meridians, which connect points with the same longitude. The prime meridian, which is defined as 0° longitude, runs through Greenwich, England. Locations east of the prime meridian have positive longitudes, while locations west of the prime meridian have negative longitudes. Longitude is generally given using the geodetic normal or the gravity direction.

Some key points about longitudes are:

• Longitudes are vertical lines that measure east or west of the meridian in Greenwich, England.
• Like latitude, longitude is measured in degrees, minutes, and seconds.
• Longitude lines are furthest from each other at the equator and meet at the poles.
• Longitudes are defined in terms of meridians, which are half-circles running from the North Pole to the South Pole.
• Longitudes provide the location of a place thats east or west of the prime meridian.

In summary, longitudes are imaginary lines that run vertically around the Earth, connecting points with the same east-west position. They are measured in degrees, minutes, and seconds, and are defined in terms of meridians. The prime meridian, which runs through Greenwich, England, is defined as 0° longitude.