Definition
An angular measurement, expressed in degrees east or west of the Prime Meridian (which runs through Greenwich, England), that specifies a position's east-west location on the Earth's surface. Lines of longitude, called meridians, run from the North Pole to the South Pole and are used together with latitude to identify any point on the globe.
Plain English
How far east or west a place is on the Earth, measured from a starting line that passes through Greenwich, England. The lines you see running top-to-bottom on a globe are lines of longitude.
Context Anchor
Pilots see longitude in chart coordinates, GPS positions, airport data, and when a text compares position on a graph to position on a map.
Derivation
From the Latin longitudo, meaning 'length.' Early mapmakers thought of the Earth as longer east-to-west than north-to-south, so the east-west measurement became 'length' — longitude.
Why Pilots Care
Accurate longitude is essential for precise navigation, route planning, GPS waypoint entry, and staying within assigned airspace boundaries.
Analogy
Longitude is like one half of an address for a place on Earth. By itself it does not give the whole location, but it tells the east-west part of that location.
Intuition Check
Longitude does not mean the length of the airplane, the runway, or the route. In navigation, it means a position measurement: how far east or west of the Prime Meridian a place is.
Example Sentence 1
The airport's position is given as 40° N latitude and 74° W longitude.
Example Sentence 2
On the sectional chart the airplane's location was shown as 35 degrees north latitude and 118 degrees west longitude.