Definition
Meridians are the imaginary north-south lines drawn on a globe or chart that run from the geographic North Pole to the geographic South Pole. Each meridian marks a specific longitude, measured in degrees east or west of the Prime Meridian (0°), which passes through Greenwich, England. Meridians cross the equator at right angles and converge at the poles.
Plain English
Meridians are the up-and-down lines on a map that go from the top of the Earth to the bottom. They are used to show how far east or west a place is.
Context Anchor
Seen on aeronautical charts and in latitude-and-longitude position descriptions.
Derivation
From the Latin meridianus, meaning 'of midday' or 'noon.' At any point along a single meridian, the sun reaches its highest point in the sky at the same moment — local noon. That is why a north-south line came to be called a meridian.
Why Pilots Care
Meridians give the east-west half of every latitude-longitude fix, enabling precise navigation, position reports, and course plotting.
Analogy
On a globe, meridians are like the curved seams running from the top to the bottom of a beach ball.
Intuition Check
Do not think of meridians as east-west lines. Meridians run north and south, but they measure how far east or west a position is.
Example Sentence 1
The pilot noted that the airport sat just west of the 95° west meridian.
Example Sentence 2
All courses on the chart are measured from true north along the nearest meridian.