Definition
Imaginary north–south lines drawn on the Earth's surface that connect the geographic North Pole to the geographic South Pole, used to measure angular distance east or west of the Prime Meridian (0°) running through Greenwich, England. Meridians are numbered from 0° to 180° east and 0° to 180° west.
Plain English
The vertical lines you see on a globe or chart that run from the top of the Earth to the bottom. They are used to describe how far east or west a place is.
Context Anchor
Seen in magnetic compass error discussions when comparing true north, based on the Earth’s geographic poles, with magnetic north, based on the Earth’s magnetic field.
Derivation
From the Latin meridianus, meaning 'of midday,' because the sun crosses a given meridian at local noon. 'Longitude' comes from the Latin longitudo, meaning 'length' — the long lines running the length of the globe from pole to pole.
Why Pilots Care
They establish true north so pilots can compare it with magnetic north and apply the correct variation when planning headings.
Analogy
On a globe, meridians of longitude are like the curved seams on a beach ball that run from the top to the bottom. They are not real marks on the Earth, but they give everyone the same reference lines to measure from.
Intuition Check
Do not think of meridians of longitude as magnetic lines or physical lines on the ground. They are map reference lines tied to the geographic poles, while a magnetic compass responds to the Earth’s magnetic field.
Example Sentence 1
When plotting a course on the sectional, the pilot measured the true track against the nearest meridian of longitude.
Example Sentence 2
True course is measured along the meridians of longitude while the compass shows magnetic course.