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Case study · UK

Greenwich meridian

London

Longitude zero is a political choice laid on a rotating sphere. The 15°/hour arithmetic is not.

The Royal Observatory at Greenwich — a political line on a spinning sphere.
Longitude is a bargain about a line, not a physical ridge.Wikimedia Commons, CC BY-SA

Locator · OpenStreetMap / OpenFreeMap · 51.48°, -0.00°

The Prime Meridian at the Royal Observatory is the civil fiction. Solar noon is when the Sun crosses your local meridian. Time zones are a human overlay on 360° / 24 h = 15° per hour. The terminator ignores the jogs in the zone boundaries.

Stand on the meridian line and you have not stopped the planet. You have labelled a longitude. The rotation lab’s 15° callout is the physics. Greenwich is the plaque.

The graticule lab puts this line on a 3D Earth with the rest of the net. Students who think the equator is ‘a line painted on the sea’ need that lab more than this plaque, but the plaque helps.

Open this state in the lab

Exam-style question

Why is 15° of longitude equal to one hour, and why is the Greenwich meridian not required for that arithmetic?

Teacher hint: 360/24 = 15. Zero longitude is a convention; the spin is not.

Process labs

Earth rotation and the terminatorGraticule and coordinates

Sources

  • Royal Observatory Greenwich