What Is Longitude?

Longitude is the angle, measured in degrees, east or west of the prime meridian. It runs from 0° at Greenwich to ±180° at the antimeridian, with eastern longitudes positive and western negative.

Longitude is the east-west angular coordinate, and it has a far stranger history than latitude. Latitude is naturally anchored by Earth's rotation axis; longitude has no natural zero, and the world spent two centuries debating whose meridian should serve as the global reference before the 1884 Washington conference fixed Greenwich. This article defines longitude precisely, runs the convergence numbers that distort every flat map, traces the 1984 shift that moved 0° about 102 metres east of the brass strip in the Royal Observatory courtyard, and addresses the misconceptions that conflate longitude with time and time zones. The companion pillar /learn/what-is-latitude-and-longitude covers the full coordinate system; /learn/what-is-latitude is the north-south axis.

Definition

Per the NOAA NGS overview, longitude is the angular distance east or west of a chosen prime meridian. The angle is measured in the equatorial plane, from 0° at the prime meridian to ±180° at the antimeridian on the opposite side of Earth. Lines of constant longitude — called meridians — are great circles running pole to pole, and all meridians meet at the geographic North and South Poles.

Longitude differs from latitude in two structurally important ways. First, its zero is conventional rather than physical: latitude's 0 is fixed by Earth's rotation axis, but longitude's 0 sits where international agreement put it. Second, meridians are not parallel — they converge to a single point at each pole — so 1° of longitude is not a fixed distance the way 1° of latitude very nearly is.

How longitude is measured

Modern longitude comes out of satellite-based positioning — GPS and the other GNSS constellations (Galileo, GLONASS, BeiDou) reference the WGS-84 frame, in which 0° is the IERS Reference Meridian. Pre-electronic longitude was determined astronomically: the observer measured local solar time (when the Sun crossed the local meridian) and compared it with the time at the prime meridian, carried on a marine chronometer. The difference, multiplied by 15° per hour, gave longitude east or west of the reference.

The compression of error by roughly six orders of magnitude in three centuries is the longitude story: a hard problem, solved in stages by mechanical engineering, by radio, and finally by physics. Modern receivers don't need to solve any longitude problem — the satellites broadcast their positions in the WGS-84 frame, and the receiver triangulates passively.

The prime meridian

Before 1884, different nations used different prime meridians for their own charts — usually a meridian through the national observatory. The proliferation of national meridians made international map-making, telegraphy and timekeeping logistically painful, and at the 1884 International Meridian Conference in Washington DC, 25 nations met to settle on one.

The conference adopted Greenwich on the practical argument that 72% of world shipping already used Greenwich-based charts. The vote was 22 in favour, 1 against (San Domingo), and 2 abstentions (France and Brazil). France retained the Paris meridian for some internal cartography until 1911; the Soviet Union retained Pulkovo until 1925. The current frame — the IERS Reference Meridian — replaced the Airy line in 1984 for high-precision use, while the historic brass strip remains in place at Greenwich as a tourist site.

Why 0° is not exactly at the Airy Transit Circle

The modern 0° longitude is not the brass strip in the Royal Observatory courtyard. It is the IERS Reference Meridian, defined by satellite and quasar observations, and it lies 102.478 m east of the Airy Transit — a shift of 5.31 arcseconds of longitude. The divergence has a single-sentence explanation and a paragraph-length one.

The 1984 shift was a side effect of switching from an astronomical definition (which relies on local vertical and is biased by deflection of the vertical from gravity anomalies, local atmospheric refraction, and the precession of Earth's axis between the observation epoch and now) to a satellite-and-quasar definition (which sits at the geocentre and is free of those biases). When the geodesy community reduced century-old historic transit-circle observations into the new frame, the best-fit zero meridian came out about 102 metres east of the Airy line. The IERS kept the new zero rather than realigning to Airy — consistency with space-based positioning won over consistency with the brass strip.

How longitude shrinks toward the poles

Meridians are not parallel: they converge to a single point at each geographic pole. The east-west surface distance corresponding to 1° of longitude follows cos(latitude) almost exactly, with a small ellipsoidal correction that never exceeds 0.34%.

Three practical consequences flow from this curve. The first: maritime and aviation charts plot longitude lines at fixed angular spacing, but the implied surface distance varies with the chart's latitude — a 5°-of-longitude box on a chart of Canada is much smaller than a 5° box on a chart of Indonesia. The second: cardinal directions distort near the poles — one metre from the geographic North Pole, walking “east” traces a tiny circle of about 6.3 m circumference. The third: every flat map projection must distort somewhere, because flattening the convergent meridian grid onto a plane is geometrically impossible without tearing or stretching.

Longitude, time, and the antimeridian

Longitude and time are deeply related but not the same thing. Earth rotates 360° in 24 hours, so each degree of longitude corresponds to 4 minutes of mean solar time — or, equivalently, each hour of solar time corresponds to 15° of longitude.

Modern time zones approximate this 15°-per-hour conversion but follow political and administrative boundaries, not exact longitude lines. Nepal sits at UTC+05:45, India at UTC+05:30, Newfoundland at UTC−03:30 — the offsets exist in 15-minute granularity, not just whole hours. The IANA Time Zone Database codifies the modern set; the /learn/time-zones-explained pillar covers the structure in depth.

Worked example: longitude of the Eiffel Tower

The Eiffel Tower sits at 2.2945° E longitude (and 48.8584° N latitude). Decomposed:

• 2.2945° is the magnitude, in decimal degrees.
• E indicates the eastern hemisphere — equivalent to a positive sign on the decimal value.
• The line of constant longitude at 2.2945° E runs north-south through the tower, crossing northern France, the English Channel, Cambridgeshire in eastern England (about 25 km east of Cambridge), the North Sea, and onward toward Svalbard.

At 48.86° N, 1° of longitude corresponds to about 73.20 km of east-west surface distance — the cos(48.86°)-times-111.32 km relationship. The tower's longitude therefore puts it about 168 km east of the IERS Reference Meridian, measured along the local parallel. The Paris time zone (UTC+01:00 standard, +02:00 in summer) is loosely anchored to this longitude but is set by French law, not by the 2.2945° value directly.

Common misconceptions