What Is Latitude?

Latitude is the angle, measured in degrees, between the equatorial plane and a point on Earth's surface. It runs from 0° at the equator to ±90° at the poles, with northern latitudes positive and southern negative.

Latitude is one half of the geographic coordinate pair, and on its own it already encodes a lot: a single latitude value tells you which climate zone you are in, which stars are above your horizon, how long the longest day of the year is, and (with the year and a chronometer) what time the Sun crosses the meridian. This article defines latitude precisely, distinguishes the geodetic and geocentric forms, runs the numbers behind the special parallels, and explains the 1929 decision that made one nautical mile equal to one arcminute. The companion pillar /learn/what-is-latitude-and-longitude covers the full coordinate system; /learn/what-is-longitude is the matching east-west axis.

Definition

Per the NOAA National Ocean Service, latitude is the angle between the equatorial plane and a line from a surface point. The angle is measured in degrees, decimal minutes, or arcseconds, with 0° at the equator and ±90° at the poles. Lines of constant latitude are parallels: each is a circle whose plane is parallel to the equatorial plane and whose circumference shrinks toward the poles.

The definition has a refinement on Earth's actual shape: because the planet is an oblate ellipsoid, flattened by 1/298.257223563 per the WGS-84 specification, the "line from the surface point to the centre" is not unique. The two candidate lines (the local surface normal and the geocentric line) produce two distinct definitions of latitude, and modern coordinates have settled on the first.

How latitude is measured

Latitude is measured by determining the direction perpendicular to the local horizontal — the surface normal — and reading its angle above the equatorial plane. A reading of 40.7484° N means the local surface normal is 40.7484° above the equator. Modern receivers do this with satellite ranging; pre-electronic navigators did it by sighting the Sun, Polaris or other reference stars with a sextant or astrolabe and applying the appropriate declination correction.

Each generation roughly compressed the error by 1-2 orders of magnitude. Pre-Harrison celestial latitude was already accurate to a few arcminutes because Sun-and-star sighting only requires a stable vertical reference; the longitude problem (covered in /learn/what-is-longitude) was the harder one because longitude required precise timekeeping at sea.

Geodetic versus geocentric latitude

The two definitions agree exactly at the equator and at the poles and diverge most at mid-latitudes. The divergence comes from the ellipsoid's flattening: the surface normal tilts slightly farther from the equatorial plane than a line to the centre would, because the ellipsoid bulges outward at the equator.

The peak deviation of 11′ 33″ corresponds to a north-south displacement of about 21.4 km if the two definitions are accidentally swapped — large enough to put a city in the wrong county. Civilian GPS, every modern map and every commercial coordinate API use the geodetic form. Geocentric latitude appears only in specialised software: satellite-orbit computation (where Earth's gravitational centre is the natural origin), some classical astrometric software, and certain geodesy texts.

The named parallels

A handful of parallels are special enough to have proper names. Their exact latitudes are set by Earth's axial tilt (obliquity of the ecliptic), which the IERS publishes and which drifts by about 46.81 arcseconds per century.

The tropics and circles move about 14 m per year north or south as obliquity decreases — slowly enough to ignore for human-scale work but fast enough that the published values changed observably between the 1970s and the 2020s. The boundaries also bracket climate zones: the tropical zone lies between the tropics, the temperate zones between the tropics and the polar circles, and the polar zones beyond the circles.

The collapse of the parallel circumference toward each pole is the same effect that drives the 1°-of-longitude collapse: meridians converge, so the perpendicular cross-section (the parallel) shrinks. At the 60th parallel the circumference is exactly half the equatorial length — a useful mental anchor.

One arcminute equals one nautical mile

The nautical mile has an explicit connection to latitude. By international agreement at the 1929 IHC, one nautical mile equals 1,852 m exactly; the BIPM lists it as a non-SI unit accepted for use; the ICAO and IHO both maintain the same definition for aviation and maritime work. The 1,852 m value was chosen so that one arcminute of latitude on Earth's surface equals one nautical mile to high precision.

The 1,852 m international nautical mile sits almost exactly at the 45° value — the global mean of the meridional arc. The maximum deviation in either direction is about half a percent, well within the needs of navigation. For comparison, the older UK nautical mile (1,853.184 m, retired 1970) and the US nautical mile (1,853.248 m, retired 1954) were both based on slightly different ellipsoid models.

How latitude length varies across the ellipsoid

One degree of latitude does not correspond to exactly the same surface distance at the equator and at the poles. The variation is about 1.12 km between extremes — small in everyday terms, but real for surveying, geodesy and any work where the difference between 110 km and 112 km matters.

The variation has the opposite sign from what most people expect. On an oblate ellipsoid, the meridional radius of curvature is smallest at the equator (where the surface curves more sharply per arc) and largest at the poles (where the surface is flatter). A given arc-angle therefore sweeps a longer surface distance at the poles than at the equator — counterintuitive, but a direct consequence of the same flattening that drives every other ellipsoidal effect on this site.

The opposite intuition — "the Earth bulges at the equator, so a degree there should be longer" — applies to longitude, not latitude. One degree of longitude shrinks from 111.32 km at the equator to 0 km at the poles. One degree of latitude grows by about 1%. Both effects come from the same ellipsoidal shape, but they apply to different axes. For precise distance work, use /tools/distance-calculator, which runs Vincenty's formula on the WGS-84 ellipsoid and accounts for both variations directly.

Worked example: latitude of the Empire State Building

The Empire State Building sits at 40.7484° N. Three things follow from that single value, before longitude even enters.

• Climate / day length: the building is at a mid-latitude, well above the Tropic of Cancer (23.44° N) and well below the Arctic Circle (66.56° N). The summer solstice gives ~15 hours of daylight; the winter solstice gives ~9 hours.
• Star visibility: Polaris stands 40.75° above the northern horizon (its altitude equals the observer's latitude). The southern celestial sphere within ~49° of the south celestial pole is permanently below the horizon.
• Meridian arc: the building is 2,444.904 arcminutes of latitude north of the equator — equivalently, 2,444.904 nautical miles (about 4,527 km on the WGS-84 ellipsoid, after the per-arcminute correction across that span).

A change of 0.0001° in latitude — from 40.7484 to 40.7485 — corresponds to about 11.1 m of north-south displacement on the ground at this latitude. Four decimal places name the building's footprint; six name a specific spot within it.

Common misconceptions