Latitude

Latitude measures how far north or south a point is on Earth or another celestial body. It’s expressed as an angle, from 0° at the Equator up to 90° at the North Pole (often written as 90° N or +90°), and down to −90° at the South Pole (90° S or −90°). The lines that connect points of equal latitude are called parallels; they run east–west as circles parallel to the Equator. Latitude and longitude together form a coordinate pair that pinpoints any location on Earth’s surface.

Usually, “latitude” refers to geodetic latitude. This is the angle between the plane of the Equator and a line that’s perpendicular (or normal) to an ellipsoidal model of Earth at that point. To define latitude and longitude, geographers work in two steps. First, they model the physical surface using a geoid—a surface that matches mean sea level over the oceans and extends under the land.

Background

Second, they simplify the geoid into a mathematically easier reference surface, often a sphere or, more accurately, an ellipsoid of revolution. On these reference surfaces, lines of constant latitude and longitude form a grid called a graticule. The latitude of a point on the actual surface is taken from the corresponding point on the reference surface, found by following the normal line from the physical point to the reference surface. Latitude, longitude, and a height specification together make up a geographic coordinate system, as defined by the ISO 19111 standard.

Because many different reference ellipsoids exist, the exact latitude of a feature isn’t fixed. The ISO standard stresses that without specifying the full coordinate reference system, latitude and longitude are ambiguous at best and meaningless at worst. This matters for high-accuracy uses like GPS, but in everyday situations where precision isn’t critical, the reference ellipsoid is usually left out. In English texts, latitude is often denoted by the Greek letter phi (ϕ or φ).

Determination

It’s measured in degrees, minutes, and seconds (or decimal degrees), with north or south of the Equator indicated. Latitude can be determined in celestial navigation using the meridian altitude method. More precise measurements require understanding Earth’s gravitational field, either to set up theodolites or to calculate GPS satellite orbits.

The graticule on the sphere

The study of Earth’s shape and gravity field is geodesy. On a spherical reference surface, the graticule is built around Earth’s rotation axis. The poles are where the axis meets the surface. Planes containing the axis cut the surface along meridians (lines of constant longitude), and the angle between any meridian and the Prime Meridian defines longitude.

The plane through Earth’s center perpendicular to the axis cuts the surface at the Equator, a great circle. Planes parallel to the Equator create circles of constant latitude—the parallels. The Equator is at 0°, the North Pole at 90° N, and the South Pole at 90° S. For any point, spherical latitude is the angle between the equatorial plane and the radial line (normal to the sphere’s surface). This is called spherical latitude to distinguish it from geodetic latitude and other auxiliary types.

Named latitudes on the Earth

Beyond the Equator, four other parallels are notable. Earth’s orbital plane around the Sun is the ecliptic; the plane perpendicular to Earth’s rotation axis is the equatorial plane. The angle between them is the axial tilt (or obliquity), denoted by i. The tropics lie at latitude i, and the polar circles at 90° − i.

The axis tilt changes slowly over time; the values given here are for the current epoch. At the December solstice, the Sun is directly over the Tropic of Capricorn; latitudes below the Antarctic Circle experience daylight, while those above the Arctic Circle are in night. The opposite happens at the June solstice, when the Sun is over the Tropic of Cancer. Only between the two tropics can the Sun ever be directly overhead.