World map with the equator

What the equator actually is

The equator is the set of points halfway between the North and South Poles — the circle you get by slicing the planet through its centre, perpendicular to its axis of rotation. Every other line of latitude is a small circle that shrinks towards the poles; only the equator is a great circle, the largest circle that can be drawn on a sphere. It is the zero from which all latitude is counted, which is why it is the one parallel with a name rather than a number.

Its length, 40,075 km, is not the same as a circle drawn through both poles, which comes out about 67 km shorter. The Earth is not a sphere: it spins once a day, and that spin has pushed its middle outwards. The planet's radius is 21 km greater at the equator than it is at the poles. One consequence is a favourite piece of trivia — the summit of Chimborazo in Ecuador, only 6,263 m above sea level, is the point on the Earth's surface farthest from its centre, beating Everest by about two kilometres, purely because it sits on the bulge.

Why the equator has no seasons

Two things set equatorial climate apart, and both come down to geometry. The first is the angle of the sunlight: at latitude zero the sun is never far from vertical, so a given beam of sunlight lands on the smallest possible patch of ground and delivers the most energy per square metre. The second is constancy. Because the equator is the axis the Earth's tilt pivots around, day length there barely moves all year — roughly twelve hours of daylight and twelve of darkness, every day, with a variation of only a few minutes.

So the equator does not have a summer and a winter; it has a wet season and a dry season, and often two of each. What changes through the year is not the heat but the rain, as the Intertropical Convergence Zone — the band where the trade winds of the two hemispheres collide and force air upwards — drifts north and south with the sun. Where that band lingers, you get the rainforests: the Amazon, the Congo Basin, the forests of Borneo and Sumatra. It is no coincidence that the equator threads through the three great rainforest blocks on Earth, or that several of the countries it crosses appear on our list of megadiverse countries.

Altitude undoes all of it, though. Quito sits 24 km from the equator at 2,850 m and is mild the year round; and on Volcán Cayambe, an hour north of the city, the equator runs straight across a glacier at about 4,600 m. It is the only point on the whole line where there is permanent ice.

Standing on the line

The equator is marked in a dozen countries, and the most famous marker is in the wrong place. The Mitad del Mundo monument outside Quito, built in 1979 on a survey dating to the French Geodesic Mission of the 1730s, carries a painted line that visitors queue to straddle. Satellite positioning has since shown the real equator to be about 240 metres north of it. The error is a fair one — the 18th-century surveyors were working with sightings of the stars and got within a few hundred metres — but it does mean the photographs are taken in the southern hemisphere.

Twenty kilometres away at Cayambe, the Quitsato sundial is built on the line as GPS finds it, and a phone will show 0°00'00" there. Elsewhere: a monument stands on Ilhéu das Rolas in São Tomé and Príncipe, where the crossing is only 1.6 km wide; Pontianak in Indonesia has marked its equator since 1928; and the highway between Nanyuki and Nairobi carries the sign where most visitors to Kenya meet the line — along with the basin-and-bucket demonstration discussed below.

The draining-water trick

At equator markers from Kenya to Ecuador, a guide will carry a basin a few paces north of the line and let it drain clockwise, carry it a few paces south and let it drain anticlockwise, then set it on the line and show it draining straight down. It is a good trick and it is entirely fake.

The Coriolis effect is real: because the Earth rotates, anything moving across its surface is deflected, right in the northern hemisphere and left in the southern. It is why hurricanes spin the way they do and why ocean gyres turn. But its strength scales with the size and duration of the motion, and in a basin of water it is something like ten million times weaker than the swirl left over from how the water was poured in. Move the basin twenty metres and nothing measurable changes — what changes is the direction the guide pours, and the tilt of their hands. The effect also falls to exactly zero at the equator, which makes the middle part of the demonstration the only honest bit.

Why latitude zero is worth money

The equator is not only a line on a map. The ground there is moving east at 465 metres per second as the planet turns — faster than anywhere else on Earth — and a rocket launched eastward keeps every bit of that as free velocity. It is the reason Europe launches from Kourou in French Guiana rather than from Europe, and one reason Cape Canaveral sits at the southern end of the United States.

There is a second reason, which matters more. A satellite in geostationary orbit — hovering above a fixed point on the ground, which is what television and weather satellites need — can only do so directly above the equator, at 35,786 km up. Any orbit inclined away from that plane makes the satellite wander north and south through the day. Launching from close to the equator means starting almost in the right plane, and every degree of latitude you start from is fuel you have to spend correcting for.

The equator and the tropics

The equator is often drawn with two companions, and they are frequently confused with it. The Tropic of Cancer (23.44° N) and the Tropic of Capricorn (23.44° S) mark the furthest points at which the sun can ever be directly overhead — a limit set entirely by the tilt of the Earth's axis. Between them lies the tropical zone, shown as the warm band on the map above; outside them, the sun is always at least slightly off vertical, at every hour of every day of the year.

On the equator itself, the sun passes directly overhead twice a year, on the two equinoxes in March and September. At the tropics it happens once, at a solstice. And the two figures are not fixed: the Earth's tilt drifts slowly, so the tropics are creeping towards the equator at about 15 metres a year. The Arctic and Antarctic Circles, at 66.56° north and south, are the same geometry from the other end — the limits of where the sun can stay up, or stay down, for a full 24 hours.