Equator, hemispheres and tropics
Five imaginary lines divide the Earth into climate zones and determine the seasons, day length and the tropical rain belt. They explain why Australia has summer when Europe has winter — and why the Sun in the tropics is never at a low angle.
The five key lines on a globe
The diagram shows the equator, the two tropics and the two polar circles at their fixed latitudes.
The equator: 0°
The equator is the imaginary circle at 0° latitude that divides the Earth exactly in two: a northern and a southern hemisphere. It lies equidistant from the North Pole (90°N) and the South Pole (90°S).
The equator is 40,075 kilometres long. Countries it crosses include Ecuador (named after the Spanish word for "equator"), Congo, Kenya and Uganda in Africa, Indonesia in Asia and Brazil in South America. On the equator the day is almost always exactly twelve hours long; there are no real seasons, but there is a dry and a wet season.
On the days of the equinox (around 20/21 March and 22/23 September) the Sun is directly overhead at noon on the equator — at the zenith. There are no shadows at midday.
Northern and southern hemispheres
The northern hemisphere encompasses Europe, North America, most of Asia and the northern portions of Africa and South America. The vast majority of the world's population — more than 90% — lives in the northern hemisphere.
The southern hemisphere encompasses the southern part of Africa, South America, Australia and the rest of Oceania, and Antarctica. It consists overwhelmingly of ocean: only a small fraction is dry land.
Alongside the North–South division there is also an eastern and western hemisphere, divided by the prime meridian (Greenwich, 0° longitude) and the 180° meridian (the date line in the Pacific). That division matters less for climate but is central to time zones.
The tropics: 23.5°
The Earth does not stand upright in its orbit around the Sun: the axis of rotation tilts 23.5° relative to the plane of the orbit. That tilt causes the seasons and determines where the tropics lie.
The Tropic of Cancer lies at 23.5°N. At the northern summer solstice (around 21 June) the Sun is directly overhead this line — the most northerly point at which the Sun can ever be at the zenith.
The Tropic of Capricorn lies at 23.5°S. At the southern summer solstice (around 21 December) the Sun is directly overhead this line. Between the two tropics — the tropics zone — the Sun is always high and the climate is warm year-round.
The polar circles: 66.5°
The Arctic Circle lies at 66.5°N (or more precisely: 90° − 23.5°). North of this line the Sun remains above the horizon for at least one full day per year (midnight sun) and also fails to rise for at least one full day (polar night). The closer to the pole, the longer those periods last.
The Antarctic Circle lies at 66.5°S. The same principle applies, but in reversed seasons. On Antarctica — entirely south of the Antarctic Circle — the polar night lasts for months during winter.
Norway, Sweden, Finland, Iceland, Canada, Alaska and Russia all extend partly above the Arctic Circle. In Tromsø (Norway) the Sun shines for 24 hours a day from mid-May to late July; during the polar night (November–January) it does not rise at all.
Reversed seasons: why summer and winter swap
The 23.5° tilt of Earth's axis is the key. As the Earth orbits the Sun, the axis keeps pointing in the same direction (towards the Pole Star). During the northern hemisphere's summer (June–August), the north tilts towards the Sun: the Sun is higher, the days are longer, and solar radiation strikes the surface more directly — giving more heat.
At the same time the southern hemisphere tilts away from the Sun: the Sun is low, days are short and radiation strikes at a shallow angle — it is winter there. Six months later the situation is exactly reversed.
Concrete exampleWhen it is summer in Europe (July), it is winter in Australia. Children in Sydney go to school in heavy coats. When Europeans are decorating their Christmas trees in December, Australians are heading to the beach — it is the height of summer. Oceania lies almost entirely in the southern hemisphere; its seasonal pattern is the mirror image of Europe's.
At the equinox (around 20 March and 23 September) the Sun is directly above the equator. North and South then receive equal sunlight and the day lasts exactly twelve hours everywhere on Earth.
The tropical zone and the temperate zones
The zone between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S) is called the tropics. Here the Sun is always high in the sky, the climate is warm and humid, and most rainforests and savanna belts are found. Large parts of Africa, South America and Asia lie in the tropics.
Between the tropics and the polar circles lie the temperate zones — the latitudes with four distinct seasons. Europe, most of North America, China and Japan lie here. Seasons are pronounced but not extreme.
Above the polar circles to the poles lie the polar zones: cold, with long winters and short, cool summers. Antarctica — the only continent lying entirely in the southern hemisphere's polar zone — has an average winter temperature of −60 °C on the interior plateau.
Frequently asked questions
What is the equator?
The equator is the imaginary circle at 0° latitude that divides the Earth into a northern and a southern hemisphere. It lies equidistant from the two geographical poles and is 40,075 km long.
Why are seasons reversed in the southern hemisphere?
Earth's axis tilts 23.5° relative to its orbit around the Sun. When the northern hemisphere tilts towards the Sun (summer in Europe), the southern hemisphere tilts away — and experiences winter. Six months later the situation is exactly reversed.
What are the tropics?
The Tropic of Cancer (23.5°N) is the most northerly point at which the Sun can ever be directly overhead (at the northern summer solstice). The Tropic of Capricorn (23.5°S) is the most southerly point — at the southern summer solstice.
What is the difference between a polar circle and a tropic?
The tropics (23.5°) mark the extreme latitudes at which the Sun can ever be directly overhead. The polar circles (66.5°) mark the zones where the Sun remains above or below the horizon for at least one full day per year — the midnight sun and the polar night.
Sources
- NASA — Earth's axial tilt and seasons (public educational resources)
- USNO — United States Naval Observatory, astronomical definitions
- Encyclopaedia Britannica — "Equator", "Tropic of Cancer", "Arctic Circle"
- CIA World Factbook — geographical location of countries by climate zone
- Royal Netherlands Meteorological Institute (KNMI) — season definitions