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Home›Weather and climate

Seasons and climate zones

Earth is closest to the Sun in early January, in the middle of the northern winter. Seasons come from the 23.44° tilt of Earth's axis. Move the Earth around its orbit, pick a latitude and read off day length and the height of the noon Sun, then see how those numbers sort the world into climate zones.

Progress

About this course

Earth is closest to the Sun in early January, in the middle of the northern winter. Seasons do not come from distance. They come from the 23.44° tilt of Earth's axis, which keeps pointing the same way in space, towards the North Star, all year. In June the north end of the axis leans towards the Sun, and six months later it leans away.

The six units are why seasons happen, day length by latitude, polar day and polar night, the tropics and the subsolar point, climate zones in the Köppen groups, and seasons in the southern hemisphere. The model lets you move Earth around its orbit, pick a latitude and read off day length and the height of the noon Sun. Set the latitude to 0° and the day length barely moves; at 60° N it rides a wave from under 6 hours to almost 19 hours. Special latitudes, the Arctic and Antarctic Circles at 66.56° and the Tropics of Cancer and Capricorn at 23.44°, are all set by the tilt. Because the noon Sun is always high in the tropics, many tropical places have seasons set by rain instead, and Australia's Bureau of Meteorology uses a wet season from October to April and a dry season from May to September.

The Köppen groups are checked in a fixed order. First ask whether the place is dry enough to be B, because a desert can be hot or cold. If not, a place where even the coldest month averages 18 °C or more is A, tropical, and one where even the warmest month stays below 10 °C is E, polar. The final unit mirrors all of this in the south: the Tropic of Capricorn crosses Australia, southern Africa and Brazil, and the Antarctic Circle surrounds almost all of Antarctica.

Before you start

Seasons come from the tilt of Earth's axis as it circles the Sun, so The solar system is a useful warm-up. Latitude appears in nearly every unit, and Continents and oceans explains it if lines of latitude are unfamiliar.

When you finish

You will be able to explain why Earth is closest to the Sun in January yet it is winter in Seoul, compare day length in London and on the equator, and say why Utqiaġvik has 84 days without sunset. You can find when the noon Sun is overhead in the tropics, give a Köppen code from monthly temperatures and contrast seasons in Sydney and Seoul.

  1. 1Why seasons happenOn 3 January 2026 Earth was 147.1 million km from the Sun, about 5 million km closer than in July, and it was winter in Seoul, London and New York.
  2. 2Day length by latitudeOn the June solstice London gets about 16 hours 38 minutes of daylight and a town on the equator about 12 hours 7 minutes.
  3. 3Polar day and polar nightOn 10 May 2026 the Sun rose over Utqiaġvik, Alaska, and did not set again for 84 days.
  4. 4The tropics and the subsolar pointIn Honolulu, at 21.3° N, the noon Sun is straight overhead twice a year: around 26 May and 15 July.
  5. 5Climate zones: the Köppen groupsA place where every month averages 18 °C or more is tropical in the Köppen system; one where no month reaches 10 °C is polar.
  6. 6Seasons in the southern hemisphereSydney gets about 14 hours 25 minutes of daylight on 21 December and about 9 hours 54 minutes on 21 June.

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Sources for this course (14)
  • NASA Space Place, What Causes the Seasons? (spaceplace.nasa.gov/seasons)
  • NASA Science, Basics of Space Flight ch. 2: perihelion 147,090,000 km about 4 January, aphelion 152,100,000 km (science.nasa.gov/learn/basics-of-space-flight/chapter2-1/)
  • EarthSky and timeanddate.com perihelion/aphelion 2026: 3 Jan 2026, 147,099,894 km
  • 6 Jul 2026, 152,087,775 km (earthsky.org/tonight/earth-comes-closest-to-sun-every-year-in-early-january/, timeanddate.com/astronomy/perihelion-aphelion-solstice.html)
  • timeanddate.com equinox and solstice times 2025-2026 UTC (timeanddate.com/calendar/seasons.html)
  • NOAA GML General Solar Position Calculations and glossary, declination ±23.44°, sunrise zenith 90.833° (gml.noaa.gov/grad/solcalc/solareqns.PDF, gml.noaa.gov/grad/solcalc/glossary.html)
  • US Naval Observatory, Approximate Solar Coordinates (aa.usno.navy.mil/faq/sun_approx)
  • NOAA Office of Education, Meteorological and astronomical seasons (noaa.gov/education/multimedia/infographic/infographic-meteorological-and-astronomical-seasons)
  • Met Office, Why do we have seasons? (metoffice.gov.uk/blog/2025/why-do-we-have-seasons)
  • Bureau of Meteorology, Solstices, equinoxes and the seasons (bom.gov.au/news-and-media/solstices-equinoxes-and-the-seasons)
  • FOX Weather (10 May 2026)
  • Utqiagvik 84 days without sunset from 10 May to 2 August 2026 (foxweather.com/earth-space/alaska-final-sunrise-midnight-sun-sunset)
  • Oregon State University open textbook, Köppen-Geiger classification descriptions (open.oregonstate.education/permaculturedesign/back-matter/koppen-geiger-classification-descriptions)
  • Beck et al. 2018, Present and future Köppen-Geiger climate classification maps at 1-km resolution, Scientific Data 5:180214 (nature.com/articles/sdata2018214).

Checked: 2026-10-17