Try 0°: no seasons. Try 90° (like Uranus): each pole has months of non-stop daylight and then months of darkness.
Not to scale: the Earth is drawn about 2,000 times larger than it would be on this orbit, and the Sun about 35 times larger. The orbit’s real shape is used: it is very nearly a circle.
Shine a torch straight down and you get a small, bright spot; tilt it and the same light spreads over a bigger, dimmer patch. Sunlight works the same way. The pictures use the city and the date chosen above or in Day Length in Your City.
Calculated in your browser for any city and date. The green line is the length of daylight through the year; the faint line is the equator, where it hardly changes.
Times are for the city’s own time zone, at sea level with a flat horizon; sunrise and sunset are when the top of the Sun touches the horizon, including the bending of light by the air.
No. The Earth’s orbit is very nearly a circle, and the Earth is actually closest to the Sun in early January, early in the Northern Hemisphere’s winter.
1. The distance changes by only about 3% (147 to 152 million km). That changes the sunlight reaching Earth by about 7%, far less than the difference between summer and winter.
2. The Northern Hemisphere has winter when Earth is closest to the Sun.
3. The two hemispheres have opposite seasons at the same time. If distance were the cause, the whole Earth would have summer together.
The seasons diagram you draw and label in a test: the Earth at the solstices and equinoxes, with its axis always leaning the same way. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed.
A printable worksheet: label the orbit diagram, sort summer and winter, work out the height of the noon Sun, multiple choice and explain questions, with an answer key.
The Earth spins once a day around an axis through the North and South Poles, and travels around the Sun once a year. The axis is not upright compared with the orbit: it leans by 23.4°, and it always points towards the same spot in the sky, near the North Star (Polaris).
When the Sun is high, its light hits the ground steeply and is concentrated on a small area. When the Sun is low, the same light is spread out over a much larger area and also passes through more air. In summer the noon Sun is high; in winter it is low. At 40°N the noon Sun reaches about 73° in June but only about 27° in December.
The hemisphere leaning towards the Sun also spends more of each spin in daylight. London gets about 16½ hours of daylight in June and less than 8 in December; near the equator the days stay close to 12 hours all year. More hours of more concentrated sunlight add up to much more energy: that is why summer is warmer.
Around 21 June (June solstice) the North Pole leans most towards the Sun and the noon Sun is overhead at the Tropic of Cancer. Around 21 December the South Pole leans most towards the Sun and the Sun is overhead at the Tropic of Capricorn. Around 20 March and 22–23 September (equinoxes) neither pole leans towards the Sun, the Sun is overhead at the equator, and day and night are about equal almost everywhere.
Press Play a year and watch the yellow equator and your city’s green circle move into and out of the sunlight. Jump to the Closest to the Sun date and check the season in your city. Then set the tilt to 0°: the daylight readout stays near 12 hours all year.
| Date (about) | Name | Noon Sun overhead at | Northern Hemisphere | Southern Hemisphere |
|---|---|---|---|---|
| 20 March | March equinox | Equator (0°) | Start of spring; day ≈ night | Start of autumn (fall); day ≈ night |
| 21 June | June solstice | Tropic of Cancer (23.4°N) | Longest day; start of summer | Shortest day; start of winter |
| 22–23 September | September equinox | Equator (0°) | Start of autumn (fall); day ≈ night | Start of spring; day ≈ night |
| 21 December | December solstice | Tropic of Capricorn (23.4°S) | Shortest day; start of winter | Longest day; start of summer |
These are the astronomical seasons. Weather services often use meteorological seasons of whole months instead (in the north: winter = December, January, February; summer = June, July, August). The warmest and coldest weeks usually come a few weeks after the solstices, because land and oceans take time to heat up and cool down.
| Line | Latitude | What is special there |
|---|---|---|
| Equator | 0° | About 12 hours of daylight every day of the year; noon Sun overhead at the equinoxes |
| Tropic of Cancer / Capricorn | 23.4°N / 23.4°S | The farthest north / south the Sun can be directly overhead (at the June / December solstice) |
| Arctic / Antarctic Circle | 66.6°N / 66.6°S | At least one day a year with 24 hours of daylight (midnight Sun) and one when the Sun does not rise (polar night), ignoring the bending of light by the air, which lets the Sun peep over the horizon for a short time |
| Poles | 90°N / 90°S | About six months of daylight followed by about six months of darkness |
| Place | Latitude | June solstice | December solstice |
|---|---|---|---|
| Singapore | 1.4°N | 12 h 12 min | 12 h 03 min |
| Dhaka | 23.8°N | 13 h 36 min | 10 h 40 min |
| New York | 40.7°N | 15 h 06 min | 9 h 15 min |
| London | 51.5°N | 16 h 38 min | 7 h 49 min |
| Sydney | 33.9°S | 9 h 54 min | 14 h 24 min |
| Reykjavik | 64.1°N | 21 h 09 min | 4 h 07 min |
Calculated with the same method as the lab (sunrise and sunset include the bending of light by the air, which makes every day a few minutes longer).
The noon Sun’s height is 90° − |latitude − declination|, where the Sun’s declination (the latitude where it is overhead) swings between +23.4° and −23.4° during the year. Sunlight arriving at an angle h above the horizon is spread over 1/sin h times the area of an overhead beam, so each square metre receives sin h as much power. Averaged over a whole day at the top of the atmosphere, the North Pole at the June solstice receives about 520 W/m², more than the equator (about 385 W/m² that day) because the Sun never sets, although the ice reflects much of it. Earth’s orbit has an eccentricity of 0.017, so the Sun–Earth distance varies by about 3.3% and the sunlight by about 6.9%. Over tens of thousands of years the tilt (between about 22.1° and 24.5°), the shape of the orbit and the direction of the axis slowly change: these Milanković cycles help to pace the ice ages.
Because Earth's axis is tilted by about 23.4° and always points the same way in space. As Earth orbits the Sun, each hemisphere in turn leans towards the Sun. That hemisphere gets the Sun higher in the sky, so sunlight is more concentrated, and it gets longer days: summer. The other hemisphere has a low Sun and short days: winter.
Key takeaway: the tilt of the axis, not the distance from the Sun.No. Earth's orbit is nearly a circle; the distance changes by only about 3%. Earth is closest to the Sun in early January, during the Northern Hemisphere's winter, and the two hemispheres have opposite seasons at the same time, which distance could not explain.
Key takeaway: Earth is closest to the Sun in January.The solstices (about 21 June and 21 December) are when one pole leans most towards the Sun: the longest day in one hemisphere and the shortest in the other. The equinoxes (about 20 March and 22-23 September) are when neither pole leans towards the Sun: the Sun is overhead at the equator and day and night are roughly equal almost everywhere.
Key takeaway: solstices: longest and shortest days; equinoxes: day ≈ night.A beam of sunlight from a high Sun lands on a small patch of ground, so each square metre gets a lot of energy. From a low Sun the same beam is spread over a larger area, so each square metre gets less. At 30° above the horizon the beam covers twice the area of an overhead beam, so the ground gets half as much energy per square metre.
Key takeaway: low Sun = the same light spread over more ground.When the North Pole leans towards the Sun (June), the South Pole leans away, so it is summer in Europe, North America and most of Asia and winter in Australia, South Africa and Argentina. Six months later the situation is reversed. That is why Christmas is in summer in Australia.
Key takeaway: when one hemisphere leans towards the Sun, the other leans away.Near the equator the noon Sun is always high in the sky (at the equator itself it is never lower than about 66°), and the days stay close to 12 hours long all year. The amount of sunlight changes little, so there are no real hot and cold seasons; many tropical places have wet and dry seasons instead.
Key takeaway: the Sun is always high and days are about 12 hours.North of the Arctic Circle (66.6°N), around the June solstice, the North Pole leans so far towards the Sun that the Sun stays above the horizon all day and night. In December the same places have polar night, with no sunrise at all. The same happens south of the Antarctic Circle six months apart.
Key takeaway: inside the polar circles the Sun can stay up, or down, for 24 hours.With no tilt, the Sun would be overhead at the equator all year, every place would have about 12 hours of daylight every day, and there would be almost no seasons. With a very large tilt, like Uranus at about 98°, the poles would face the Sun for months on end and seasons would be extreme. You can try both in the lab with the tilt slider.
Key takeaway: no tilt, no seasons; more tilt, stronger seasons.The most sunlight arrives around the June solstice in the north, but land and especially oceans take weeks to warm up. As long as more energy arrives each day than is lost, temperatures keep rising, so the warmest weeks are usually in July or August (in the north). This is called seasonal lag.
Key takeaway: the ground and oceans take time to heat up.Not noticeably. During a year the axis keeps pointing at the same place in the sky, near the North Star. Over about 26,000 years it slowly traces a circle (precession), and over about 41,000 years the tilt itself varies between about 22.1° and 24.5°. These slow changes affect the climate over very long times.
Key takeaway: within a year the axis points the same way.