PHYSICS · Stage 7 · Chapter 5: The Earth in Space
Physics 5.2 Our planet: seasons — Axial Tilt (23.5°) and Solar Hemisphere Heating
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Lesson summary
5.2 Our planet: seasons — Axial Tilt (23.5°) and Solar Hemisphere Heating — Alhussan International Academy — Physics (Stages 7–9) notes: Explain how the tilt of the Earth's axis causes seasons, changes in day length, temperature and the height of the Sun in the sky.Orbit — the Ea…
- Near the poles
- Near the equator
- Changing day length
- Over the year
- Summer in the north
- Winter in the south
Section 1 of 6The year and the seasons
- Orbit
- the Earth orbits the Sun once every year, or 365 days.
- Seasons
- the year is divided into spring, summer, autumn and winter.
- Near the poles
- countries have a big difference in weather between summer and winter, with longer, warmer days in summer and shorter, colder days in winter.
- Near the equator
- countries between the Tropic of Cancer and the Tropic of Capricorn do not notice much change.
- Changing day length
- in most places the length of the day changes over the year,
- so the Sun sets earlier each night as winter approaches and later each night as summer approaches.
Section 2 of 6The tilt of the Earth's axis
- Tilt
- the axis is tilted by an angle of 23.5°, and this angle does not change.
- It always points to the same place in space as the Earth orbits.
- Over the year
- for 6 months the North Pole is tilted towards the Sun, and for 6 months it is tilted away.
- Summer in the north
- the North Pole is tilted towards the Sun, so the northern hemisphere receives more direct sunlight.
- The Sun stays in the sky for longer each day as the Earth spins, so the day is longer than the night.
- Winter in the south
- at the same time the southern hemisphere is tilted away from the Sun.
- The Sun stays in the sky for less time each day until midwinter.
- Six months later
- the situation is reversed, so the hemispheres have opposite seasons.
- What the tilt explains
- different day lengths, different temperatures and the varying height of the Sun in the sky in summer and winter.
Section 3 of 6Changing temperature
- Why summer is hotter
- the Sun is in the sky for longer, and the rays from the Sun are concentrated over a smaller area.
- Rays at 90°
- if the rays hit the surface at 90°, more energy hits each square metre per second,
- so the surface heats up faster and the air temperature rises more quickly.
- Winter
- the Sun's light is spread over a bigger area, such as 2 square metres instead of 1,
- so it is less intense, and the days are shorter,
- so there is less heating.
Section 4 of 6Changing height of the Sun
- Highest at noon
- during the day the Sun is highest in the sky at noon.
- Over the year
- the height the Sun reaches at noon is higher in summer than in winter, again because of the tilted axis.
- Shadows
- a fence post makes a shorter shadow at noon in summer than in winter, because the Sun is higher.
Section 5 of 6Midnight Sun and polar night
- Midnight Sun
- in regions in the north inside the Arctic Circle, such as Finland, Norway and Canada, the Sun does not set in summer,
- so they are known as the "Land of the Midnight Sun".
- The same happens in Antarctica during its summer, but no one lives there to see it.
- Polar night
- in winter in these places the Sun does not rise at all.
| Feature | Summer (tilted towards Sun) | Winter (tilted away) |
|---|---|---|
| Sun at noon | High | Low |
| Day length | Longer than night | Shorter than night |
| Heating | More | Less |
Section 6 of 6A common misconception
- Distance from the Sun
- some people think it is hotter in summer because the Earth is closer to the Sun, which is not true.
- Orbit
- the Earth's orbit is nearly circular, and the Earth is slightly closer to the Sun in January each year.
- No tilt
- if the axis was not tilted, day length would not change over the year and there would be no seasons.
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