School › Day, night, seasons › Ages 10–11
Day, night and the reasons for the seasons
How Earth's spin gives us day and night, how its tilt gives us seasons, and why distance from the Sun is NOT the reason. With a fair-test torch experiment.
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The Earth spins on its axis, an imaginary line through the North and South Poles. One complete turn takes about 24 hours (23.9 hours, to be exact). The half facing the Sun has day; the other half has night.[1][2]
At the same time, the Earth orbits the Sun. One orbit takes 365.25 days. Because of that extra quarter day, we add a leap day every four years.[2]
Earth's axis is tilted by about 23.4°. In June the northern half of the Earth leans towards the Sun and has summer; in December the southern half leans towards the Sun and the north has winter.[2][3]
The tilt changes two things. First, in summer the Sun climbs higher in the sky and its light hits the ground more directly, so each patch of ground gets more energy. In winter the light comes in at a slant and is spread over a bigger area. Second, summer days are longer: in parts of the USA the Sun is up about 15 hours in June but only about 9 hours in December.[4][1]
A common mistake is to think summer happens because Earth is closer to the Sun. In fact Earth is closest to the Sun in January (about 147 million km) and farthest in July (about 152 million km). The difference is only about 3%.[1][3][4]
Special days mark the year. At the solstices (around 21 June and 21 December) one hemisphere is tilted most towards the Sun: in the north, 21 June is the longest day. At the equinoxes (around 21 March and 21 September) every place has about 12 hours of day and 12 of night. 'Equinox' means 'equal night'.[5][4]
Try it: Fair test: straight or slanted sunlight?
You need
- A torch
- Squared paper (1 cm squares)
- A ruler
- A protractor
- A pile of books to rest the torch on
- A pencil
Safety
Don't shine the torch into anyone's eyes. Never look at the real Sun, not even through sunglasses.
What to do
- Put the squared paper on the table. Hold the torch exactly 20 cm above it, pointing straight down (90°).
- Draw around the patch of light and count the squares inside it.
- Now tilt the torch so it shines at 45° to the paper, keeping the front of the torch 20 cm from the middle of the patch. Draw around the patch and count the squares.
- Repeat at 30°.
- Keep the same torch, the same distance and the same paper each time: only the angle changes.
- Which patch looks brightest? Which is biggest?
What happens, and why?
The torch gives out the same amount of light each time. When it shines straight down, the light falls on a small patch, so each square gets a lot of light. At a slant the same light is spread over a bigger patch, so each square gets less. Summer sunlight is like the straight torch; winter sunlight is like the slanted one.[4][1]
Play: Summer or winter in the north?
Think about the northern half of the Earth (for example Europe). Is this true in summer or in winter?
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Quiz
Other ages: Ages 6–7 · Ages 15–16