Chapter 4 of 8

All 8 chapters
  1. 1The planets, readunderstood
  2. 2How big and how far, readunderstood
  3. 3Gravity and orbits, readunderstood
  4. 4Tilt and the seasons, readunderstood
  5. 5Seasons around the world, readunderstood
  6. 6Day length and the Sun's height, readunderstood
  7. 7Moons, asteroids and comets, readunderstood
  8. 8Exploring space, readunderstood

S3.12.4 Chapter 4

Tilt and the seasons

Why is it hot for part of the year and cold for another part? Is the Earth nearer the Sun in summer?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain seasons using the tilt of the Earth's axis
  • Model seasons with a tilted ball going round a lamp

Through the year the weather changes. So does the length of the day. A season is a part of the year with its own kind of weather.

The Earth spins round a line through its middle, from the North Pole to the South Pole. This line is the axis.

The axis does not stand upright. It leans over by about 23.4 degrees. We say the Earth is tilted.

Draw it first

Draw the tilted Earth

  1. The Sun on the left and the Earth on the right. In the first picture a dotted upright line runs through the Earth. In the second picture the axis leans away from the upright line by 23.4°, with N at the top and S at the bottom. In the third picture three arrows of sunlight go from the Sun to the Earth, and are labelled sunlight. Labels: Sun, Earth, N, S, 23.4°, sunlight.SunEarth

    1 Draw the Sun and the Earth. Draw an upright dotted line through the Earth, to show what upright means.

  2. The Sun on the left and the Earth on the right. In the first picture a dotted upright line runs through the Earth. In the second picture the axis leans away from the upright line by 23.4°, with N at the top and S at the bottom. In the third picture three arrows of sunlight go from the Sun to the Earth, and are labelled sunlight. Labels: Sun, Earth, N, S, 23.4°, sunlight.SunEarthNS23.4°

    2 Draw the axis leaning by 23.4°. Write N at the north end and S at the south end.

  3. The Sun on the left and the Earth on the right. In the first picture a dotted upright line runs through the Earth. In the second picture the axis leans away from the upright line by 23.4°, with N at the top and S at the bottom. In the third picture three arrows of sunlight go from the Sun to the Earth, and are labelled sunlight. Labels: Sun, Earth, N, S, 23.4°, sunlight.SunEarthNS23.4°sunlight

    3 Draw the sunlight going from the Sun to the Earth. The north end of the axis leans towards the Sun.

The axis points the same way in space all year. It always points to the same far-off spot in the sky. The Earth goes round the Sun, but the axis does not turn.

The Sun in the middle and the Earth drawn at four places round it, A, B, C and D. In every place the axis leans the same way. At A the north end leans towards the Sun. At C the south end leans towards the Sun. At B and D the axis leans neither towards nor away. Labels: N, A, B, C, D, Sun, A north tipped towards the Sun, B north and south lit equally, C south tipped towards the Sun, D north and south lit equally.NANBNCNDSunA north tipped towards the SunB north and south lit equallyC south tipped towards the SunD north and south lit equally
The Earth at four places on its path. The axis leans the same way each time. N shows the north end.

The Earth has two halves. The half north of the equator is the northern hemisphere. The half south of it is the southern hemisphere.

At A, the northern hemisphere leans towards the Sun. It has summer. The southern hemisphere leans away. It has winter.

Half a year later the Earth is at C. Now the south leans towards the Sun and has summer. The north leans away and has winter.

The Earth with the Sun far to the left. The axis leans so that the north end tips towards the Sun. The lit half faces the Sun and the dark half is cross hatched. Rings across the Earth are named: Arctic Circle, Tropic of Cancer, equator, Tropic of Capricorn, Antarctic Circle. The Arctic Circle ring is light all day, and the Antarctic Circle ring is dark all day. Arrows on the side show sunlight. Labels: N, S, Arctic Circle, Tropic of Cancer, equator, Tropic of Capricorn, Antarctic Circle, light all day, dark all day.NSArcticCircleTropic ofCancerequatorTropic ofCapricornAntarcticCirclelight all daydark all day
The north is tipped towards the Sun. The north has summer. The rings across the Earth are lines of latitude. The middle one is the equator.

Why does the tip make the difference? There are two reasons. The Sun climbs higher in the sky at midday. And the day is longer.

A high Sun is better at warming. Look at one beam of light, 10 cm wide, from a high Sun and from a low one.

See it three ways

As a picture

Two pictures side by side. In the left one the Sun is high, at 90 degrees, and a beam of light 10 cm wide covers 10 cm of ground. In the right one the Sun is low, at 30 degrees, and the same beam covers 20 cm of ground. Labels: Sun high, Sun low, 90°, 30°, 10 cm, 20 cm.10 cmSun high90°20 cmSun low30°

In words

The same beam of light covers more ground when the Sun is low. So each square centimetre gets less light and less warmth. A high Sun gives a stronger warming.

In symbols

At 30° the patch is 10 × 2 = 20 cm long. The light is spread twice as thin.

Away from the equator, in summer the Sun is high at midday, and the days are long. So the ground has more time to warm, and it warms faster. In winter the Sun is low and the days are short.

The Earth does not change in distance very much. The table shows how far it is from the Sun at its nearest and its farthest.

A table of the Earth's distance from the Sun. The head row says place and million km. The nearest place is 147. The farthest place is 152. Labels: place, million km, nearest, farthest, 147, 152.placemillion kmnearest147farthest152
The Earth is nearest the Sun in early January. It is farthest in early July.

The difference is only about 5 million km out of 150 million km. That is tiny. And in early January it is winter in the north and summer in the south.

Try it

Make seasons with a ball and a lamp

You need: a torch or battery lamp, a ball or an orange, a stick or pencil to push through the ball, a pen, a table in a room you can make dark, a sticky note for the wall.

Be safe: A lamp can get hot. Keep it away from paper and do not touch the bulb. Never use a lamp plugged into a wall socket. Never shine a torch into anyone's eyes.

  1. Push the stick through the ball from top to bottom. The ends are the poles. Lean the stick over, about a quarter of the way to flat.
  2. Draw a dot on the upper half of the ball for the north. Draw another on the lower half for the south.
  3. Put the lamp in the middle of the table. Stick the note on one wall of the room.
  4. Carry the ball in a big circle round the lamp. Keep the top of the stick leaning towards the note all the way. Never turn it.
  5. Stop at four places, a quarter of the way apart. At each place, see which dot gets more light.

What you will see: In one place the top of the stick leans towards the lamp. The north dot gets more light. Half way round it leans away, and the south dot gets more light. In the two places between, both dots are lit about the same.

If you cannot try it, this is what happens.
A ball lit from the right, as in the model. Its axis leans so that the south end tips towards the lamp. The lit half faces the lamp and the dark half is cross hatched. Rings across the ball are named: Arctic Circle, Tropic of Cancer, equator, Tropic of Capricorn, Antarctic Circle. The Antarctic Circle ring is light all day, and the Arctic Circle ring is dark all day. Arrows on the side show sunlight. Labels: N, S, Arctic Circle, Tropic of Cancer, equator, Tropic of Capricorn, Antarctic Circle, light all day, dark all day.NSArcticCircleTropic ofCancerequatorTropic ofCapricornAntarcticCircledark all daylight all day

A wrong idea many people have

It is summer when the Earth is nearer the Sun.

Why it feels right: A fire feels hotter when you are nearer. So the Sun seems to work the same way.

The test that breaks it: The Earth is nearest the Sun in early January. But it is winter in the north then. When the north has summer, the south has winter, yet both are the same distance from the Sun.

What is true: The tilt causes the seasons. The half of the Earth tipped towards the Sun has summer. The distance changes very little.

Key words

season
a part of the year with its own kind of weather.
axis
the line through the Earth from pole to pole, which it spins round.
tilted
leaning over. The Earth's axis leans by about 23.4 degrees.
hemisphere
one half of the Earth: north or south of the equator.
summer
the season when your hemisphere is tipped towards the Sun.
winter
the season when your hemisphere is tipped away from the Sun.

Back to your guess

The answer is C: The Earth's axis is tilted, so each half takes turns leaning towards the Sun.

The Earth's axis is tilted. The half that leans towards the Sun has summer. The half that leans away has winter. The distance to the Sun hardly changes.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What causes the seasons?

    Tap your answer.

    1. Yes, that is right. The axis points the same way all year. The half tipped towards the Sun has summer, and the half tipped away has winter.

    2. Not this one. The distance changes by only about 5 million km out of 150 million km. That is far too small, and the north has winter when the Earth is nearest. Go to S3.12.4. Try another.

    3. Not this one. The Sun shines about as strongly all year. The seasons come from how the Earth leans. Go to S3.12.4. Try another.

    See the answer

    The tilt of the Earth's axis

    The axis points the same way all year. The half tipped towards the Sun has summer, and the half tipped away has winter.

    • If you chose B: The distance changes by only about 5 million km out of 150 million km. That is far too small, and the north has winter when the Earth is nearest. Go to S3.12.4.
    • If you chose C: The Sun shines about as strongly all year. The seasons come from how the Earth leans. Go to S3.12.4.

  2. The northern hemisphere is having summer. What season is the southern hemisphere having?

    Tap your answer.

    1. Not this one. The two halves do not get the same light. When the north tips towards the Sun, the south tips away. Go to S3.12.4. Try another.

    2. Yes, that is right. When the north is tipped towards the Sun, the south is tipped away. So the south has winter.

    3. Not this one. The distance is almost the same, but distance does not decide the season. The tilt does. Go to S3.12.4. Try another.

    See the answer

    Winter

    When the north is tipped towards the Sun, the south is tipped away. So the south has winter.

    • If you chose A: The two halves do not get the same light. When the north tips towards the Sun, the south tips away. Go to S3.12.4.
    • If you chose C: The distance is almost the same, but distance does not decide the season. The tilt does. Go to S3.12.4.

  3. You carry a tilted ball round a lamp, as in the try it. The stick leans the same way all the time. The ball is at four stops, A, B, C and D. The Sun stands for the lamp. At which stop is the south half tipped towards the lamp?

    The Sun, standing for the lamp, in the middle and the tilted ball drawn at four stops round it, A, B, C and D. The stick leans the same way at each stop. Labels: N, A, B, C, D, Sun.NANBNCNDSun
    Tap your answer.

    1. Not this one. At stop A the north end of the stick leans towards the lamp. Look at which way the stick leans, compared with the lamp. Go to S3.12.4. Try another.

    2. Not this one. At stop B the stick leans sideways, neither towards nor away from the lamp. Go to S3.12.4. Try another.

    3. Not this one. At stop D the stick leans sideways, neither towards nor away from the lamp. Go to S3.12.4. Try another.

    4. Yes, that is right. The stick leans the same way at every stop, to the right. At stop C the lamp is on the left. So the south end of the stick leans towards it.

    See the answer

    Stop C

    The stick leans the same way at every stop, to the right. At stop C the lamp is on the left. So the south end of the stick leans towards it.

    • If you chose A: At stop A the north end of the stick leans towards the lamp. Look at which way the stick leans, compared with the lamp. Go to S3.12.4.
    • If you chose B: At stop B the stick leans sideways, neither towards nor away from the lamp. Go to S3.12.4.
    • If you chose C: At stop D the stick leans sideways, neither towards nor away from the lamp. Go to S3.12.4.

  4. Why is the Sun's light weaker on the ground in winter?

    Tap your answer.

    1. Not this one. The distance hardly changes. The Earth is nearest the Sun in early January, which is winter in the north. Go to S3.12.4. Try another.

    2. Not this one. The Sun is the same size all year. A low Sun spreads its light over more ground. Go to S3.12.4. Try another.

    3. Yes, that is right. A low Sun makes a long patch of light on the ground. The same light is spread thinner, so each square centimetre is warmed less.

    See the answer

    The Sun is lower, so the same light is spread over more ground.

    A low Sun makes a long patch of light on the ground. The same light is spread thinner, so each square centimetre is warmed less.

    • If you chose A: The distance hardly changes. The Earth is nearest the Sun in early January, which is winter in the north. Go to S3.12.4.
    • If you chose B: The Sun is the same size all year. A low Sun spreads its light over more ground. Go to S3.12.4.

  5. The Earth is 147 million km from the Sun at its nearest. It is 152 million km at its farthest. What does this tell us about the seasons?

    Tap your answer.

    1. Yes, that is right. 152 − 147 = 5 million km. That is a tiny change next to 150 million km. And the two halves have opposite seasons at the same distance.

    2. Not this one. The Earth is nearest in early January. That is winter in the north. So nearness cannot be the cause. Go to S3.12.4. Try another.

    3. Not this one. The Earth is farthest in early July. That is winter in the south, but it is summer in the north at the same time. Go to S3.12.4. Try another.

    See the answer

    The distance changes by only 5 million km, so it cannot cause the seasons.

    152 − 147 = 5 million km. That is a tiny change next to 150 million km. And the two halves have opposite seasons at the same distance.

    • If you chose B: The Earth is nearest in early January. That is winter in the north. So nearness cannot be the cause. Go to S3.12.4.
    • If you chose C: The Earth is farthest in early July. That is winter in the south, but it is summer in the north at the same time. Go to S3.12.4.

  6. From an earlier chapter

    What do we call the curved path of the Earth round the Sun?

    Tap your answer.

    1. Not this one. The axis is the line the Earth spins on. The path round the Sun is the orbit. Go to S3.12.4. Try another.

    2. Yes, that is right. The orbit is the curved path of one body going round another. The Earth's orbit round the Sun takes one year.

    3. Not this one. A year is how long one trip takes. The path itself is the orbit. Go to S3.12.3. Try another.

    See the answer

    Its orbit

    The orbit is the curved path of one body going round another. The Earth's orbit round the Sun takes one year.

    • If you chose A: The axis is the line the Earth spins on. The path round the Sun is the orbit. Go to S3.12.4.
    • If you chose C: A year is how long one trip takes. The path itself is the orbit. Go to S3.12.3.

  7. A beam of sunlight 8 cm wide falls on flat ground. The Sun is low, at 30 degrees. The patch is 2 times as long as the beam is wide. How long is the patch?

    See the answer

    16 cm

    8 × 2 = 16 cm. A low Sun stretches the same beam over a longer patch.

Keep it

Copy this into your exercise book.

The Earth's axis is tilted about 23.4° and points the same way all year. The half tipped towards the Sun has summer. The other half has winter.

Draw: The Sun, and the Earth at two opposite places with the axis tilted the same way. Mark summer and winter in each half.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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