Chapter 5 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

S3.12.5 Chapter 5

Seasons around the world

Why is it summer in one place while it is winter in another? Why is it warm all year at the equator?

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 why the two hemispheres have opposite seasons
  • Explain why places near the equator stay warm all year
  • Explain how the rain belt follows the overhead Sun to make wet seasons

In some places the year goes from hot to cold and back again. Near the equator it stays warm. There the seasons are wet and dry. Why is the Earth so different from place to place?

First, the two halves of the Earth. In the last chapter you saw that the north and the south take turns leaning towards the Sun. When one half leans in, the other leans out.

The Earth with the Sun far to the left. The axis leans so that the south 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. 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
The south is tipped towards the Sun. The south has summer and the north has winter.

When the south leans towards the Sun, it has summer. At the same time the north leans away and has winter. When the north has its longest day, the south has its shortest. The two halves have opposite seasons.

Now look at the middle of the Earth. The equator is the line round the middle, halfway between the poles. Places near it are warm all year.

At the equator the Sun is always high at midday. Even on its lowest day it is more than 66 degrees above the horizon. That is not far from straight above you.

A stick on flat ground at the equator, at midday, on a day when the Sun is lowest there. The Sun is high in the north, at 66.6 degrees. The stick has a short shadow pointing south. The ground is named north on the Sun side and south on the other. Labels: north, south, stick, shadow, 66.6°.stickshadow66.6°northsouth
The lowest the midday Sun gets at the equator. It is still very high, so the shadow is short.

The days near the equator are about the same length all year. The table gives the hours of day at the equator. It is about 12 hours of day and 12 hours of night, in every month.

See it three ways

As a picture

A table of the hours of day at the equator in four months. month, hours of day. Jan, 12. Apr, 12. Jul, 12. Oct, 12. Labels: month, hours of day, and every month and number in the table.monthhours of dayJan12Apr12Jul12Oct12

In words

Near the equator the Sun is high at midday. The day is about 12 hours long all year. The ground gets strong sunlight for a long time every day, so it stays warm.

In symbols

At the equator: 12 hours of day + 12 hours of night = 24 hours.

There is more. Between two special lines the midday Sun can stand straight overhead. Overhead means straight above you. One is the Tropic of Cancer, 23.4 degrees north of the equator. The other is the Tropic of Capricorn, 23.4 degrees south of it.

The band between the two lines is called the tropics. Outside the tropics the midday Sun is never overhead.

The place where the midday Sun is overhead moves. It goes north to the Tropic of Cancer and back south. It crosses the equator on the way. Then it goes to the Tropic of Capricorn and back. This takes one year.

A table of where the midday Sun is overhead on four days. date, overhead at. 21 March, equator. 21 June, 23.4° north. 22 September, equator. 21 December, 23.4° south. Labels: date, overhead at, and every date and place in the table.dateoverhead at21 Marchequator21 June23.4° north22 Septemberequator21 December23.4° south
The overhead Sun on four days of the year. It goes north, then south, then north again.

Now rain. Where the Sun is high, the ground gets very hot. The hot ground warms the air above it. Warm, moist air rises. As it rises it cools, and clouds and heavy rain form.

So rain falls most under the overhead Sun. There is a belt of cloud and rain round the Earth near the equator. It is the rain belt. It follows the overhead Sun north and south, a little behind it.

Draw it first

Draw the rain belt following the Sun

  1. The Earth seen from the side, with the Sun on the left. The equator and the two tropics are drawn across it. In the first picture the Sun is overhead at the equator and a hatched rain belt lies along it. In the second picture the Sun is overhead at 23.4 degrees north and the belt has moved north. In the third picture the Sun is overhead at 23.4 degrees south and the belt has moved south. Labels: N, S, Tropic of Cancer, equator, Tropic of Capricorn, Sun overhead, rain belt.NSTropic ofCancerrain beltequatorSunoverheadTropic ofCapricorn

    1 Draw the Earth with the equator and the two tropics. When the Sun is overhead at the equator, draw the rain belt along the equator.

  2. The Earth seen from the side, with the Sun on the left. The equator and the two tropics are drawn across it. In the first picture the Sun is overhead at the equator and a hatched rain belt lies along it. In the second picture the Sun is overhead at 23.4 degrees north and the belt has moved north. In the third picture the Sun is overhead at 23.4 degrees south and the belt has moved south. Labels: N, S, Tropic of Cancer, equator, Tropic of Capricorn, Sun overhead, rain belt.NSrain beltTropic ofCancerSunoverheadequatorTropic ofCapricorn

    2 Move the overhead line to the Tropic of Cancer. The belt moves north with it.

  3. The Earth seen from the side, with the Sun on the left. The equator and the two tropics are drawn across it. In the first picture the Sun is overhead at the equator and a hatched rain belt lies along it. In the second picture the Sun is overhead at 23.4 degrees north and the belt has moved north. In the third picture the Sun is overhead at 23.4 degrees south and the belt has moved south. Labels: N, S, Tropic of Cancer, equator, Tropic of Capricorn, Sun overhead, rain belt.NSTropic ofCancerequatorrain beltSunoverheadTropic ofCapricorn

    3 Move the overhead line to the Tropic of Capricorn. The belt moves south.

These pictures show the idea. The real belt is not a neat band. Land, sea and mountains push it about. We say it moves roughly with the overhead Sun.

When the rain belt is over a place, it is the wet season. When the belt has moved away, it is the dry season. The belt passes over a place near the equator twice a year. That place has two wet seasons. A place farther from the equator gets it once a year, so it has one wet season. Some places near the tropics are past where the belt reaches. They stay dry, and many are deserts.

Try it

Move the overhead Sun with a torch

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

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. Draw the equator round the middle with the pen.
  2. Draw one line about a quarter of the way up from the equator, for the Tropic of Cancer. Draw one the same distance down, for the Tropic of Capricorn.
  3. In the dark, hold the torch a hand's width from the ball. Shine it at the equator. The brightest spot is where the Sun is overhead.
  4. Lean the top of the stick towards the torch, about a quarter of the way to flat. Keep the torch still, shining at the middle of the ball. Watch the bright spot.
  5. Lean the stick the other way, so the bottom leans towards the torch. Watch the spot again.

What you will see: The bright spot is on the equator when the stick is upright. It moves north when you lean the top towards the torch. It moves south when you lean the other way. It never goes beyond the two tropic lines.

If you cannot try it, this is what happens.
The Earth side on with the Sun on the left. The midday Sun is overhead at 10 degrees north, and the rain belt lies round that line. Labels: N, S, Tropic of Cancer, equator, Tropic of Capricorn, Sun overhead, rain belt.NSTropic ofCancerrain beltSunoverheadequatorTropic ofCapricorn

A wrong idea many people have

The equator is warm because it is nearer the Sun.

Why it feels right: A map or a ball shows the equator sticking out towards the Sun. Nearer things feel hotter.

The test that breaks it: The side of the Earth that faces the Sun is at most about 6 400 km nearer. The Sun is 150 million km away. Such a tiny difference cannot matter.

What is true: The equator is warm because the midday Sun is always high there. The days are about 12 hours long all year. The sunlight is strong and it lasts.

Key words

equator
the line round the middle of the Earth, halfway between the poles.
overhead
straight above you.
tropics
the band between the Tropic of Cancer and the Tropic of Capricorn.
rain belt
the belt of cloud and heavy rain that follows the overhead Sun.
wet season
the time when the rain belt is over a place.
dry season
the time when the rain belt has moved away.

Back to your guess

The answer is C: When the north leans towards the Sun, the south leans away. So the two halves have opposite seasons.

The axis leans one way. When the north is tipped towards the Sun, the south is tipped away. So the two halves have opposite seasons, at the same time.

Check yourself

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

  1. The southern half of the Earth is having winter. What season is the northern half having?

    Tap your answer.

    1. Yes, that is right. When the south is tipped away from the Sun, the north is tipped towards it. So the north has summer.

    2. Not this one. The Earth does not have one season at a time. The two halves lean opposite ways. Go to S3.12.5. Try another.

    3. Not this one. Both halves have seasons. They are opposite: when one leans towards the Sun, the other leans away. Go to S3.12.4. Try another.

    See the answer

    Summer

    When the south is tipped away from the Sun, the north is tipped towards it. So the north has summer.

    • If you chose B: The Earth does not have one season at a time. The two halves lean opposite ways. Go to S3.12.5.
    • If you chose C: Both halves have seasons. They are opposite: when one leans towards the Sun, the other leans away. Go to S3.12.4.

  2. Why is it warm all year near the equator?

    Tap your answer.

    1. Not this one. The side facing the Sun is at most about 6 400 km nearer. The Sun is 150 million km away. That is far too small to cause the heat. Go to S3.12.5. Try another.

    2. Yes, that is right. At the equator the midday Sun is never lower than 66 degrees. Day and night are both about 12 hours. So the ground always gets strong sunlight for a long time.

    3. Not this one. A place does not have a tilt of its own. The whole Earth is tilted, and the equator stays in strong sunlight. Go to S3.12.4. Try another.

    See the answer

    The midday Sun is always high, and the days are about 12 hours long all year.

    At the equator the midday Sun is never lower than 66 degrees. Day and night are both about 12 hours. So the ground always gets strong sunlight for a long time.

    • If you chose A: The side facing the Sun is at most about 6 400 km nearer. The Sun is 150 million km away. That is far too small to cause the heat. Go to S3.12.5.
    • If you chose C: A place does not have a tilt of its own. The whole Earth is tilted, and the equator stays in strong sunlight. Go to S3.12.4.

  3. Where on the Earth can the midday Sun be straight overhead?

    Tap your answer.

    1. Not this one. Outside the tropics the midday Sun is never straight overhead, even in summer. It stays south or north of you. Go to S3.12.5. Try another.

    2. Not this one. The Sun is overhead at the equator on two days a year. But it is also overhead at other places in the tropics, as the spot moves. Go to S3.12.5. Try another.

    3. Not this one. At the poles the Sun is low in the sky and never gets overhead. Go to S3.12.5. Try another.

    4. Yes, that is right. The overhead Sun moves between 23.4 degrees north and 23.4 degrees south of the equator. Outside that band it is never straight above.

    See the answer

    Only between the Tropic of Cancer and the Tropic of Capricorn

    The overhead Sun moves between 23.4 degrees north and 23.4 degrees south of the equator. Outside that band it is never straight above.

    • If you chose A: Outside the tropics the midday Sun is never straight overhead, even in summer. It stays south or north of you. Go to S3.12.5.
    • If you chose B: The Sun is overhead at the equator on two days a year. But it is also overhead at other places in the tropics, as the spot moves. Go to S3.12.5.
    • If you chose C: At the poles the Sun is low in the sky and never gets overhead. Go to S3.12.5.

  4. What makes a wet season?

    Tap your answer.

    1. Not this one. The Earth's distance from the Sun does not decide the rain. The rain belt follows the overhead Sun. Go to S3.12.5. Try another.

    2. Not this one. Winter does not always bring rain. Rain comes where the Sun is high, the air is warm and moist air rises. Go to S3.12.5. Try another.

    3. Yes, that is right. Where the Sun is high, hot ground warms moist air. It rises, cools and makes clouds and heavy rain. The belt follows the overhead Sun, so the rain comes to different places at different times.

    See the answer

    The rain belt moves over the place, following the overhead Sun.

    Where the Sun is high, hot ground warms moist air. It rises, cools and makes clouds and heavy rain. The belt follows the overhead Sun, so the rain comes to different places at different times.

    • If you chose A: The Earth's distance from the Sun does not decide the rain. The rain belt follows the overhead Sun. Go to S3.12.5.
    • If you chose B: Winter does not always bring rain. Rain comes where the Sun is high, the air is warm and moist air rises. Go to S3.12.5.

  5. The midday Sun is overhead at about 20° north. A town lies at 20° north and another at 20° south. Which town is more likely to have the rain belt over it now?

    Tap your answer.

    1. Yes, that is right. The rain belt follows the overhead Sun, a little behind it. The Sun is overhead near 20° north, so the belt is near there. The northern town is more likely to be wet.

    2. Not this one. The belt follows the overhead Sun, roughly. The Sun is overhead in the north now, so the belt is in the north too. Go to S3.12.5. Try another.

    3. Not this one. The belt is a band near the overhead Sun. It cannot be over both places at once. Go to S3.12.5. Try another.

    See the answer

    The town at 20° north

    The rain belt follows the overhead Sun, a little behind it. The Sun is overhead near 20° north, so the belt is near there. The northern town is more likely to be wet.

    • If you chose B: The belt follows the overhead Sun, roughly. The Sun is overhead in the north now, so the belt is in the north too. Go to S3.12.5.
    • If you chose C: The belt is a band near the overhead Sun. It cannot be over both places at once. Go to S3.12.5.

  6. From an earlier chapter

    A friend says it is summer when the Earth is nearer the Sun. What do you say?

    Tap your answer.

    1. Not this one. The Earth is nearest the Sun in early January, when it is winter in the north. Nearness does not cause the seasons. Go to S3.12.4. Try another.

    2. Yes, that is right. The distance changes by only about 5 million km out of 150 million km. The tilt makes one half lean towards the Sun, and that half has summer.

    3. Not this one. The Earth is the same distance from the Sun for both halves. The tilt gives each half its own season. Go to S3.12.4. Try another.

    See the answer

    That is not true. The tilt of the axis causes the seasons.

    The distance changes by only about 5 million km out of 150 million km. The tilt makes one half lean towards the Sun, and that half has summer.

    • If you chose A: The Earth is nearest the Sun in early January, when it is winter in the north. Nearness does not cause the seasons. Go to S3.12.4.
    • If you chose C: The Earth is the same distance from the Sun for both halves. The tilt gives each half its own season. Go to S3.12.4.

  7. Tariq notes that the overhead Sun is at 10° south. Some weeks later it is overhead at 20° north. How many degrees has it moved?

    See the answer

    30 degrees

    From 10° south to the equator is 10 degrees. From the equator to 20° north is 20 degrees. 10 + 20 = 30.

Keep it

Copy this into your exercise book.

North and south have opposite seasons. Near the equator the Sun is always high, so it stays warm. The rain belt follows the overhead Sun and makes the wet seasons.

Draw: The Earth with the equator and two tropics. Draw the overhead line and the rain belt on it.

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

No internet tomorrow? Save the whole book as one file (911 KB). It opens on any phone.