Chapter 7 of 7

All 7 chapters
  1. 1Sun, Earth and Moon, readunderstood
  2. 2Why we have day and night, readunderstood
  3. 3The Sun seems to move, readunderstood
  4. 4A year: going round the Sun, readunderstood
  5. 5The Moon goes round the Earth, readunderstood
  6. 6Phases of the Moon, readunderstood
  7. 7Eclipses, readunderstood

S2.9.7 Chapter 7

Eclipses

Why does the Sun sometimes go dark in the day?

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 how the Moon blocks the Sun's light in an eclipse
  • Make a pinhole viewer. Watch an eclipse facing away from the Sun
  • Explain how the Earth's shadow causes an eclipse of the Moon

Sometimes the day goes dim. Sometimes the Moon turns dark red. Nothing is wrong. These are eclipses.

An eclipse is when one ball is in the shadow of another.

The Sun is the lamp. The Moon and the Earth are balls. Each ball has a shadow on the side away from the Sun.

The Moon can pass between the Sun and the Earth. Then the Moon blocks the Sun's light. This is an eclipse of the Sun. Its other name is a solar eclipse.

Draw it first

Draw an eclipse of the Sun

  1. A view from the side. The Sun is on the left, the Moon is in the middle and the Earth is on the right, all in a line. Two arrows of sunlight pass above and below the Moon and reach the Earth. A third arrow stops at the Moon. Two lines from the Moon's edges meet at a point on the Earth. A small black patch lies where they meet. A dashed line joins the label shadow to the lines. Labels: Sun, Moon, Earth, sunlight, shadow. The picture is not to scale.SunMoonEarthnot to scale

    1 Draw the Sun, the Moon and the Earth in a line. Write their names.

  2. A view from the side. The Sun is on the left, the Moon is in the middle and the Earth is on the right, all in a line. Two arrows of sunlight pass above and below the Moon and reach the Earth. A third arrow stops at the Moon. Two lines from the Moon's edges meet at a point on the Earth. A small black patch lies where they meet. A dashed line joins the label shadow to the lines. Labels: Sun, Moon, Earth, sunlight, shadow. The picture is not to scale.SunMoonEarthsunlightnot to scale

    2 Draw sunlight from the Sun. Let the middle arrow stop at the Moon.

  3. A view from the side. The Sun is on the left, the Moon is in the middle and the Earth is on the right, all in a line. Two arrows of sunlight pass above and below the Moon and reach the Earth. A third arrow stops at the Moon. Two lines from the Moon's edges meet at a point on the Earth. A small black patch lies where they meet. A dashed line joins the label shadow to the lines. Labels: Sun, Moon, Earth, sunlight, shadow. The picture is not to scale.SunMoonEarthsunlightshadownot to scale

    3 Draw the Moon's shadow reaching the Earth. Shade the small patch where it lands.

The Moon's shadow is small. It touches only a narrow strip of the Earth.

People in the strip see the Sun fully covered. The day goes dim. People near the strip see part of the Sun covered. People far away see no change.

An eclipse of the Sun can happen only at new Moon. Then the Moon is between the Earth and the Sun.

The Moon is small, but it is near. The Sun is about 400 times as wide as the Moon. It is also about 400 times as far away. So they look the same size in the sky.

You can watch an eclipse of the Sun safely. Use a pinhole viewer. It makes a small picture of the Sun on paper. You look at the paper, never at the Sun.

Try it

Make a pinhole viewer

You need: a piece of card, about the size of a book, a sheet of paper, a pin, a grown up to help, a sunny day.

Be safe: Never look at the Sun. Not through the hole, not past the card. Look only at the paper. Only a grown up uses the pin.

  1. A grown up pushes a pin through the middle of the card. The hole must be small and round.
  2. Put the paper on the ground in the sunlight.
  3. Stand with your back to the Sun. Never turn round to look at it.
  4. Hold the card up over your shoulder, in the sunlight.
  5. Let the card's shadow fall on the paper. Find the bright spot in it.
  6. Move the card higher or lower. Stop when a small round picture shows.

What you will see: A small round picture of the Sun shows on the paper. Hold the card higher and the picture grows bigger and dimmer. In an eclipse, the picture shows a bite out of the circle.

If you cannot try it, this is what happens.
A view from the side. The Sun is on the left. Two arrows of sunlight stop at a card with a small hole in it. A third arrow goes through the hole and lands on a sheet of paper on the right. A small round spot on the paper is the picture. Labels: Sun, card with hole, paper, picture.Suncard with holepaperpicture

Small gaps between leaves work like pinholes. Look at the ground under a tree. The bright round spots are pictures of the Sun. In an eclipse they show the bite.

Sometimes the Earth is between the Sun and the Moon. The Earth's shadow falls on the Moon. This is an eclipse of the Moon. Its other name is a lunar eclipse.

A view from the side. The Sun is on the left. Three arrows of sunlight go from the Sun to the Earth in the middle. Behind the Earth, two lines show the Earth's shadow stretching to the right and slowly narrowing. The Moon is on the right, inside the shadow. A dashed line joins the label shadow to the lines. Labels: Sun, sunlight, Earth, shadow, Moon. The picture is not to scale.SunsunlightEarthMoonshadownot to scale
The Earth's shadow falls on the Moon.

An eclipse of the Moon can happen only at full Moon. Then the Earth is between the Sun and the Moon.

Sun, Moon and Earth can line up at new or full Moon.

Seen from above. The Sun is on the left, labelled Sun. The Earth is in the middle, labelled Earth. A ring round the Earth is the path of the Moon. A small ball sits on the ring on the left, between the Sun and the Earth, labelled new Moon. A second small ball sits on the right of the ring, labelled full Moon. A dashed line runs from the Sun through the Earth and both balls. At the bottom are the words not to scale. Labels: Sun, Earth, new Moon, full Moon, not to scale.SunEarthnew Moonfull Moonnot to scale
On the dashed line, an eclipse can happen. Not to scale.

Anyone on the night side of the Earth can see it. The Moon often turns dark red. It is safe to look at an eclipse of the Moon.

See it three ways

As a picture

Two rows. The first row is labelled eclipse of the Sun. It shows the Sun, then the Moon, then the Earth. The second row is labelled eclipse of the Moon. It shows the Sun, then the Earth, then the Moon. Labels: eclipse of the Sun, eclipse of the Moon, Sun, Moon, Earth.eclipse of the SunSunMoonEartheclipse of the MoonSunEarthMoon

In words

In an eclipse of the Sun, the Moon is in the middle. In an eclipse of the Moon, the Earth is in the middle.

Why is there no eclipse every month? The path of the Moon is tilted a little.

Seen from the side. The Sun is on the left, labelled Sun. Arrows of sunlight go right. One arrow, labelled sunlight, reaches the Earth, labelled Earth, at the top right. A slanting line, labelled Moon's path, runs straight through the Earth, from lower left to upper right. The Moon, labelled Moon, sits on the line below the arrow. Two lines from the Moon go right and pass under the Earth. They are labelled shadow misses. At the top are the words tilt drawn bigger. At the bottom are the words not to scale. Labels: Sun, sunlight, Earth, Moon, Moon's path, shadow misses, tilt drawn bigger, not to scale.SunsunlightEarthMoonMoon's pathshadow missestilt drawn biggernot to scale
The tilt is drawn much bigger than it is. Most months the Moon is above or below the line. Its shadow misses the Earth.

A wrong idea many people have

There is an eclipse at every new Moon and every full Moon.

Why it feels right: The Moon passes between the Sun and the Earth every month. It seems it must block the Sun.

The test that breaks it: Do this in a dark room. Use a torch for the Sun and a ball for the Earth. Hang a small stone on a thread for the Moon. Swing the stone round the ball in a flat circle. It passes in front of the torch each time. Its shadow falls on the ball. Now tilt the circle a little. The stone misses the line between the torch and the ball. No shadow reaches the ball.

What is true: The Moon's path is tilted a little. Most months it misses the line from the Sun to the Earth. So there is no eclipse.

Key words

eclipse
when one ball is in the shadow of another.
eclipse of the Sun
when the Moon blocks the Sun's light.
eclipse of the Moon
when the Earth's shadow falls on the Moon.
pinhole viewer
a card with a tiny hole. It shows a picture of the Sun on paper.

Back to your guess

The answer is B: The Moon passes in front of the Sun.

The Moon passes between the Sun and the Earth. It blocks the Sun's light. This happens only at new Moon.

Check yourself

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

  1. At which phase can an eclipse of the Sun happen?

    Tap your answer.

    1. Yes, that is right. At new Moon the Moon is between the Earth and the Sun. Then it can block the Sun.

    2. Not this one. At full Moon the Earth is between the Sun and the Moon. That can bring an eclipse of the Moon. Try another.

    3. Not this one. At half Moon the Moon is off to the side. It is not in a line with the Sun and the Earth. Go to S2.9.6. Try another.

    See the answer

    New Moon

    At new Moon the Moon is between the Earth and the Sun. Then it can block the Sun.

    • If you chose B: At full Moon the Earth is between the Sun and the Moon. That can bring an eclipse of the Moon.
    • If you chose C: At half Moon the Moon is off to the side. It is not in a line with the Sun and the Earth. Go to S2.9.6.

  2. What causes an eclipse of the Moon?

    Tap your answer.

    1. Not this one. A cloud is in our air. It cannot shade the Moon. The Earth's shadow does. Try another.

    2. Not this one. A ball cannot shade its own lit side. The Earth's shadow falls on the Moon. Try another.

    3. Yes, that is right. The Earth is between the Sun and the Moon. Its shadow reaches out and covers the Moon.

    See the answer

    The Earth's shadow falls on the Moon.

    The Earth is between the Sun and the Moon. Its shadow reaches out and covers the Moon.

    • If you chose A: A cloud is in our air. It cannot shade the Moon. The Earth's shadow does.
    • If you chose B: A ball cannot shade its own lit side. The Earth's shadow falls on the Moon.

  3. Tariq wants to watch an eclipse of the Sun. What is safe?

    Tap your answer.

    1. Yes, that is right. You look at a picture on paper. You never look at the Sun.

    2. Not this one. Sunglasses do not make the Sun safe. Even a moment can harm your eyes. Try another.

    3. Not this one. A phone camera is not safe. Never look at the Sun this way. Try another.

    See the answer

    A pinhole viewer, with his back to the Sun

    You look at a picture on paper. You never look at the Sun.

    • If you chose B: Sunglasses do not make the Sun safe. Even a moment can harm your eyes.
    • If you chose C: A phone camera is not safe. Never look at the Sun this way.

  4. Who can see an eclipse of the Moon?

    Tap your answer.

    1. Not this one. That is an eclipse of the Sun. The Earth's shadow is big enough to cover the whole Moon. Try another.

    2. Not this one. At full Moon the Moon is on the night side. People in the day cannot see it. Try another.

    3. Yes, that is right. The Moon is up for everyone on the night side. They can all see the shadow on it.

    See the answer

    Anyone on the night side of the Earth

    The Moon is up for everyone on the night side. They can all see the shadow on it.

    • If you chose A: That is an eclipse of the Sun. The Earth's shadow is big enough to cover the whole Moon.
    • If you chose B: At full Moon the Moon is on the night side. People in the day cannot see it.

  5. From an earlier chapter

    Why can we not see a new Moon?

    Tap your answer.

    1. Not this one. The Earth's shadow falls on the Moon only in an eclipse. Go to S2.9.6. Try another.

    2. Not this one. The Moon is still there. We look at its dark half. Go to S2.9.6. Try another.

    3. Yes, that is right. At new Moon the Moon is between us and the Sun. We see the half that the Sun does not light.

    See the answer

    Its lit half faces away from us.

    At new Moon the Moon is between us and the Sun. We see the half that the Sun does not light.

    • If you chose A: The Earth's shadow falls on the Moon only in an eclipse. Go to S2.9.6.
    • If you chose B: The Moon is still there. We look at its dark half. Go to S2.9.6.

  6. Draw an eclipse of the Moon. Add the Earth's shadow.

    See the answer

    The Earth is between the Sun and the Moon. Its shadow reaches the Moon.

    The Earth is in the middle. Its shadow falls on the Moon.

  7. Mei thinks every new Moon brings an eclipse of the Sun. Why does it not?

    Tap your answer.

    1. Not this one. The Moon looks as wide as the Sun in the sky. When it is in line, it can cover the Sun. Try another.

    2. Not this one. The Moon stays near the Earth. It keeps going round it. The path is tilted. Try another.

    3. Not this one. The Earth's shadow reaches the Moon only at full Moon. At new Moon the Moon is between us and the Sun. Try another.

    4. Yes, that is right. Most months the Moon is above or below the Sun-Earth line. Its shadow misses the Earth.

    See the answer

    The Moon's path is tilted. Most months it misses the line to the Sun.

    Most months the Moon is above or below the Sun-Earth line. Its shadow misses the Earth.

    • If you chose A: The Moon looks as wide as the Sun in the sky. When it is in line, it can cover the Sun.
    • If you chose B: The Moon stays near the Earth. It keeps going round it. The path is tilted.
    • If you chose C: The Earth's shadow reaches the Moon only at full Moon. At new Moon the Moon is between us and the Sun.

Keep it

Copy this into your exercise book.

The Moon blocks the Sun at new Moon. That is an eclipse of the Sun. The Earth's shadow falls on the Moon at full Moon. That is an eclipse of the Moon. Never look at the Sun.

Draw: Draw the Sun, the Moon and the Earth in a line. Draw the Moon's shadow touching the Earth.

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