Chapter 3 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

S3.12.3 Chapter 3

Gravity and orbits

Why does the Earth not fall into the Sun, or fly off into space?

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 that gravity keeps planets and moons in orbit
  • Explain why planets far from the Sun take longer to go round

The Earth goes round the Sun. The Moon goes round the Earth. What keeps them on their paths?

Gravity is the pull that every body has on every other body. The more mass a body has, the stronger its pull. The Sun is huge. It holds nearly all the mass of the solar system.

So the Sun pulls on every planet. The pull is strong enough to hold the Earth, even from 150 million km away.

If the pull were all, the planets would fall straight into the Sun. But each planet is also moving sideways, very fast. The Earth moves about 30 km every second.

Draw it first

Draw why a planet curves

  1. The Sun on the left and a planet on the right. In the first picture an arrow from the planet to the Sun is labelled gravity. In the second picture a second arrow points up from the planet and is labelled its motion. In the third picture a dotted curve runs from the planet up and round towards the left, labelled orbit. Labels: Sun, planet, gravity, its motion, orbit.Sunplanetgravity

    1 Draw the Sun and a planet. Draw an arrow from the planet to the Sun. It is the pull of gravity.

  2. The Sun on the left and a planet on the right. In the first picture an arrow from the planet to the Sun is labelled gravity. In the second picture a second arrow points up from the planet and is labelled its motion. In the third picture a dotted curve runs from the planet up and round towards the left, labelled orbit. Labels: Sun, planet, gravity, its motion, orbit.Sunplanetgravityits motion

    2 Draw a second arrow from the planet, at right angles to the first. It shows the planet moving sideways.

  3. The Sun on the left and a planet on the right. In the first picture an arrow from the planet to the Sun is labelled gravity. In the second picture a second arrow points up from the planet and is labelled its motion. In the third picture a dotted curve runs from the planet up and round towards the left, labelled orbit. Labels: Sun, planet, gravity, its motion, orbit.Sunplanetgravityits motionorbit

    3 The pull keeps bending the path. The planet follows a curve round the Sun. Draw the curve and call it the orbit.

The curved path round a bigger body is an orbit. The pull never lets the planet go, and the sideways motion never lets it fall in. So it goes round and round.

Without the pull, a planet would not go round. It would fly off in a straight line. You can see this with a ball on a string.

Try it

Swing a ball on a string

You need: a light, soft ball made of rolled cloth or a knotted sock, a string about 1 m long, a wide, open, flat place with nobody near.

Be safe: Use only a light, soft ball. Never use a stone. Swing and let go well away from people, animals and windows.

  1. Tie the string firmly to the soft ball. Check that nobody is within 4 m.
  2. Hold the end of the string. Swing the ball round in a flat circle at about waist height.
  3. Feel the string pull your hand inwards. The pull is like gravity.
  4. Slow down and stop. Then swing again and let go on purpose, with nobody in the way. Watch where the ball goes.

What you will see: While you hold the string, the ball goes round in a circle. The string pulls it inwards all the time. When you let go, the pull stops. The ball flies off in a straight line, not round and round.

If you cannot try it, this is what happens.
Seen from above. A hand in the middle of a ring. A string joins the hand to a ball at the top of the ring. The ball goes round the ring. From the ball an arrow points straight to the right and is labelled flies off. Labels: ball, string, hand, flies off.ballstringhandflies off

The Sun is like your hand, and gravity is like the pull of the string. Let the pull go, and a planet leaves its orbit in a straight line.

Gravity also holds moons. The Earth pulls on the Moon, and the Moon moves sideways, so the Moon goes round the Earth.

The Earth in the middle. A ring goes round it. The Moon sits on the ring at the right. An arrow from the Moon points to the Earth and is labelled gravity. Labels: Earth, Moon, gravity.EarthMoongravity
The Earth pulls the Moon inwards. The Moon moves sideways. So it goes round the Earth.

Now think about time. One trip round the Sun is a year. The Earth takes about 365 days, so our year has 365 days.

A planet that is nearer the Sun has a shorter way round, and it moves faster. A planet that is farther out has a longer way round, and it moves slower.

See it three ways

As a picture

The Sun at the top. Three rings go round it, drawn to scale. The nearest ring has Mercury, the next has Venus and the farthest has Earth, in bold. The labels say sizes not to scale. Labels: Sun, Mercury, Venus, Earth, sizes not to scale.MercuryVenusEarthSunsizes not to scale

In words

The nearer a planet is to the Sun, the shorter its orbit and the faster it moves. So Mercury takes the shortest time to go round. It takes only 88 days. The Earth takes 365 days.

In symbols

Mercury: 88 days. Venus: 225 days. Earth: 365 days.

A bar chart called days to go round the Sun. The bars are: Mercury 88, Venus 225, Earth 365, Mars 687. The scale goes from 0 to 700 in steps of 100. Labels: days to go round the Sun, Mercury, Venus, Earth, Mars, 0, 100, 200, 300, 400, 500, 600, 700.0100200300400500600700MercuryVenusEarthMarsdays to go round the Sun
The year of each rocky planet, in Earth days. The farther the planet, the longer the year.
A bar chart called Earth years to go round. The bars are: Jupiter 11.9, Saturn 29.5, Uranus 84, Neptune 165. The scale goes from 0 to 180 in steps of 30. Labels: Earth years to go round, Jupiter, Saturn, Uranus, Neptune, 0, 30, 60, 90, 120, 150, 180.0306090120150180JupiterSaturnUranusNeptuneEarth years to go round
The year of each giant, in Earth years. Neptune takes 165 Earth years to go round once.

There are two reasons. A far planet has a longer way round. It also moves more slowly, because the Sun pulls less far out. The Earth moves about 30 km a second. Neptune moves only about 5 km a second.

Worked example

Venus takes 225 days to go round the Sun. How many times does it go round in 900 Earth days?

  1. One trip takes 225 days.
  2. Two trips take 2 × 225 = 450 days.
  3. Four trips take 4 × 225 = 900 days.
  4. So 900 ÷ 225 = 4. Venus goes round 4 times.

Answer: 4 times

A wrong idea many people have

There is nothing to hold a planet, so it just floats where it is.

Why it feels right: Space looks empty, and there are no ropes or walls to see.

The test that breaks it: Swing the ball on its string. The ball goes round only while the string pulls. Let go, and it flies away. A planet stays on its path because something pulls it all the time.

What is true: Gravity is the invisible pull. It bends the path of each planet into an orbit round the Sun.

Key words

gravity
the pull that every body has on every other body.
orbit
the curved path of one body going round another.
year
the time a planet takes to go round the Sun once.

Back to your guess

The answer is B: The Sun pulls the Earth, and the Earth is moving sideways, so it keeps curving round the Sun.

The Sun pulls the Earth with gravity. The Earth is also moving sideways. The pull bends its path into a curve, so the Earth keeps going round the Sun.

Check yourself

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

  1. What keeps the Earth going round the Sun?

    Tap your answer.

    1. Not this one. There is almost no air in space, so nothing pushes the Earth along. The pull of gravity bends its path. Go to S3.12.3. Try another.

    2. Not this one. The spin gives day and night. It is not what holds the Earth in its orbit. Go to S2.9.2. Try another.

    3. Yes, that is right. The Sun's gravity pulls the Earth all the time. The Earth moves sideways, so its path curves into an orbit.

    4. Not this one. If nothing pulled on it, the Earth would fly off in a straight line. Go to S3.12.3. Try another.

    See the answer

    The pull of the Sun's gravity

    The Sun's gravity pulls the Earth all the time. The Earth moves sideways, so its path curves into an orbit.

    • If you chose A: There is almost no air in space, so nothing pushes the Earth along. The pull of gravity bends its path. Go to S3.12.3.
    • If you chose B: The spin gives day and night. It is not what holds the Earth in its orbit. Go to S2.9.2.
    • If you chose D: If nothing pulled on it, the Earth would fly off in a straight line. Go to S3.12.3.

  2. Suppose the Sun's pull on the Earth stopped suddenly. What would the Earth do?

    Tap your answer.

    1. Yes, that is right. The Earth is moving sideways. Without the pull, nothing bends its path, so it goes straight, like the ball you let go.

    2. Not this one. A thing that is moving keeps moving. It would not stop. The ball on the string did not stop when you let go. Go to S3.12.3. Try another.

    3. Not this one. Falling in needs a pull. With no pull, nothing draws the Earth towards the Sun. Go to S3.12.3. Try another.

    See the answer

    Fly off in a straight line

    The Earth is moving sideways. Without the pull, nothing bends its path, so it goes straight, like the ball you let go.

    • If you chose B: A thing that is moving keeps moving. It would not stop. The ball on the string did not stop when you let go. Go to S3.12.3.
    • If you chose C: Falling in needs a pull. With no pull, nothing draws the Earth towards the Sun. Go to S3.12.3.

  3. Which planet takes the longest time to go round the Sun?

    Tap your answer.

    1. Not this one. Mercury is nearest the Sun, so it has the shortest way round. Its year is only 88 days. Go to S3.12.3. Try another.

    2. Yes, that is right. Neptune is the farthest planet. It has the longest way round and moves slowly, so its year is the longest, 165 Earth years.

    3. Not this one. Jupiter is the biggest planet, but size does not decide the year. Its year is 11.9 Earth years. Neptune is farther, so its year is longer. Go to S3.12.3. Try another.

    See the answer

    Neptune

    Neptune is the farthest planet. It has the longest way round and moves slowly, so its year is the longest, 165 Earth years.

    • If you chose A: Mercury is nearest the Sun, so it has the shortest way round. Its year is only 88 days. Go to S3.12.3.
    • If you chose C: Jupiter is the biggest planet, but size does not decide the year. Its year is 11.9 Earth years. Neptune is farther, so its year is longer. Go to S3.12.3.

  4. Why does Neptune take so long to go round the Sun?

    Tap your answer.

    1. Not this one. Cold does not slow a planet. Its speed depends on the pull of the Sun, which is weaker far out. Go to S3.12.3. Try another.

    2. Not this one. Size does not decide the year. Jupiter is much bigger than Neptune, yet its year is shorter. Go to S3.12.3. Try another.

    3. Yes, that is right. Far from the Sun, the path is longer. The Sun's pull is weaker, so the planet moves slowly. Both make the year longer.

    See the answer

    It is far away, so it has a long way to go and moves slowly.

    Far from the Sun, the path is longer. The Sun's pull is weaker, so the planet moves slowly. Both make the year longer.

    • If you chose A: Cold does not slow a planet. Its speed depends on the pull of the Sun, which is weaker far out. Go to S3.12.3.
    • If you chose B: Size does not decide the year. Jupiter is much bigger than Neptune, yet its year is shorter. Go to S3.12.3.

  5. Mercury goes round the Sun once in 88 days. How many times does it go round in 264 Earth days?

    Tap your answer.

    1. Not this one. You took 88 away from 264. To find how many trips, divide the days by the days in one trip. Go to S3.12.3. Try another.

    2. Not this one. You added 88 and 264. To find how many trips fit, divide. Go to S3.12.3. Try another.

    3. Not this one. You multiplied the two numbers. That makes a huge number. Divide the days by the days in one trip. Go to S3.12.3. Try another.

    4. Yes, that is right. 264 ÷ 88 = 3. Mercury goes round 3 times.

    See the answer

    3 times

    264 ÷ 88 = 3. Mercury goes round 3 times.

    • If you chose A: You took 88 away from 264. To find how many trips, divide the days by the days in one trip. Go to S3.12.3.
    • If you chose B: You added 88 and 264. To find how many trips fit, divide. Go to S3.12.3.
    • If you chose C: You multiplied the two numbers. That makes a huge number. Divide the days by the days in one trip. Go to S3.12.3.

  6. From an earlier chapter

    Mei makes a string model where 1 m stands for 200 million km. The Earth is about 150 million km from the Sun. How far along the string is the Earth?

    Tap your answer.

    1. Yes, that is right. Model distance = real distance ÷ scale = 150 ÷ 200 = 0.75 m.

    2. Not this one. You divided by 20, not by 200. Check how many zeros the scale has. Go to S3.12.2. Try another.

    3. Not this one. You divided by 2 000. The scale is 200, so divide 150 by 200. Go to S3.12.2. Try another.

    4. Not this one. You multiplied. A model is smaller than the real thing, so divide the real distance by the scale. Go to S3.12.2. Try another.

    See the answer

    0.75 m

    Model distance = real distance ÷ scale = 150 ÷ 200 = 0.75 m.

    • If you chose B: You divided by 20, not by 200. Check how many zeros the scale has. Go to S3.12.2.
    • If you chose C: You divided by 2 000. The scale is 200, so divide 150 by 200. Go to S3.12.2.
    • If you chose D: You multiplied. A model is smaller than the real thing, so divide the real distance by the scale. Go to S3.12.2.

  7. What keeps the Moon going round the Earth?

    Tap your answer.

    1. Not this one. The Sun does pull on the Moon. But the Earth's gravity is what holds the Moon in orbit round the Earth. Go to S3.12.3. Try another.

    2. Yes, that is right. The Earth pulls on the Moon all the time. The Moon moves sideways, so its path curves into an orbit round the Earth.

    3. Not this one. There is no air out there. The Moon stays on its path because of the Earth's pull. Go to S3.12.3. Try another.

    See the answer

    The Earth's gravity pulls on it.

    The Earth pulls on the Moon all the time. The Moon moves sideways, so its path curves into an orbit round the Earth.

    • If you chose A: The Sun does pull on the Moon. But the Earth's gravity is what holds the Moon in orbit round the Earth. Go to S3.12.3.
    • If you chose C: There is no air out there. The Moon stays on its path because of the Earth's pull. Go to S3.12.3.

Keep it

Copy this into your exercise book.

Gravity pulls planets towards the Sun. A planet also moves sideways, so its path curves into an orbit. Planets far out go slower and have a longer year.

Draw: The Sun and a planet. Add an arrow of pull to the Sun, an arrow sideways, and a curved orbit.

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.