Sun, Moon, and Stars: Astronomy for Beginners · Part I, Ch. III, part 1 · Part 1: Part I: Our family in the sky, and its neighbors
3. What binds the family together: gravity
Goal: Your child can explain that gravity pulls the planets toward the Sun, and that their forward speed keeps them from falling in, so they travel in orbits.
You'll need
- The book, open to Part I, Chapter III
- A soft ball or a sock tied in a knot, on a string about two feet long
- An apple or a small ball to drop
- A plastic cup half full of water and a towel
What's in the book
The first half of the chapter explains attraction, or gravity: why an apple falls, why heavy things are heavy, and how Newton saw the same pull holding the planets to the Sun. A ball whirled on a string and a glass of water carried round a corner show why the Earth does not fall into the Sun.
The lesson, step by step
Hook question
SayThe Sun pulls hard on the Earth. So why doesn't the Earth fall into the Sun?
Let your child guess. Don't correct yet.
Read together
Read the first part of the chapter, up to the sentence about the earth's speed letting her wander away for ever. The book asks:
What causes an apple to fall to the ground when it drops from the branch?
Drop the apple. Say: The Earth pulls it down. That pull is called gravity.
Try the book's string experiment
Go outside or to a clear space. Swing the soft ball on its string in a circle beside you (not over anyone's head). Feel it pull outward.
SayThe string is like the Sun's pull. The ball is like the Earth, always trying to fly off in a straight line.
Let go of the string once, away from people and windows, and watch the ball fly off straight.
The water and the corner
Over a towel, walk quickly with the half-full cup and turn sharply. Some water keeps going straight and slops out, just as in the book.
Read the book's rule together:
everything which is once set moving must continue moving in a straight line until checked.
Put it together
SayThe Sun pulls the Earth inward. The Earth's speed carries it forward. The two together make it go round and round. That path is called an orbit.
The book says the Earth goes more than eighteen miles every second.
What we know now
The book says two people floating far out in space would be pulled together and unable to stay apart. The pull between two people is real but extremely tiny; they would drift together only very slowly.
Everything else here is still how scientists explain orbits. Newton's ideas about gravity still guide spacecraft today.
Tell it back
Ask your child to explain, using the ball on the string, why the Earth goes round the Sun instead of falling in or flying away.
Check questions
Who first showed that the same pull that drops an apple also holds the planets?
What would happen if the Earth suddenly went much faster?
What would happen to life on Earth without the Sun?
Afterwards
Roll a marble around the inside of a round cake pan or pie plate. The rim keeps it turning, like the string. Lift the rim away at one point (or use a pan with a gap) and watch the marble roll off straight.
Tip
Keep the swinging ball soft and the string short, and swing it beside the body, not overhead.
Answers to the book's problems
Isaac Newton. She would slowly move farther from the Sun, and at about 25 miles a second she would escape it for ever. Without the Sun, plants would wither, water would freeze and animals and people would die. Model answer: the Sun's pull is like the string, the Earth's speed is like the ball's pull outward; balanced together, the Earth keeps circling.
Worked out for this site from the scan. If a number in your copy differs, trust the book.