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One day Isaac was reading a book under an 

apple tree on the farm. An apple fell out of 

the tree - bonk! Ow!

Now, for most people that would be the end 

of the story, but not for Isaac. Not for 

somebody who just couldn't stop asking 

why all the time.

Why did the apple fall out of the tree? Does 

everything fall? What makes things fall? Can 

anything stop things from falling? Are the 

sun, moon, and stars falling? Why don't they 

ever hit the ground?

So many questions. Newton spent many years 

answering these questions by thinking and doing 

experiments. He made up the law of gravity. 

According to this law everything pulled everything 

else to itself by a force called gravity. How strong 

that force is depends on how heavy the things are 

and how close together.

So even two apples pull toward each other. But, 

the force is so small that you need a very careful 

experiment to measure it. The reason that things 

fall toward the ground is that the earth we live on 

is so very heavy, and we are so close to it.

Newton's law of gravity not only explained how 

things fall on earth, but how planets move 

around the sun and how moons move around 

planets. A friend of his, Edmund Halley, decided 

to try Newton's theory on comets. People had 

been studying comets for hundreds of years 

without figuring them out, so he decided to 

study their reports and compare them to 

Newton's theories.

Up til then people had thought that comets just 

came and went, and that nobody could know 

when or why. But Newton's law of gravity gave 

rules that Halley could use to study the records 

of comets. He found some reports of a big 

bright comet that he was sure was the same 

one, coming back every 75 years. He predicted 

when it would come back next.

If anybody still didn't 

believe Newton, then the 

appearance of Halley's 

Comet just when Halley 

had predicted it using 

Newtons ideas was 

enough to convince them.

 Halley's comet has come a few 

times since then, always right 

on schedule. You'll be able to 

see it on its next trip near the 

sun and earth when you're old 

enough to be a grandparent

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