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Space · The Sun and its family

Gravity and orbits

Falling and always missing the target

Only spacecraft can see it

Gravity and orbits
The harder the stone is thrown, the farther it lands: at a certain speed it never touches the ground.

How it happens, step by step

  1. Let it drop

    A stone you let go falls straight down at your feet. Gravity always pulls everything toward the center of the Earth.

  2. Throw it sideways

    If you throw it, it still falls, but meanwhile it has moved sideways. It lands farther from where you stand.

  3. Throw it harder

    The harder you throw it, the farther away it lands. And the Earth, being round, starts to curve away beneath it.

  4. The ground slips away

    At a certain speed the ground drops as fast as the stone comes down. It never reaches the ground again.

  5. That is an orbit

    The stone keeps falling forever without touching anything. It is exactly what the Moon and the satellites are doing.

To get an idea

An orbit is a slightly squashed circle, and the Sun is not in the middle but off to one side.

A planet races when it is close to the Sun and slows down when it is far away: it does not always go at the same speed.

Fun facts

These are not in the book: there was no room on paper.

  1. Before Kepler, the motions of the planets were explained with circles inside other circles, called epicycles.

  2. That system worked, but to stay accurate it needed more and more circles and became clumsy.

  3. Newton showed that Kepler's laws all follow from a single law of gravitation.

  4. Einstein later explained that gravity is really space itself curving around heavy masses.

  5. To send a probe to a planet, Newton's math is still used, because it is much simpler.

  6. There is a fixed relation between how far a planet is from the Sun and how long it takes to go around it.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

Why doesn't the Moon fall onto the Earth?

Question 2 of 5

What shape are the orbits of the planets?

Question 3 of 5

Where is the Sun in a planet's orbit?

Question 4 of 5

Does a planet always travel at the same speed?

Question 5 of 5

What is gravity according to Einstein?

Riddles

I have been falling for billions of years and I have never landed anywhere, because the floor slips away beneath me at my own speed. What am I doing?

I am not a circle, even though for centuries everyone insisted I had to be. I am a little squashed, and the star in charge of me is not at my center. What am I?

When I get closer I race, when I move away I slow down, and nobody ever pushes me: I do it all by myself because something invisible holds my hand. What do I do?

Try it yourself

Activity 1

Throw a ball harder and harder in the yard and mark where it lands each time. The spot moves away: keep imagining until it comes back from behind.

Activity 2

Draw an ellipse with two thumbtacks, a string and a pencil. Then put a marble on one thumbtack: that is the Sun, and it is not in the center.

The words

ellipse
A squashed circle, with two points inside called foci.
free fall
The motion of a body acted on only by gravity, with nothing holding it up.

Sources: Wikidata (Q4130), Wikipedia (Orbita).