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How we found all this out

Nobody has been further than the Moon. This is the story of how the rest was worked out from here, and of what everything turned out to be made of.

11 lessons in 3 parts. Every one can be read again at a deeper level.

Begin with lesson 1

Seven pictures of the Moon in a row: full, then with a curved dark bite growing across it, then wholly dark red.
A photograph. The Earth’s curved shadow crossing the Moon on 14 March 2025: the oldest evidence that the Earth is a ball. Credit: NASA/Eric Bordelon.

Part 1

The Earth moves

Two thousand years of argument, settled by a telescope and a stubborn planet.

  1. 1
    Why the sky turns

    The stars wheel overhead every night because the ground under you is turning. One point in the sky stays still, and how high it sits tells you where you are.

    With moving pictures and photographs

  2. 2
    From a still Earth to a moving one

    For two thousand years almost everyone was sure the Earth stood still in the middle of everything. Five people, a telescope and one stubborn planet changed that.

    With moving pictures and photographs

  3. 3
    Kepler's three laws

    Three short rules that describe every orbit, found from years of watching Mars.

  4. 4
    What gravity is

    Everything pulls on everything else. The pull grows with mass and fades with distance, and it never quite reaches zero.

    With moving pictures and photographs

Part 2

How far, how big

From the Moon to the edge of what can be seen, one step at a time.

  1. 5
    Finding the size of everything

    Nobody can stretch a tape to the stars. The size of the universe was found one step at a time, over two thousand years, and every step made it far bigger.

    With moving pictures and photographs

  2. 6
    Measuring distance: kilometres, au, light years and parsecs

    Why astronomers keep four different rulers, and when each one is used.

  3. 7
    Variable stars and the distance ladder

    Some stars brighten and fade on a steady beat, and the beat gives away how bright they truly are. That is how the distances to other galaxies were first measured.

  4. 8
    How big is the universe?

    There is an edge to what can be seen, 46 billion light years away in every direction. It is not the edge of the universe, and nobody knows how far the rest goes.

    With moving pictures and photographs

Part 3

What it is all made of

Atoms, the four forces, and how light tells us what the stars are.

  1. 9
    Atoms, and the four forces

    Everything you can touch is built from three kinds of particle, and everything that ever happens to it is the work of just four forces. Stars and galaxies follow from that.

    With moving pictures and photographs

  2. 10
    Reading starlight

    Spread a star's light into a rainbow and it is crossed by dark lines. They tell you what the star is made of, how hot it is and how it is moving.

  3. 11
    Where the elements come from

    The first minutes of the universe made hydrogen and helium. Almost every other kind of atom, in the Earth and in you, was made by a star.

    With moving pictures and photographs

Still being written

This pack is not finished. These lessons are planned and will be added to it: The people who changed the picture: Leavitt, Payne, Bell Burnell, Rubin, Finding Neptune with a pencil.

Go and look