Heliocodex

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Gravity and getting about

One rule that covers a dropped ball, the Moon, the tides and every spacecraft ever flown.

12 lessons in 4 parts. Every one can be read again at a deeper level.

Begin with lesson 1

The International Space Station, with its long solar panels, above the blue and white Earth.
A photograph. The International Space Station, falling round the Earth once every 93 minutes. Credit: NASA.

Part 1

The idea

What gravity is, and why an orbit is a fall that never lands.

  1. 1
    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

  2. 2
    What an orbit is

    An orbit is a fall that never reaches the ground. Newton's cannonball shows how.

    With moving pictures and photographs

  3. 3
    Kepler's three laws

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

Part 2

What it does

Tides, moons kept in step, and places where pulls balance.

  1. 4
    Tides

    Why there are two high tides a day, why the Moon keeps one face to us, and what keeps Io erupting.

  2. 5
    Moons in step

    Some moons keep time with each other, lapping their neighbours in exact whole numbers. That rhythm heats Io, carves gaps in rings and makes two moons swap places.

  3. 6
    Balance points: where gravity holds things still

    Near any two orbiting bodies there are five places where a small object can keep station. Telescopes are parked at two of them, and asteroids gather at two more.

Part 3

Using it, and its limits

How spacecraft travel, and what happens when gravity itself makes waves.

  1. 7
    How a rocket works

    A rocket moves by throwing hot gas out of its back. Reaching orbit takes so much speed that nine tenths of a big rocket is fuel, and most of it is dropped on the way.

    With moving pictures and photographs

  2. 8
    Leaving and arriving: how spacecraft get about

    Getting into space is about speed, not height. After that a spacecraft mostly coasts, and borrows speed from planets on the way.

  3. 9
    Gravitational waves

    When very heavy things swing round each other, space itself ripples. Since 2015 those ripples can be caught, a way of studying the universe that uses no light.

Part 4

Einstein’s picture

Why time and space depend on how you move, and what that does to gravity.

  1. 10
    Space and time are not fixed

    Light always travels at the same speed, whoever measures it. To make that true, moving clocks have to run slow, and nothing can go faster than light.

    With moving pictures and photographs

  2. 11
    Gravity as bent space and time

    Einstein's second idea: mass bends space and time, and what we call gravity is things following that bend. It has passed every test for more than a century.

    With moving pictures and photographs

  3. 12
    Gravity as a lens

    A galaxy's gravity bends the light passing it, so it can stretch, copy and brighten whatever lies behind. Astronomers use it as a telescope.

    With moving pictures and photographs

Go and look