Heliocodex

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The sky above you

For anyone who wants to go outside and understand what they are looking at: why the sky changes, what the patterns are, and which of the lights are which.

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

Begin with lesson 1

A great glowing cloud of pink, orange and grey gas with a bright hollow near the middle.
A photograph. The Orion Nebula, which you can see by eye as the fuzzy middle "star" of Orion’s sword. Credit: NASA, ESA, M. Robberto (STScI/ESA) and the Hubble Space Telescope Orion Treasury Project Team, CC BY 4.0.

Part 1

Getting your bearings

How to look, how far away things are, and the units used to say so.

  1. 1
    Looking up for yourself

    You need no telescope to begin. Find somewhere dark, give your eyes half an hour, and know what to look for.

    With moving pictures and photographs

  2. 2
    How big is space

    The true spacing of the Solar System and the stars, on a scale you can walk.

    With moving pictures and photographs

  3. 3
    Measuring distance: kilometres, au, light years and parsecs

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

Part 2

What changes, and why

The night, the year, the month, and the shadows that cause eclipses.

  1. 4
    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. 5
    The sky through the year

    Winter and summer have different stars because the Earth's night side looks out a different way each season. The Sun's path among them is the zodiac.

    With moving pictures and photographs

  3. 6
    Seasons

    Why it is the tilt of the Earth, and not its distance from the Sun, that makes summer and winter.

  4. 7
    Phases and eclipses

    Why the Moon changes shape every month, and why eclipses are much rarer than that.

  5. 8
    Tides

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

Part 3

The stars overhead

The patterns, the colours, and what the eye can and cannot see.

  1. 9
    Constellations

    A constellation is a pattern of stars as seen from the Earth, and one of 88 areas the sky is divided into. Its stars are rarely together in space.

  2. 10
    The stars you can see

    A few thousand stars are bright enough to see by eye. All belong to our galaxy, most lie within 1,000 light years, and the patterns they make are only patterns.

  3. 11
    Colour and brightness

    A star's colour tells you how hot it is, and its true brightness tells you how big. Plot one against the other and the stars fall into order.

  4. 12
    Star clusters

    Stars are born in groups. Some groups drift apart within the galaxy's disc, and some are tight balls of very old stars that have held together for most of the age of the universe.

Go and look