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GAIA

Astronomy guide

First steps to find your way around the sky and get to know space: no instruments needed, starting tonight.

10 chapters, about 47 minutes of reading in all

Start with chapter one

  1. Chapter 1: Finding your way in the sky

    Cardinal points, height above the horizon and how to measure the sky with your hand.

    5 min read

  2. Chapter 2: Finding Polaris

    Every observer's first exercise: from the Big Dipper to Polaris.

    4 min read

  3. Chapter 3: The moving sky

    Why stars rise and set, and why constellations change with the seasons.

    5 min read

  4. Chapter 4: Getting to know the Moon

    Its phases, when to observe it and why the full Moon is not the best time.

    5 min read

  5. Chapter 5: Planet or star?

    How to recognise Venus, Jupiter, Mars and Saturn with the naked eye.

    4 min read

  6. Chapter 6: Your first constellations

    Which ones to learn first, season by season, from the northern hemisphere.

    6 min read

  7. Chapter 7: Darkness and light pollution

    Letting your eyes adapt to the dark and choosing the right place and night.

    4 min read

  8. Chapter 8: Naked eye, binoculars, telescope

    What to start with and what to look for when buying your first instrument.

    6 min read

  9. Chapter 9: Meteors, satellites and what to report

    Recognising what moves across the sky and sending a good report.

    5 min read

  10. Chapter 10: Observing safely

    The Sun, eclipses and the rules that are not negotiable.

    3 min read

Glossary

The words you will meet most often, explained in one line.

Magnitude
A measure of apparent brightness: the lower the number, the brighter the object. Sirius is −1.5; from a dark sky the naked eye reaches about +6.
Zenith
The point in the sky straight above your head.
Azimuth
Direction along the horizon, measured in degrees from north through east (east = 90°, south = 180°).
Ecliptic
The path the Sun follows across the sky during the year; the Moon and planets stay close to it.
Astronomical unit (AU)
The mean Earth–Sun distance: about 149.6 million km.
Light-year
The distance light travels in a year: about 9.46 trillion km. It is a distance, not a time.
Opposition
When an outer planet is on the opposite side of Earth from the Sun: it is up all night and at its brightest.
Conjunction
When two objects appear close together in the sky as seen from Earth; in space they can be very far apart.
Elongation
A planet's angular distance from the Sun. For Mercury and Venus, greatest elongation is the best time to observe them.
Radiant
The point in the sky from which a shower's meteors appear to come; it names the shower (Perseids from Perseus).
ZHR
Zenithal Hourly Rate: the theoretical number of meteors per hour under a perfect sky with the radiant overhead. In practice you see fewer.
Terminator
The line separating the lit and dark parts of a planet or the Moon.