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Glossary

The most important terms from astronomy and space travel, explained in simple words.

Albedo: How much light a surface reflects: 0 = black as coal, 1 = white as fresh snow. Enceladus reflects almost everything, our Moon only about 12%.
Asteroid: A small, rocky or metallic body that orbits the Sun – most of them in the Asteroid Belt between Mars and Jupiter.
Astronomical unit (AU): The average distance between Earth and the Sun: around 150 million km. Handy for giving distances in the Solar System – Jupiter, for example, is 5.2 AU from the Sun.
Axial tilt: How far a planet's axis of rotation is tilted. Earth's 23.4° are the reason for the seasons.
Comet: A “dirty snowball” of ice and dust. Near the Sun, ice turns to vapour and forms a glowing coma and tails.
Corona: The outermost, extremely hot layer of the Sun's atmosphere (over 1 million °C). Visible to the naked eye only during a total solar eclipse.
Cryovolcanism: Volcanism with ice instead of rock: instead of lava, water, ammonia or methane comes out – suspected on Pluto and Ceres, for example.
Dwarf planet: Round and orbiting the Sun like a planet, but it has not cleared its orbit. Examples: Pluto, Ceres, Eris.
Eccentricity: A measure of how much an orbit differs from a circle: 0 = perfect circle, just under 1 = very elongated ellipse (as with comets).
Ecliptic: The plane in which Earth travels around the Sun. Almost all the planets orbit roughly in this plane – which is why the Solar System looks like a flat disc.
Exosphere: An “atmosphere” so thin that the gas particles hardly ever collide – as on Mercury, the Moon or many icy moons.
Gas giant: A giant planet made of hydrogen and helium with no solid surface: Jupiter and Saturn.
Heliosphere: The “bubble” that the solar wind blows around the Solar System. Voyager 1 and 2 have already left it.
Ice giant: A large planet made mainly of “ice” in the astronomers' sense – water, ammonia, methane: Uranus and Neptune.
Kepler's laws: Around 1610 Johannes Kepler found that planets move on ellipses, faster when near the Sun, and that the further away they are, the longer one orbit takes. This app calculates in exactly this way.
Kuiper Belt: A zone of icy bodies beyond Neptune (30-50 AU), home to Pluto and many comets.
Light-year: The distance light travels in one year: around 9.5 trillion km. So a light-year is a distance, not a time.
Magnetic field: An invisible force field created by flowing metal in the interior. It protects against the solar wind and steers it towards the poles – where auroras glow.
Meteor and meteorite: A meteor (shooting star) is the streak of light from a grain of dust burning up in the atmosphere. If a chunk reaches the ground, it is called a meteorite.
Nuclear fusion: The fusing of light atomic nuclei into heavier ones. In the Sun, hydrogen nuclei are fused into helium – this is where all its energy comes from.
Oort Cloud: A gigantic, spherical cloud of trillions of icy bodies far out in space – reaching more than a light-year from the Sun. Long-period comets come from there.
Orbital period: The time a body takes to travel once all the way around the Sun (or around its planet) – one year for Earth.
Perihelion and aphelion: Perihelion is the point of an orbit closest to the Sun, aphelion the point furthest from it. Earth is at perihelion in January – so winter has nothing to do with distance.
Planet: Since 2006 the rule has been: a planet orbits the Sun, is round because of its own gravity and has “cleared” its orbit of other bodies.
Roche limit: If a moon comes too close to its planet, tidal forces tear it apart. That is why rings, not moons, are found inside this limit.
Shepherd moon: A small moon whose gravity holds ring particles together or clears a gap – like a shepherd with a flock.
Solar wind: A constant stream of charged particles that the Sun blows into space at 400-800 km/s. It creates auroras and comet tails.
Swing-by (gravity slingshot): A probe flies close past a planet and lets the planet's gravity speed it up and change its course – like a free turbo boost.
Tholins: Reddish-brown organic substances that form when UV light alters methane and nitrogen. They colour Pluto, Titan and many Kuiper Belt objects.
Tidal force: Gravity pulls more strongly on the near side of a body than on the far side – this stretches it slightly. On Earth this causes the tides, and on Io it heats the volcanoes.
Tidal locking: A moon turns exactly once on its own axis while it travels once around its planet – so it always shows the same face. Our Moon does this.