Glossary of space, Earth and navigation terms
Key words from the articles on this site, in alphabetical order. Where an article explains a term in more depth, the definition links to it. Figures are rounded.
Asteroid
A small rocky or metallic body that orbits the Sun but is too small to be a planet or dwarf planet. Most asteroids in our Solar System are found in the asteroid belt between Mars and Jupiter.
Astronomical unit (AU)
The average distance between the Earth and the Sun, about 150 million km. It is a handy unit for distances within the Solar System, as used in our planets table.
Aurora
Glowing curtains of light in the sky, usually seen near the poles, caused when charged particles from the Sun collide with gas particles high in the atmosphere. In the north it is called the aurora borealis (northern lights), and in the south the aurora australis.
Axis
The imaginary line through the North and South Poles around which the Earth spins once a day. The Earth's axis is tilted by about 23.4°, which is the cause of the seasons.
Bearing
A direction measured clockwise from north, in degrees, and written with three figures, so east is 090° and south-west is 225°. See how a compass works.
Black hole
A region of space where gravity is so strong that nothing, not even light, can escape. One can form when the core of a very massive star collapses at the end of its life.
Chronometer
A clock accurate enough to keep time at sea for weeks or months. Comparing a chronometer set to Greenwich time with local time lets a navigator work out longitude.
Circumnavigation
A voyage all the way around the world. The first known circumnavigation was completed in 1522 by the ship Victoria, under Juan Sebastián Elcano, on the Magellan–Elcano expedition.
Constellation
A recognised pattern of stars and the area of sky around it. The International Astronomical Union recognises 88 constellations, which together cover the whole sky.
Convection current
A slow circular movement in a liquid, gas or soft solid, caused by hotter material rising and cooler material sinking. Convection currents in the mantle help move the Earth's tectonic plates.
Core
The centre of the Earth, made mainly of iron and nickel. The outer core is liquid and its movement produces the Earth's magnetic field. The inner core is solid because of the enormous pressure.
Crescent
A phase of the Moon in which less than half of the side facing us is lit, so it looks like a thin curved sliver.
Crust
The thin, rocky outer layer of the Earth. Oceanic crust is thinner and denser than continental crust. See inside the Earth.
Declination (magnetic)
The angle between true north and magnetic north at a particular place, also called magnetic variation. It differs from place to place and changes slowly over time.
Dwarf planet
A body that orbits the Sun and is round, but has not cleared other objects from the region around its orbit. Pluto has been classed as a dwarf planet since 2006.
Eclipse
An event in which one body moves into the shadow of another, or blocks our view of it. In a solar eclipse the Moon passes between the Sun and the Earth; in a lunar eclipse the Earth's shadow falls on the Moon.
Equator
The line of latitude at 0°, halfway between the North and South Poles. It divides the Earth into the Northern and Southern Hemispheres.
Equinox
One of the two days each year, around 20 March and 22 September, when the Sun is directly overhead at the equator at noon and day and night are roughly equal in length everywhere.
Escape velocity
The speed something must reach to escape a planet's gravity without further push. For the Earth it is about 11.2 km per second. See how rockets work.
Galaxy
A huge collection of stars, gas and dust held together by gravity. Our Sun is one of hundreds of billions of stars in the Milky Way galaxy.
Gibbous
A phase of the Moon in which more than half, but not all, of the side facing us is lit.
Gravity
The force of attraction between any two objects with mass. It holds the Moon in orbit around the Earth and the planets in orbit around the Sun.
Hemisphere
Half of a sphere. The Earth can be split at the equator into Northern and Southern Hemispheres, or at the prime meridian and the 180° line into Eastern and Western Hemispheres.
Kármán line
A boundary often used to mark where space begins, 100 km above sea level. Some organisations use a lower figure of about 80 km. See the layers of the atmosphere.
Latitude
The angle north or south of the equator, from 0° at the equator to 90° at each pole. Lines of latitude run east to west and are also called parallels. See latitude and longitude.
Light year
The distance light travels in one year, about 9.5 million million km. It is a measure of distance, not of time.
Longitude
The angle east or west of the prime meridian, from 0° to 180°. Lines of longitude, called meridians, run from pole to pole.
Lunar module
The part of an Apollo spacecraft that landed on the Moon. On Apollo 11 the lunar module was called Eagle.
Magnetic north
The direction a compass needle points, towards the Earth's north magnetic pole. This pole is in the Arctic but is not at the geographic North Pole, and it moves over time.
Main sequence
The long, stable stage of a star's life, when it fuses hydrogen into helium in its core. The Sun has been a main sequence star for about 4.6 billion years. See the life cycle of stars.
Mantle
The thickest layer of the Earth, about 2,900 km deep, lying between the crust and the core. It is almost entirely solid rock, but it can flow very slowly over millions of years.
Meteor
The streak of light seen when a small piece of space rock or dust enters the atmosphere at high speed and burns up. The piece of rock itself, while still in space, is a meteoroid, and one that reaches the ground is a meteorite.
Microgravity
The condition in which people and objects appear weightless, as on the International Space Station. Gravity is still present; astronauts float because they and the station are falling around the Earth together.
Nebula
A cloud of gas and dust in space. Stars form when parts of a nebula collapse under their own gravity.
Nuclear fusion
The joining of small atomic nuclei, such as hydrogen, to make larger ones, such as helium, releasing large amounts of energy. Fusion in the core is what makes a star shine.
Orbit
The curved path one object takes around another because of gravity, such as the Earth around the Sun or a satellite around the Earth.
Ozone layer
A region of the stratosphere, mostly between about 15 and 35 km up, with a higher concentration of ozone gas. It absorbs much of the Sun's harmful ultraviolet light.
Phase (of the Moon)
The shape of the lit part of the Moon as seen from Earth. The phases repeat about every 29.5 days. See the phases of the Moon.
Planet
A body that orbits the Sun, is massive enough for its gravity to pull it into a round shape, and has cleared the area around its orbit. This definition was agreed by the International Astronomical Union in 2006.
Plate boundary
The place where two tectonic plates meet. Plates move apart at constructive boundaries, together at destructive boundaries, and past each other at conservative boundaries.
Polaris
The Pole Star, which lies almost directly above the Earth's North Pole, so it barely moves in the night sky. Its height above the horizon is roughly equal to the observer's latitude. See stargazing for beginners.
Prime meridian
The line of 0° longitude, passing through the Royal Observatory at Greenwich in London. It was chosen at an international conference in 1884.
Red giant
A large, cool, very bright star in a late stage of its life, after the hydrogen in its core runs low. The Sun is expected to become a red giant in roughly 5 billion years.
Resultant force
The single force that has the same effect as all the forces acting on an object together. A rocket lifts off only when its thrust is greater than its weight, giving an upward resultant force.
Rover
A wheeled robot vehicle that explores the surface of a planet or moon. Sojourner, which landed in 1997, was the first rover on Mars. See Mars rovers.
Satellite
Any object that orbits a planet. The Moon is a natural satellite. The first artificial satellite was Sputnik 1, launched by the Soviet Union in 1957.
Solstice
One of the two days each year, around 21 June and 21 December, when the Sun is directly overhead at one of the tropics at noon. In the Northern Hemisphere the June solstice is the longest day and the December solstice the shortest.
Spring and neap tides
Spring tides, with the largest difference between high and low water, happen around new and full Moon, when the Sun, Earth and Moon are in line. Neap tides, with the smallest difference, happen around the quarter Moons. Spring tides have nothing to do with the season.
Stage (rocket)
A section of a rocket with its own engines and fuel, dropped once its fuel is used up so the rest of the rocket carries less weight. The Saturn V used for Apollo had three stages.
Stratosphere
The layer of the atmosphere above the troposphere, from about 12 km to 50 km up. Temperature rises with height here because ozone absorbs ultraviolet light.
Subduction
The sinking of a denser oceanic plate beneath another plate at a destructive boundary. It produces deep ocean trenches, earthquakes and explosive volcanoes.
Supernova
The huge explosion of a massive star at the end of its life, which can briefly outshine a whole galaxy. It may leave behind a neutron star or a black hole.
Tectonic plate
One of the large, rigid pieces into which the Earth's crust and uppermost mantle are broken. The plates move a few centimetres a year.
Thrust
The forward or upward force produced by an engine. In a rocket, hot gas is pushed out downwards and the gas pushes the rocket upwards, as Newton's third law describes.
Troposphere
The lowest layer of the atmosphere, from the ground to about 12 km on average. Nearly all weather happens here, and temperature falls as you go up.
True north
The direction towards the geographic North Pole, the point where the Earth's axis meets its surface. Lines of longitude on a map point to true north.
Waxing and waning
A waxing Moon is growing, with more of it lit each night, from new Moon towards full Moon. A waning Moon is shrinking, from full Moon back towards new Moon.
Weight
The force of gravity acting on an object's mass, measured in newtons. Weight changes from planet to planet, but mass stays the same.
White dwarf
The small, hot, dense core left behind when a star like the Sun has shed its outer layers. It no longer carries out fusion and slowly cools over a very long time.