SCIENTIFIC VOCABULARY · UPPER PRIMARY AND LOWER SECONDARY
The Solar System glossary
What is an orbit, an astronomical unit or a dwarf planet? Here are 45 short definitions to explain the terms used in the atlas and in class.
A
Albedo
The fraction of incoming light reflected by a surface. Light-coloured surfaces generally have a higher albedo than dark ones.
Aphelion
The point in an orbit farthest from the Sun. Its opposite is perihelion.
Asteroid
A small, mainly rocky or metallic body orbiting the Sun. Many lie between Mars and Jupiter.
Astronomical unit (AU)
A unit of length equal to exactly 149,597,870.7 kilometres, close to the average distance between Earth and the Sun.
Atmosphere
A layer of gas held around a celestial body by gravity. Its composition, pressure and thickness vary greatly between worlds.
Aurora
A glow produced when charged particles from space interact with the upper atmosphere, guided by a magnetic field.
B
Barycentre
The shared centre of mass around which two or more bodies orbit. Earth and the Moon orbit their barycentre.
C
Comet
A small body rich in ice and dust. Near the Sun, its ice turns into gas and can form a coma and tails.
D
Dwarf planet
A body that orbits the Sun and is nearly round, but has not cleared its orbital neighbourhood and is not a satellite.
LEARN MORE →E
Eccentricity
A number describing how much an orbit differs from a perfect circle. A value near zero means a nearly circular orbit.
Ecliptic
The plane of Earth's orbit around the Sun, used as a reference when describing positions in the Solar System.
Ellipse
A closed curve resembling a stretched circle. Planetary orbits are ellipses with the Sun at one focus.
Equinox
The time of year when the Sun appears to cross the plane of Earth's equator. Day and night are then of roughly equal length.
Exoplanet
A planet orbiting a star other than the Sun.
G
Gas giant
A giant planet dominated by hydrogen and helium. Jupiter and Saturn belong to this group.
LEARN MORE →Gravitation
The attractive interaction between objects with mass. It keeps planets in orbit and gives massive celestial bodies a nearly round shape.
H
Habitable zone
The region around a star where temperatures could allow liquid water on a planet's surface, if it also has suitable atmospheric conditions.
Heliosphere
The enormous bubble formed by the solar wind and the Sun's magnetic field in the interstellar medium.
I
Ice giant
A giant planet whose interior contains a large proportion of materials such as water, ammonia and methane. Uranus and Neptune are ice giants.
LEARN MORE →Impact crater
A depression formed when an object strikes a surface at high speed. Craters preserve part of a world's geological history.
K
Kuiper Belt
A vast region beyond Neptune populated by icy bodies. Pluto is one of its best-known members.
LEARN MORE →L
Light-year
A unit of distance equal to how far light travels in a year: about 9.46 trillion kilometres. It is not a length of time.
M
Magnetosphere
The region around a celestial body where its magnetic field controls the movement of charged particles.
Main asteroid belt
The region between Mars and Jupiter containing many small bodies and the dwarf planet Ceres.
LEARN MORE →Meteor
The streak of light seen when a small body enters an atmosphere at high speed, commonly called a shooting star.
Meteorite
A fragment from space that passes through an atmosphere and reaches the ground without being completely destroyed.
Meteoroid
A small rocky or metallic fragment travelling through space before any entry into an atmosphere.
N
Natural satellite
A body that naturally orbits a planet, dwarf planet or another small body. The Moon is Earth's natural satellite.
LEARN MORE →Nuclear fusion
A reaction in which light nuclei combine and release energy. In the Sun's core, hydrogen mainly fuses to form helium.
LEARN MORE →O
Occultation
The temporary disappearance of a celestial body when another object passes directly in front of it from an observer's viewpoint. Some eclipses are occultations.
Orbit
The path a body follows around another under the influence of gravity. A planet follows an orbit around the Sun.
Orbital inclination
The angle between an orbital plane and a reference plane, usually the ecliptic for the Solar System.
Orbital revolution
The motion of a body around another. Earth's orbit around the Sun takes about 365.25 days.
P
Perihelion
The point in an orbit nearest the Sun. Its opposite is aphelion.
Planet
In the Solar System, a body that orbits the Sun, is nearly round and is gravitationally dominant in its orbital neighbourhood.
LEARN MORE →Plasma
A state of matter containing ionised atoms. The Sun is a sphere of plasma, not a solid surface.
R
Rocky planet
A planet made mainly of rock and metal, with a solid surface. Mercury, Venus, Earth and Mars are rocky planets.
LEARN MORE →Rotation
The motion of a body around its own axis. Earth's rotation produces the cycle of day and night.
S
Solar nebula
The cloud of gas and dust whose collapse formed the Sun and the disc in which the planets assembled, about 4.6 billion years ago.
Solar System
The Sun and all objects gravitationally bound to it: planets, moons, dwarf planets, asteroids, comets, dust and gas.
LEARN MORE →Solar wind
A continuous flow of charged particles and magnetic fields escaping from the Sun's outer atmosphere.
Solstice
The time when Earth's axial tilt produces the longest or shortest daylight period of the year in a hemisphere.
Star
A massive, luminous celestial body that produces energy through nuclear fusion. The Sun is the star of our Solar System.
LEARN MORE →T
Terminator
The moving boundary between the day and night sides of a celestial body.
Transit
The apparent passage of a smaller body in front of a larger one. From Earth, we can observe transits of Mercury or Venus across the Sun.
SOURCES AND REFERENCE DEFINITIONS