Here you will find the definitions of a lot of the terms that you will hear about when you study the planets and space in general.


A rocky body in space that orbits the Sun. Asteroids can be up to 1 km in diameter or a lot smaller. Many appear in the main asteroid belt.


The atmosphere typically surrounds a planet but it can surround another large body in space. It is the gaseous mantle of a planet or other body.

Big Bang

The big bang is the event that is believed to have triggered the creation of the universe.

Black hole

A black hole is a massive region in space that is so dense that no light or radiation can escape from it.


A comet is a small icy object that orbits the Sun in a very long eccentric orbit.


A constellation is a group of stars that are named after a mythological character or object. Examples include Gemini, Orion, Sagittarius, etc.


The corona is the outermost part of the Sun’s atmosphere. This area can be seen during solar eclipses.


Cosmology is the study of the universe as a whole. It includes all individual disciplines.


A day is the total time that it takes a planet to complete one full spin on its axis.


This is a term that you will often hear when studying orbital paths. An eccentric orbit occurs when a planet, moon or object does not orbit in a perfect circle.


The equator is an imaginary line that is drawn down a planet where there is an equal distance at all point on the line between the north and south poles of the planet.


A collection of stars dust and gas that are held together by gravity. Different galaxies can be different shapes such as elliptical, spiral or irregular.

Gas giant

The gas giants refer collectively to the four outermost planets in our solar system. These are Jupiter, Saturn, Uranus and Neptune.


Gravitation is the force of attraction between all particles of matter in the known universe.

Hubble Space Telescope

The Hubble Space Telescope is a large telescope that we launched into space so that we can view objects many light years away from our own planet.

Inferior planet

An inferior planet is one that is closer to the Sun from the position of the observer. So from the Earth, Mercury and Venus are called inferior planets.


This is the total brightness of an object.

Lunar eclipse

A lunar eclipse occurs when the Earth lies directly between the Sun and the Moon.


Mass is the amount of matter that is contained within a body.


A meteor is a small piece of rock that travels through space which when it enters a planet’s atmosphere it burns up.


A meteorite is a meteor that is typically larger in size because a meteorite does not burn up in the atmosphere of a planet, it will actually hit the surface of the planet.

Milky Way

This is the galaxy that we are a part of which also contains billions of other objects including our own planet.


Our own natural satellite is called the Moon.


An orbit is the path that one object takes around another object in space.


This is the closest a celestial body such as a planet gets to the Sun during its orbit.


A planet is a body that is in orbit around the Sun or another star.

Reflecting telescope

This is a type of telescope that works by a mirror collecting light and magnifying it.

Refracting telescope

This is similar to a reflecting telescope but a lens is used which provides a stronger magnification.

Satellite or Moon

A moon (sometimes called a satellite) is a body in the orbit of a larger parent body. Typically a moon will orbit a planet.

Solar eclipse

A solar eclipse occurs when the Moon passes directly between the Sun and the Earth and a shadow is cast on the Earth.

Solar system

The solar system is the group of objects that contains the Sun and any other object that is subject to the gravitational pull of the Sun. This can include planets, moons, asteroids, dust, gas, etc.


A star is a massive luminous ball of plasma that produced light and energy from its core through a series of nuclear reactions.


The Sun is the star which the Earth and the planets within our solar system orbit around.


These are darker, cooler patches that appear on the surface of the Sun and appear in 11 year cycles.

Superior planet

A superior planet is a planet that is further away from the Sun than the Earth.

Terrestrial planets

The terrestrial planets are the four planets that are closest to the Sun. These are Mercury, Venus, Earth and Mars.


The universe is considered as everything that has existed, currently exists and will exist in the future.

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