Astronomy Astronomy Terms Basic Astronomy Astronomy Definitions

Whether you are a beginning astronomer or just a person who would like to know what it means when the newspaper talks about an eclipse, a knowledge of the very basic, fundamental terms of astronomy can be very useful in life and is worth picking up. Here is an alphabetical list of the most common terms thrown around regarding the topic of astronomy.

Asteroid – a small rock body anywhere from 1 mile up to 500 miles long (about the size of Kansas) that orbits a star. Almost all of the asteroids found in our solar system are located in the Asteroid Belt, an area between Mars’s orbit and Jupiter’s orbit estimated to contain at least a million asteroids.

Astronomical Unit (A.U.) – a unit of measurement equal to the distance between the Sun and the Earth, 93 million miles. Since using ‘miles’ in space gets difficult when you are considering long distances, astronomers use their own unit. For example, the distance between the Sun and Neptune is 2.8 billion miles but it’s easier to just say that the distance is 30.1 AU (30.1 x 93 million), or about 30 times larger than the distance between the Sun and Earth.

Aurora Borealis – a beautiful display of colorful light mainly seen from regions near the north and south poles of the Earth, caused by particles from the Sun interacting with Earth’s magnetic field.

Binary Star – the collective name given to a system of two stars that orbit each other around a central point of gravity in between themselves.

Black Hole – a theoretical gravitational ‘vacuum’ that forms when a large star collapses onto itself (theoretical because scientists haven’t actually seen one but have detected it with instruments). When a very large star dies, its insides stop making nuclear energy and cause an imbalance – the pressure inside a star versus its gravity outside. This causes the gravity to crush the star until it is condensed to a tiny little dot (commonly called a singularity). Because of this, the gravity is so strong that it sucks in the surrounding material, even capturing light so that it cannot escape from it, hence the term ‘black’ hole.

Comet – a small body surrounded by dust and gas that orbits a star and forms a gaseous tail when it gets close to that star. (Note the difference between an asteroid and a comet – asteroids do NOT have gaseous tails!)

Constellation – an area of the night sky that is recognized by the shape its stars make when you pretend to connect lines between them, just like the borders separating states and countries. It’s just a way for astronomers to recognize which area of the sky they are looking in, like a map.

Dark Matter – a theoretical form of matter that scientists cannot see through electromagnetic radiation like light (hence the term ‘dark’ matter) but detect it from the gravitational pull it poses over other celestial objects.

Eclipse – when one celestial body’s shadow covers part of another celestial body, as seen from a planet. A lunar eclipse is when Earth is directly between the Sun and our Moon so that Earth casts a shadow on the Moon, blocking out its sunlight, giving the Moon a dark orange eerie glow. A solar eclipse is when the Moon is directly between the Sun and Earth so that when looked at from Earth, the Moon seems to entirely cover the Sun in the sky and temporarily blocks out its light. Eclipses are only seen from certain parts of Earth – you’ve got to be at the right place to see it at the right angle.

Galaxy – a large compilation of millions of stars (and gas and dust) that are held together by each other’s gravity and separated from other large compilations of stars by space. The group of stars in which our solar system resides in is called the Milky Way Galaxy.

Gas Giants – a collective term for the four outermost planets of our solar system which are made up almost entirely of gas, unlike the four innermost planets which are made up of rock. Jupiter, Saturn, Uranus, Neptune are generally called the Gas Giants (also the Jovian Planets) and Mercury, Venus, Earth and Mars are generally called the Rocky (or Terrestrial) Planets.

Gibbous – the phase of the Moon’s cycle when it is between a Half Moon and a Full Moon, so about 3/4 full.

Great Red Spot – a giant violent gas storm on Jupiter that slowly rotates as Jupiter itself rotates. It can fit about two Earths inside of it and has existed for as long as humans have been able to observe Jupiter.

Kuiper Belt – (pronounced KAI-PER) a collection of bodies past Neptune that orbit the Sun, containing lots of comets and other icy bodies (icy because they are so far away from our Sun). Much like the Asteroid Belt contains an extremely large number of rocky asteroids, the Kuiper Belt contains an extremely large number of icy comets.

Light Year – a unit of measurement for DISTANCE, not TIME. Light travels at a specific, finite speed. One light year is the distance that light travels through space in one year, about 6 trillion miles. For example, when someone says that a certain star is ‘four light years’ away, it means that it takes four years for the light from that star to reach Earth. So if that star exploded, we would still see it as a normal star in our night sky until four years later, when the light from the explosion finally reaches Earth.

Magnitude – a logarithmic system used to measure the brightness of objects we see in the night sky. The more the number of magnitude decreases (even into the negative), the brighter an object is. As seen from Earth, the Sun’s magnitude is -27 and that of the North Star (Polaris) is 2.

Meteor – a rock from space as it is falling through a planet’s atmosphere, leaving a streak of light.

Meteorite – what that piece of rock is called when it actually hits the ground.

Meteoroid – a very small rock or other piece of debris traveling through space, usually a remnant of a comet. That’s why when meteor showers happen on Earth, it is because a comet is passing nearby and Earth’s gravity sucks in the debris.

Moon – a natural small body that orbits a home planet. The Earth’s moon is generally capitalized (the Moon) to distinguish it from just the general term ‘moon.’ The moons of other planets have distinct names.

Nebula – a cloud of gas and dust in space that contains the gases that are needed to create stars. This is where new stars are born. When a certain area of a nebula gets enough gravity, it starts pulling in the surrounding gas and dust, swirling it together until it is strong enough to star a nuclear reaction and become the core of a star.

Radio Telescope – a type of telescope that detects radio waves being emitted from space.

Satellite – something that orbits a planet. The Earth has many satellites in orbit around it – one to keep track of weather, one that controls our television signals, etc. Moons can also be called ‘satellites.’

Solar Flare – an intense eruption of charged particles (or electromagnetic radiation) from the sun’s surface. It can reach all the way to Earth, disrupting telecommunication signals.

Star – an object in space that conducts nuclear fusion in its core, fusing hydrogen and helium (and heavier gases) to give itself energy to produce radiation, or light.

Supernova – when a large star comes to the end of its life, having used up all its hydrogen and helium fusion power, and explodes. The inside grows very powerful as the star quickly uses up the very last of its nuclear reaction energy, scrambling to cling on to dear life, until it can no longer produce any more and explodes, blasting its material into all directions in space.

Terrestrial Planets – a collective term for the four innermost planets of our solar system, made up of mostly rock and metals. The terrestrial planets are Mercury, Venus, Earth and Mars.

Waning – when the moon goes from a Full Moon to a a New Moon (when you can’t see the Moon), appearing to be DEcreasing in how much of its surface area is illuminated.

Waxing – when the moon goes from New Moon to Full Moon, appearing to be INcreasing in how much of its surface area is illuminated.

Zodiac – a path in the night sky that astronomers use to describe where the planets pass. As seen from Earth’s night sky, all the planets in our solar system shift from east to west in the same path in our sky, a curved line called the Zodiac.