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Below is a list of useful Astronomy words. These big words were created by scientists to explain what happens in Outer Space. It is useful to know these words, but you don’t have to memorize them all in one big chunk. That would ruin all the fun. Rather, learn these Astronomy words one by one as you explore and learn about our Universe. If you see a word you don’t know, come here and look it up, and hopefully it will stay in your memory.

Absolute magnitude

How bright a star would look if it were 32.6 light years away from the Earth.

Absolute zero

The lowest possible temperature, -273.16 degrees C.


Change in velocity (speed, or direction).


A meteorite which is stoney.


The natural glow of the night sky due to reactions that take place in the Earth’s upper atmosphere.


The albedo of an object is how much light it reflects. A perfect reflector such as a mirror would have an albedo of 100, the moon has an albedo of 7, and the Earth has an albedo of 36.

Angstrom unit

Unit used to measure the wavelength of light, and other electromagnetic radiation.


Shaped like or forming a ring.


When two stars that orbit each other are as far away from each other as they can get.


The point in an object’s orbit around the Sun when it is furthest from the Sun.


The point in an object’s orbit around the earth when it is furthest from the Earth.


A rock, or Minor Planet orbiting the Sun.


A belief that links the positions of the stars and planets to human destinies. It has no scientific background.

Astronomical Unit

The distance from the Earth to the Sun. Usually written AU.


The use of physics and chemistry in the study of Astronomy.


The gaseous area surrounding a planet or other body.


The smallest particle of any element.


Beautiful lights seen over the polar regions which are caused when energized particles from the Sun react with the Earth’s magnetic field.


An imaginary straight line on which an object rotates.

Background radiation

Weak microwave radiation coming from space in all directions. It is believed to be the remnant of the Big Bang.


The center of gravity of the Earth and moon.

Binary star

A star which is actually made up of two stars orbiting each other.

Black Hole

A region of space around a very small and extremely massive object within which the gravitational field is so strong that not even light can escape.


A brilliant meteor, which may explode during its descent through the Earth’s atmosphere.


A sensitive radiation detector.

Celestial sphere

An imaginary sphere surrounding the Earth. It is used to help astronomers explain where objects are found in the sky.


A variable star that scientists can use to determine how distant a galaxy or star cluster is.

Charge-Coupled Device (CCD)

A sensitive imaging device which is replacing photography in most branches of Astronomy.


Part of the Sun’s atmosphere, it is visible during a total solar eclipse.

Circumpolar star

A star which never sets, but can be viewed year round.


A group or stars or galaxies which are held together by their common gravity.

Color index

A measure of a star’s color, which tells scientists how hot the star’s surface is.


The hazy-looking patch surrounding the nucleus of a comet.


A small, frozen mass of dust and gas revolving around the sun.


When a planet appears to come close to another planet or star. It only appears to come close because it moves in between the other object and the Earth.


A grouping of stars which have been given names by ancient astronomers because of the way they look.


The outermost part of the Sun’s atmosphere.


A type of telescope designed to view the Sun’s Corona.

Cosmic rays

High-speed particles that reach the Earth from Outer Space.


The study of the universe.


Amount of time it takes the Earth to spin once on its axis.


The compactness of matter.

Direct Motion

Objects moving around the Sun in the same direction as the Earth are moving in direct motion, objects moving in the opposite direction are moving in retrograde motion.

Diurnal Motion

The apparent motion of the sky from East to West caused by the Earth moving from West to East.


The faint glow of the moon when the side facing Earth is dark. Caused by light reflecting off the Earth.


When our view of one object in the sky is blocked by either another object or the Earth’s shadow.


The path the Sun, Moon, and planets all follow in the sky.


The area around a star where it is just the right temperature for life to exist.


Negative particle which orbits an atom.


Substance which cannot be broken down any further. There are 92 known elements.


March 21st and September 22nd. Twice a year when the day and night are the same amount of time all around the world.

Escape Velocity

The speed an object must have in order to escape from another object’s gravity.


The outermost part of the Earth’s atmosphere.

Flares (Solar Flares)

Beautiful eruptions in the outer part of the Sun’s atmosphere.


A group of stars, gas and dust held together by gravity.

Gamma Ray

Extremely short-wavelength and energetic electromagnetic radiation.


Simply means the Earth in the Center. People used to believe the Universe was geocentric, or that the Earth was in the center of the Universe.


Study of the Earth using Physics.


When the Moon is more than half full, but less than completely full.

Hertzsprung-Russell Diagram

A diagram which helps scientists understand different kinds of stars.

HI Region

Cloud of neutral hydrogen.

HII Region

Cloud of ionized hydrogen.

Hubble Constant

The relationship between the distance of an object and the speed at which it is traveling away from us. The further away an object is the faster away from us it is traveling.

Inferior Planets

Mercury and Venus which lie closer to the Sun than the Earth are called inferior planets.


A region of the Earth’s atmosphere.


A measurement of temperature often used in astronomy. 0 degrees Kelvin equals -273 degrees Celsius and -459.4 degrees Fahrenheit.

Kepler’s Laws of Planetary Motion

1. The planets move in elliptical orbits with the Sun at one focus. 2. An imaginary line joining the center of a planet to the center of the Sun sweeps the same amount of space all the time. 3. The time it takes a planet to orbit the Sun is related to how far away from the Sun an object is.

Kirkwood Gaps

Regions in the asteroid belt where almost no asteroids can be found. This is due to the fact that the giant planet Jupiter changes the orbits of any object which enters these areas.

Light Year

The distance which a ray of light would travel in one year. This is about 6,000,000,000,000 (6 trillion) miles.


The edge of any object in Outer Space. The edge of the Moon for example.

Local Group

A group of around two dozen galaxies. It is the group to which our galaxy belongs.


Period between new moons. 29 days 12 hours 44 minutes.


Region around an object where the influence of the object’s magnetic field can be felt.


How much matter an object contains. It is not the same as weight, although an object’s mass does help determine how much it will weigh.


A shooting star, observed when a particle of dust enters into the Earth’s atmosphere.


An object from Outer Space, such as a rock, that falls into the Earth and lands on its surface.


Any small object in Outer Space, such as dust or a rock.


An extremely small object. They are so small that when they hit the Earth’s atmosphere they do not create a shooting star effect.

Milky Way

Our Galaxy. (The word “Galaxy” actually means milky way in Greek).

Minor Planet



A group of atoms linked together.

Multiple Star

A group of stars that orbit each other.


That point on the celestial sphere directly below the observer.


A cloud of gas and dust.


A very small particle with no mass or charge.

Neutron Star

The remnants of a dead star. They are incredibly compact and spin very quickly, some spin 100 times a second.


A star which suddenly flares up to many times its original brightness before fading again.

Oblate Spheroid

A planet that is not perfectly round because it is wider in the middle and shorter from top to bottom.


The covering up of one celestial body by another.


When a planet is exactly opposite the Sun so that the Earth is between them.


The path one object takes around another.


An area in the Earth’s upper atmosphere which absorbs many of the lethal radiations coming from space.


The shift of an object when it is viewed from two different places. For example if you close one eye and look at your thumb nail, and then switch eyes, you will see everything in the background move back and forth. Scientists use this to measure the distance to stars.


3.26 light years


The lighter part of a shadow found on the shadow’s edge.


When two stars that orbit each other are at their closest point.


The point in an object’s orbit around the Earth when it is closest to the Earth.


When an object which revolves around the Sun is at the closest point it gets to the Sun.


The disturbances in the orbit of a celestial object caused by the gravitational pull of another object.


The apparent change in the shape of the Moon, Mercury, and Venus due to how much of the sunlit side is facing the Earth.


The bright surface of the Sun.


An object moving around a star.

Planetary Nebula

A nebula of gas surrounding a star.


The Earth behaves like a spinning top. Its poles are spinning in circles causing the poles to point in different directions over time. It takes 25,800 years for the Earth to complete one precession.

Proper Motion

The motion of the stars across the sky as seen from Earth. Closer stars have a higher proper motion than more distant ones, just as in your car closer objects such as road signs seem to move faster than distant mountains and trees.


The center of an atom is made up of one or more protons. It has a positive charge.


A very distant, immensely bright object.


The area in the sky where during a meteor shower the meteors appear to radiate from.

Radio Galaxies

Galaxies which are extremely powerful emitters of radio radiation.

Red Shift

When an object is traveling away from the Earth, the light from this object is stretched out, making it look redder.


When something is moving in a circle around another object, such as the way the Moon Circles the Earth, it is said to revolve around that object.


When an object spins it is said to be rotating.

Saros Cycle

A period of 18 years 11.3 days in which eclipses repeat themselves.


A small object orbiting a larger one. There are many electronic objects that orbit the Earth.


Twinkling of stars. Due to the Earth’s atmosphere.


The condition of the Earth’s atmosphere at a particular time. If the sky is clear astronomers say there is good seeing.


The study of the Moon’s surface.

Seyfert Galaxies

Galaxies with small bright centers. Many Seyfert galaxies are good sources of radio waves.

Shooting Star

A light in the atmosphere caused by a meteor falling towards the Earth.

Sidereal Period

The period of time that it takes an object in space to complete one full orbit in relation to the stars.

Solar System

The system of planets and other objects orbiting the star Sol, which happens to be our Sun.

Solar Wind

A steady flow of particles streaming out from the Sun in all directions.


22 June, and 22 December. Time of the year when the day is either shortest, or longest depending on where you are.


Jets up to 16,000 kilometers in diameter in the Sun’s atmosphere.


A self-luminous object that shines through the release of energy produced by nuclear reactions at its core.


Level of the Earth’s atmosphere from about 11-64 kilometers (7-11 miles) above sea level.

Sun Spots

Dark patches on the Sun’s surface.


Ancient instrument used to tell time.

Superior Planets

The planets which lie further from the Sun than the Earth.


A super bright explosion of a star. A supernova can produce the same amount of energy in one second as an entire galaxy.

Synchronous Satellite

An artificial satellite which moves around the Earth at the same speed that the Earth rotates, so that it is always above the same part of Earth.

Synodic Period

The time it takes an object in space to reappear at the same point in relation to two other objects, e.g., the Earth and Sun.


The position of the Moon in its orbit when at new or full phase.


The line between day and night on any celestial object.


An instrument used for measuring very small quantities of heat.

Time Dilation

The idea that as you approach the speed of light time slows down and mass increases.

Trojans Asteroids

Asteroids that circle the Sun following Jupiter’s orbit.


The lowest part of the Earth’s atmosphere.


The dark inner part of a sunspot, or shadow.

Variable Stars

Stars which fluctuate in brightness.


Point directly above your head in the night sky.