Last Updated:   02/04/17


Ever wonder what the difference is between a Meteor, Meteorite, or Meteoroid? You've come to the right place.

- Astronomical Terminology and Definitions

- Telescope Terminology and Definitions

- Types of Telescopes


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Astronomical Definitions



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.


The natural glow of the night sky due to reactions that take place in the Earths 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.


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


The point in an objects orbit around the Sun when it is furthest from the Sun.


The point in an objects orbit around the earth when it is furthest from the Earth.


A meteorite which is stony.


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. Approximately 93,000,000 miles.


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 Earths 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 a 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 stars surface is.


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


A small, frozen mass of dust and gas revolving around the sun. Dust, ice and other space materials will get into the tail of the comet which can be especially long. When the comet moves from the sun when in orbit the matter on the tail will be thrown off course into space where it can disintegrate. In many cases the earth will pass through clouds of this material during its natural orbit. A meteor shower will result here.


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 Earths 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 objects gravity.


The outermost part of the Earth's atmosphere.


(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.

HI region

Cloud of neutral hydrogen.

HII region

Cloud of ionized hydrogen.

Hertzspurng-Russell Diagram

A diagram which helps scientists understand different kinds of stars.

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.


Region of the Earths atmosphere.

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 objects magnetic field can be felt.


How much matter an object contains, it is not the same as weight, although an objects 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. Shooting stars are essentially meteoroids.


An extremely small object. They are so small that when they hit the Earths 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 an 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.


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 shadows edge.


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


The point in an objects 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.


An extremely bright and distant active galactic nucleus.


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.

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.


Level of the Earths atmosphere from about 11-64 kilometers above sea level.


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


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


Ancient instrument used to tell time.

Sun spots

Dark patches on the Sun's surface.

Superior planets

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

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.


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 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.




Telescope Definitions




Defines the size of the telescopes opening, larger apertures allow more light into the telescope and is one of the most critical choices when choosing a telescope. Larger apertures are more complex and expensive, as well as make the telescope less portable.


The application of photography in astronomy. Images can build up for long periods on a photographic emulsion or charge-coupled device (CCD), revealing stars and other objects invisible to the eye, and are recorded permanently, allowing their positions and brightness to be measured accurately.

Averted Vision

At night, the periphery of the eye's retina is more sensitive to light than the center. Looking slightly to one side of a faint object (averting your vision), so that the light falls on the more sensitive outer part of the retina, usually reveals faint details otherwise lost when looking directly at the object.


Any telescope that folds the light path and directs it through a hole in the center of the primary mirror (called the Cassegrain focus) at the bottom of the telescope.


The alignment of the optical elements of a telescope at the correct angles to the light path. If not properly collimated, a telescope will deliver distorted images.


An optical defect in reflector telescopes in which in-focus star images appear progressively more triangular or comet-like the closer the get to the edge of the field of view. The faster the focal ratio, the more prominent the coma.


The angular distance of an object north or south of the celestial equator, measured in degrees. One of the two coordinates (right ascension is the other) that let you find celestial objects with the aid of telescope setting circles.

Equatorial Mount

A telescope mount designed for astronomical use. It aligns the axis of rotation of a telescope with the axis of the Earth, allowing the scope to follow the seemingly curved paths taken by the stars and planets.


Also known as an ocular, where you view the light collected by the telescope.


Low powered telescope mounted coaxially to the main telescope with a sorter focal length, allowing for more sky to be seen at one time for getting an initial track on the object you want to observe with the main telescope.

Focal length

The length of the effective optical path of a telescope or eyepiece ( the distance from the main mirror or lens where the light is gathered to the point where the prime focus image is formed). Typically expressed in millimeters.

Focal ratio

The "speed" of a telescope's optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera. Fast f/4 to f/5 focal ratios are generally best for wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either. An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio - so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.


Capability to place the telescope under computer control to find an object.


An optional part of a go-to system to help find alignment stars, while helpful, it is not critical to using a go-to system.

Lens coating

Lens treatments to reduce reflection, increase transmission, improves the limiting magnitude.

Limiting magnitude

Lighting conditions (including ambient light) that effect how faint objects can be seen.


The ability of a telescope to make a small, distant object large enough to examine in detail. If you look at the Moon (250,000 miles away) with a 125x scope, it's the same as looking at it with your bare eyes from 2000 miles away (250,000 / 125 = 2000). The same telescope used terrestrially will make an object one mile away appear to be only 42 feet away (5280 / 125 = 42).


The main light-gathering lens or mirror of a telescope.


The ability of a telescope to separate closely-spaced binary stars into two objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25 cent coin at a distance of three miles. In essence, a measure of how much detail a telescope can reveal.

Right Ascension

Typically, the angular distance of a celestial object east of the vernal equinox, measured in hours and minutes. Simply stated, one of the two coordinates (declination is the other) that let you find celestial objects by using a telescopes setting circles and a star chart or star atlas. If you face the north celestial pole, the stars will rise (ascend) on your right - hence the term "right ascension". The same point on the 360 degree celestial sphere passes overhead approximately every 24 hours, making each hour of right ascension equal to 1/24th of a circle, or 15 degrees. Each degree of sky therefore moves past a stationary telescope in four minutes - a rapid rate when observing at high power.

Setting Circles

Circular scales on an equatorial mount telescope that are used to point it at the position (in right ascension and declination) of a celestial object. Setting circles and a star chart make it possible to find objects even when they are too faint to see through the finder scope.


A measure of how dark the sky is on a given night. Transparency is affected by the amount of humidity and dust in the atmosphere, as well as by the amount of light pollution present. The four stars in the bowl of the Little Dipper are magnitudes 2.2, 3.1, 4.3, and 5.0. If all four can be seen without using averted vision (after your eyes have had ten minutes or so to become dark adapted), and you can clearly see the faint outline of the Milky Way, the transparency would be rated 5 and your observing site is probably dark enough to let you use a 10" scope without being overly affected by light pollution. If you have to use averted vision to see the fourth star, you may be limited to an 8" scope. If only three of the Little Dipper stars can be seen consistently (the faintest being magnitude 4.3), the transparency would be rated 4, and light pollution will probably limit you to a 6" scope. A transparency of 4 is only fair for deep sky observing - 5 is much more satisfactory with an 8" scope or larger.




Types Of Telescopes



Catadioptric Telescope

A telescope that uses a combination of mirrors and lenses to increase the effective focal length of the telescope while allowing it to be folded into a more convenient and compact size. The use of a correcting lens in these telescopes virtually eliminates spherical aberration, chromatic aberration, and coma. The word catadioptric is derived by combining the term for an optical system that forms images by using mirrors, (catoptric) with the one of a system that uses lenses (dioptric). The most popular catadioptric designs are the Schmidt-Cassegrain and Maksutov-Cassegrain.

Dobsonian Telescope

A conventional Newtonian reflector optical tube on an inexpensive plywood or fiberboard altazimuth mount. Nylon or Teflon bearings allow smooth telescope motion at a fingers touch, with no vibration or unsteadiness. The scope is moved by hand from object to object (there are no manual slow motion controls or motor drives) using a technique called star-hopping to locate objects. Usually it's a large aperture, fast focal ratio scope deigned for visual deep space observing - although 6" and 8" medium f/ratio (f/6 and f/8) Dobsonians are also suitable for planetary observing. Cannot be used for astrophotography. The Dobsonian is an economical way to get into large aperture astronomy at a fraction of the cost of an equatorially mounted telescope of similar aperture.


A catadioptric telescope that uses a thick and deeply-dished spherical corrector lens to correct for the spherical aberration of its spherical primary mirror - an all- spherical design that keeps its collimation virtually indefinitely. Its typically long focal ratio and small secondary obstruction yield higher contrast and resolution than any other catadioptric or reflector design.

Newtonian Reflector

This classic 300 year old Sir Isaac Newton design uses a large primary mirror at the bottom of the telescope tube, with a flat diagonal mirror at the top that brings the light out to the Newtonian focus at the side of the tube. Totally color-free, for excellent planetary observing. Offers more light-gathering aperture per dollar than any other telescope design, as well, for very good deep space performance.


Reflecting telescopes gather light with a mirror, reflecting it before directing it to the eyepiece.  Reflectors typically give you a wider aperture for the dollar.  They require more care and maintenance, however.

The simplest type of reflecting telescope is called a "Newtonian," after Sir Isaac Newton who invented them.  When a Newtonian telescope in it's simplest and most user-friendly form, it's called a Dobsonian telescope after John Dobson


A telescope that uses a lens to bring light to a focus at the end of a long tube.

Rich Field Telescope (RFT)

A fast focal ratio reflector that gives wide-angle views of star clouds, nebulas, large galaxies, etc. Most large Dobsonians are rich field telescopes.

Schmidt-Cassegrain Telescope (SCT)

A catadioptric telescope that uses a thin aspheric corrector lens to compensate for the spherical aberration of its primary mirror.  An 8-inch SCT like those sold by Celestron, Meade, and Orion have a focal length of 80 inches packed into a 12-pound tube less than 18-inches long.



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