Glossary of Astronomical Terms - Astronoscope

Glossary of Common Astronomical Terms

Astronomy is littered with abbreviations and terms which you will find helpful to understand in taking your first steps into this fascinating pastime. Here are a few of the more common ones to get you started.

Absolute Magnitude

This term ‘levels the playing field’ for stars and other celestial objects. It is the brightness of a star or other celestial body as it would appear if viewed from a distance of exactly 10 parsecs (32.6 light years). By this measure the Sun has a magnitude 4.83. The exception to this is for Solar system planets that shine in reflected light. Their Absolute Magnitude assumes a viewing distance of 1AU.

Astronomical Unit (au)

One Astronomical Unit or AU is defined as the average distance between the Earth and the Sun, about 93 million miles (149 million KM) or about 8 light minutes. This form of measurement is often used to describe planetary distances from their local star. For instance, Jupiter orbits the Sun at a distance of 5AU, so it is 5 times further way from the Sun than the Earth is.


The farthest point in a planetary or other bodies orbit around its primary star. 

Apparent Magnitude

This is a measurement of brightness of a star or other celestial body as seen from Earth. The apparent magnitude will mainly depend on the object’s actual luminosity and distance from Earth. The most obvious celestial object in our sky is the Sun, whose Apparent Magnitude is -26.74. Magnitudes with a negative value are brighter than those with a positive value.

Black Hole

A gravitational mass so powerful that nothing can escape it, not even light. Most Galaxies, including our own are thought to have a super-massive Black Hole at its centre.

Celestial Grid

A system for locating objects in the sky, like latitude and longitude used on the earth’s surface. The celestial equivalent uses Declination (DEC) and Right Ascension (RA) to locate each stars position when observed from the Earth.


A comet is small solar system body made up of a collection of rock, ice and dust. They usually measure a few miles across but can sometimes be larger, particularly those from the Oort Cloud. They can have highly eccentric orbits and orbital periods of just a few years too many thousands of years. Some Comets return regularly, some like Comet Halley have longer periods, 76 years in the case of Halley, while others may only ever appear once.


A loose formation of stars that are organised into recognisable patterns. There are 88 official constellations. The stars themselves are not necessarily close to each other even though they may appear so from Earth. 


The Celestial equivalent of Latitude, usually abbreviated to DEC. A measurement in degrees and seconds of an objects position North or South of the Celestial equator.


A measure of how much the orbit of an object deviates from being circular.

Erecting Lens

Due to the optical path that light takes through the telescope the image that you see is always upside down. When you are viewing objects in space this is not a problem, but if you want to use your telescope to view terrestrial objects, you will need to use an Erecting Lens to invert the image the right way up.



A planetary body that exists outside our solar system. So far we have identified over 4,800 exoplanets in over 3,500 different star systems. Several of these are thought to be Earth like planets that may be able to support life. Below is a small selection of identified Exoplanets with Earth and Mars as size comparators.

Light Year (ly)

This is the distance travelled by light in a vacuum in one Earth year. The speed of light is 186,000 miles per second (297,600KM) or 5.9 trillion miles (9.46 trillion KM) in one year. Casual references to astronomical distances are usually expressed in light years. Even though the term ‘year’ is used, this is a measurement of distance, not time. 

Orbital Period

This is the amount of time an object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting a star, moons orbiting planets or binary stars orbiting each other.

Oort Cloud

A theoretical expanse of space ranging from 2000 to 200,000AU from the Sun. It is thought to be made up of millions of cometary bodies.


Another measurement of distance, not time, more commonly used by professional forums and publications. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond (1/60th of a degree). A Parsec equates to 3.26 light years.

Put simply, if two lines, whose angle is one arcsecond, are extended until the two ends are 1AU apart, the lines are 1 parsec in length.


The closest point in a planetary or other bodies orbit around its primary star.

Right Ascension

The celestial equivalent of Longitude, usually abbreviated to RA. This is a measurement of an objects position East of the First point of Aries, which is the Sun’s position when it crosses the Celestial Equator during the March Equinox. It is measured in hours, minutes and seconds and forms a 24-hour sphere around the Earth. Each 1 hour of RA is equivalent to 15 degrees.

There are lot more terms and abbreviations involved, but these should help you get going.