home > article sections > astronomy articles
email this page to a friend

Basic Astronomical Terminology

Binary star

A binary star is a double star system having orbital revolution components that cause the twin stars (so called because they usually form from the same interstellar cloud) to orbit each other around a shared center of mass due to the 'mutual gravity' of the binary system.

Aphelion & Perihelion

Aphelion / Perihelion is an object's orbital point (in distance and time) around a star where the object's distance (on its elliptical orbit) from its parent star is farthest / closest. The terms apogee & perigee are used instead when referring to objects orbiting the Earth (e.g. the Moon); apoapsis & periapsis refers to orbits around all other bodies.

Astronomical Unit

This is slightly less than the mean distance from the Earth to the Sun, approx. 149,597,870.691km or about 93 million miles. The semi-major axis between the Earth and Sun is greater than one AU because one astronomical unit is the measure of an unperturbed circular orbit. If you were travelling at 160 kilometres/hour speed it would take at least a century to cover a distance of one AU.


A planet further out from the Sun than Earth is in opposition at 180 degrees from the Sun, directly on the opposite side of the Earth. A planet in opposition is at its closest to, and at its best visibility from, Earth.


Precession is a circular motion about a body's axis of rotation. Like a giant gyroscope, the Earth has an axis that passes through its poles, precessing once every 27,700 years. This slow yet uniform rotation about the Earth's axis constantly changes coordinates on sky maps. The rotational axis never points in the same direction due to lunisolar and planetary precessions, causing the celestial equinoxes to drift westward.

Wane & Wax

A waning moon refers to the time between the full and new phases; a waxing moon refers to an increase in the illuminated lunar surface area.

Division of the Sky into Constellations

The sky (including both the northern and southern hemispheres) is divided into 88 constellations. A constellation is a grouping of stars usually resembling a mythical figure from Greek or Arabic folklore. Many constellations are easily recognisable in the sky, e.g. Orion. These constellations act as guideposts to the heavens enabling astronomers, professional and amateur alike, to find their way around the night sky. So the constellations serve a practical function, rather than just being superficial patterns.

Every region of the sky is designated to a particular constellation, this facilitates the current, though rather old and peculiar, method of naming stars. In astronomy the stars are named according to their brightness (magnitude) and assigned Greek letters. For instance, the brightest star in the constellation of Cygnus - the Swan is given the name Alpha Cygni, the second brightest Beta Cygni and so on, until one runs out of Greek letters that is, then it's on to roman numerals. But this is not a hard and fast rule. Sometimes the stars were named simply in order from one direction across the constellation to the other. Also stars have changed brightness, therefore you will not always find the brightest star in a constellation named Alpha.

Many of the stars also have common names as well, e.g. Alpha Cygni is better known as Deneb. A few of the more famous constellations are: Ursa Major - the Bear / the Big Dipper, Orion - the Hunter, Cassiopeia - the W shape, and the Southern Cross.

Celestial co-ordinate system

From the Earth, the constellations seem to be struck on the inside of a hollow sphere known as the celestial sphere. This sphere appears to rotate around the Earth in an east-west direction every 24 hours. A grid of lines known as right ascension and declination help astronomers locate stars on the celestial sphere, and star maps are a projection of the imaginary sphere onto a flat surface.

Right Ascension (R.A.)

R.A. is one of the co-ordinates used to locate positions on the celestial sphere. Lines run from the North to the South celestial pole and are similar to the Earth's lines of longitude, except that they are measured in units of time.

Declination (DEC.)

DEC. is one of the co-ordinates used to locate positions on the celestial sphere and it is measured in degrees. Lines run from East to West and are linked to the Earth's lines of latitude: stars of 0° Dec., for example, lie in the same plane as the Earth's equator.

Apparent Magnitude

This is a measure of apparent brightness, which is the visible-light brightness of a celestial object observed from Earth, depending on both the distance of the object and its actual or true brightness.

Absolute Magnitude

is the magnitude (visible-light brightness) that a celestial object would have if it were observed at a standard distance of 32.6 light-years (10 parsecs). Absolute magnitude differs from apparent magnitude, which is a measure of how bright an object looks to an observer on the Earth.

Astronomers here on Earth use apparent magnitude. The brighter a star is the lower its magnitude, i.e. a star with a magnitude of 1.2 is brighter than one with mag. 3.0, and a star mag. -0.7 is brighter than one of mag. -0.1. During the 18th century the ratio between magnitudes was fixed at 2.5 (2.5118865 to be exact). This means that a star of a given magnitude is 2.5 times brighter than a star one magnitude dimmer.

To learn more about the terminology associated with telescopes please take a look at our telescope jargon guide.

back to the top   |   email this page to a friend

Author: Astronomy Today Staff

Related Articles

Moon's Influence
Meteor Showers
Near Earth Asteroids
Northern Lights
Observing Tips
Radio Astronomy
Space Art
Telescope FAQs

Other Sections

Astronomy articles
Solar System Guide
Space Exploration
Cosmology articles
Book Reviews


Night Sky Guide 2013
Buying a Telescope
Historical Eclipses
Meet Astronomers
The Constellations


Read blog posts
Our newsletter
Meet the Team