Glossary of Technical Terms

Absolute Magnitude see Magnitude

Absolute Zero approximately -273° on the centigrade scale, but zero on the Kelvin scale. To convert from centigrade to Kelvin add 273, so

Water freezes at 273K and boils at 373K

Adaptive Optics

Albedo   the ratio of the amount of solar radiation reflected from an object to the total amount incident upon it.

Angstrom A ten-billionth of a meter, i.e. 10-10 m. This unit, which departs from the normal metric 'conventions' come into use originally because of some astronomical work.

Angular Momentum Angular Momentum is conserved in the absence of any external applied forces. The usual example quoted by way of illustation is an ice skater who can spin faster or slower purely by altering the point of application of their own angular momentum. Likewise, as a planet follows its elliptical path, it will travel faster at times when it is closer to the Sun. This is more obvious if we were to consider comets.

Apogee (and Aphelion)


Astronomical Unit   the average distance from the Sun to the Earth - 149,597,870 km.

Black Body Radiation

Brown Dwarf

Doppler Shift   important effect on light red-shifting it if something is moving in our direction and blue shifting it if an obkect is moving away from us. This is usually demonstated by identical effects on sound. Fast moving objects like Formula 1 cars, ambulances etc. will noticeably change their sound when switching from moving towards us to moving away from us. Something like a whistle waved around your head on a rope will demonstrate the same effect.

Chandrasekhar Limit

Colatitude is defined as

90 - latitude


HII Region

Hydrogen Alpha (Hα)

Line in the middle of the red part of the visible spectrum. It is produced by a transition from Hydrogen's second excited state down to its first excited state. Emission will peak at about 10,000K. Above this, Hydrogen will tend to become ionised, thereby reducing electronic transitions from a mass of Hydrogen. At too low a temperature, little (if any) Hydrogen would be excited up to second ecited state. Hα filters are commonly used to view the Sun.



Meridian A great circle that passes thru both poles. A great circle is a line that divides the Earth into two equal halves. In astronomy, the observer's meridian will be the great circle in the sky which passes thru the zenith and the poles. This meridian will cut the horizon at the exact north and south points.

NGC - New General Catalog (of Nebulae)   based on the 7840 objects recorded by William and John Herschel. These were extended and published in 1888 by John Louis Emil Dreyer of Armagh Observatory.


Parsec If an object has a parallax of one second of arc (where 1 second = 1/3600 degree), it is at a distance of 1 parsec. One parsec is equal to 3.26 light years.

star's distance = 1/parallax (stated in seconds) <\center>

Proper Motion

Rotation The word 'rotation' is reserved is reserved for spin on a bodies own axis. The orbit about the Sun will be its 'revolution'.

Sidereal Period the rotational period with respect to the Stars. This period is not the one which is immediately detected by us, we tend to rely on the synodical period.

Solar Day

Spectroscopic Binary

Synodic Period   the period of a particular body, as perceived by us, in simple terms. For a more scientific definition : a synodic period is the amount of time between similar arrangements of the Earth, Sun and a third body.


Universal Time (UT) is effectively the same as Greenwich Mean Time (GMT).