Astronomical Jargon



For all you who are a little confused by the language of astrophysics, here is a list of useful words. Please feel free to suggest additions - contact us!

Below is a quick introduction to some astronomical concepts and below that is basically a dictionary of terms.

Distance measurements

It is quite often for people to have lots of trouble when it comes to describing distances in the sky. The easiest way to do this is by measuring everything as an angular measure as the sky can not be expressed in a linear manner (e.g. metres). So in Astronomy the angular measure is taken and so an Astronomer might say that tow stars are 15 degrees apart, meaning if you draw a line from your eye to each star then the two lines would form an angle of 15 degrees at your eye.

Useful ways to measure angular distance are:

If you hold your arm out at full length and look past it with one eye then this covers approximately 10 degrees of the sky.

A fingertip at arms length is approximately 1 degree.

The Sun and the moon are approximately half a degree across.

The Plough is approximately 25 degrees across.

There is smaller divisions of angular distance. One degree is made up of 60 arc minutes (60') which is in turn made up of 60 arc seconds (60'').

We are going to update this page very soon and we have the dictionary all ready but we will have to change the structure of the page so for now our new Jargon can be found in pdf form here.

Dictionary of Terms

  • Absolute Magnitude - This is the apparent magnitude that an object would have if it was at a distance of 10 parsecs.
  • Albedo- This is a measure of how much a surface reflects the solar radiation that falls upon it.
  • Altitude - This is vertical angle from the ground up to an object in the sky.
  • Angular Diameter - This is the angle an object subtends in the sky.
  • Angular Resolution - A telescope can only resolve two objects if they have are a certain minimum angular distance apart. The angular resolution q of a telescope is given by the formula: q = 1.22l/D Where l is the wavelength of light and D is the diameter of the collecting area.
  • Aperture - Diameter of the objective end of the optics (eg for a refractor the primary lens, for a reflector the primary mirror etc).
  • Aphelion - Point in a body's orbit of the sun, when it is farthest to it.
  • Apparent magnitude - Logarithmic scale of measurement of the brightness of an object as viewed from Earth. The scale works in the reverse sense so lower numbers mean brighter objects an object that is 100 times brighter than another will have an apparent magnitude difference of 5 (eg an object of magnitude 1 is 100 brighter than an object of magnitude 6).
  • Arc minute - An angular measurement equal to 1/60 of a degree
  • Arc second - This is an angular measurement which is equivalent to 1/3600 of a degree.
  • Astronomical Unit - This is a distance measurement equal to the mean distance between the Earth of the Sun. This is equivalent to 150 million kilometers (93 million miles).
  • Azimuth - This is the direction 90 degrees vertically from the ground i.e. straight up.
  • Barlow lens - Negative lens that fits between the focuser and the eyepiece to increase the magnification by a stated factor.
  • Binary Star System - This is a star system in which two stars orbit a common centre of mass. In close binary systems mass transfer occurs between the two stars.
  • Blue shift - When a source of light moves towards the receiver, the light is blue shifted, i.e its wavelength is shifted towards the blue end of the spectrum. In astronomy this is important as galaxies in the local cluster can be moving towards us due to their peculiar velocity and by measuring the amount of blue shift you can measure how quickly it is moving towards us.
  • Cassegrain Focus - This is an arrangement of a telescope in which light is reflected from a primary mirror back along the telescope onto a secondary mirror which then reflects the light through a hole in the centre of the primary mirror to an optical device such as a camera or the observer.
  • Celestial Equator - Great circle that is 90 from the celestial pole.
  • Celestial Pole - The point about which the celestial sphere appears to rotate.
  • Celestial Sphere - An imaginary sphere around the Earth containing all of the objects in the sky.
  • Chromatic Aberration - In an optical setup using lenses the refractive index of the lens varies with the wavelength of light incident upon it. This means that red and blue light are refracted (bent) by different amounts producing a distorted image. This phenomena is known as chromatic aberration.
  • Conjunction - This is the event when a planet reaches it lowest elongation from the sun. For planets inside Earth's orbit, there are two: the inferior when the planet is on the nearside of the sun and the superior when the planet is on the farside.
  • Cosmological Redshift - The expansion of the Universe causes a red-shift in the light of other galaxies.
  • Culmination - The event when an object reaches its highest point in the sky during its daily motion. (Verb: to culminate)
  • Dark adaption - Process where the eyes adjust to dark conditions and become more sensitive. It can take half an hour for the eyes to fully dark adapt.
  • Dobsonian - A style of telescope comprising a Newtonian on a Dob mount, which uses a large mount on a swiveling base rather than a tripod to make for a remarkably sturdy yet lightweight mount capable of handling large light buckets. Astrosoc owns a Dobsonian telescope, details available here.
  • Elongation - Angular seperation between a planet and the sun.
  • Equatorial Coordinates - A coordinate system that uses coordinates similar to latitude and longitude to plot the position of celestial objects. The coordinates are measured in right ascension, equivalent to longitude, and declination which is equivalent to latitude.
  • Equatorial Mount - This is a telescope mounting that has one axis parallel to the rotation axis of the Earth so that the telescope can track the stars.
  • Equinox - This is the point where the Sun passes through the intersection of the ecliptic and celestial equator.
  • Finder - This is a small scope attached to a larger one that is used to find an object in the sky before it is observed through the main telescope. The finder scope has a much lower magnification than the main scope so that it views a larger patch of sky.
  • Focal length - Distance required for the objective to bring light to a focus.
  • Focal ratio - Ratio of the focal length of a telescope to its aperture.
  • Focuser - Assembly at the focus of the telescope's main object where the eyepiece fits. It involves a moveable port for the eyepiece so it can be brought into focus.
  • Greatest eastern elongation - For a planet inside Earth's orbit, it is point where it is the furthest angular seperation east of the sun.
  • Greatest western elongation - Same thing but when the planet is west of the sun.
  • Magnification - Factor by which the image is enlarged. For a telescope, it is the ratio of the focal length of the objective to the focal length of the eyepiece.
  • Meridian - Great circle on the celestial sphere that passes through the poles.
  • Messier - This is an object in the catalogue of 110 objects compiled by Charles Messier in the 19th century.
  • Moon - Body orbiting a planet, whatever that is.
  • Nadir - This is the point on the celestial sphere directly opposite to the zenith.
  • Newtonian telescope- This type of telescope has an open objective end with a concave mirror at the base of the tube, which reflects the light back up bringing it to a focus after it has been reflected out the side near the top by a planar diagonal mirror.
  • Opposition - This is the event when a planet is on the opposite side of the the celestial sphere to
  • Perihelion - Point in a body's orbit of the sun, when it is closest to it.
  • Planet - Smeg knows! Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune are called that. Pluto is called it with some contention.
  • Refractor - Telescope that uses an objective lens to bring light to a focus at the bottom of the tube.
  • Right Ascension - Equivalent of longitude on the celestial sphere. It is measured in 24 hours numbered eastwards from the meridian of the vernal equinox. The right ascension of an object is the sidereal time it will culminate.
  • Seeing - The stability of the atmosphere in terms of how it affects seeing through it. Good seeing is when the atmosphere is stable and the images are crisp. Bad seeing is when the atmosphere is turbulent and images are blurred and shimmer.
  • Sidereal time - This is time measured with respect to the stars. One sidereal day is 23 solar hours and 56 minutes. 00h occurs when the vernal equinox reaches its highest point in the sky.
  • Sidereal year - This is the year measured with respect to the stars. It is the time it takes for the sun to return to the same position with respect to the fixed stars.
  • Solar time - This is time measured with respect to the sun. 12h is when the sun culminates.
  • Tasco - Now-defunct manufacturer of low quality telescopes, often referred to as craposcopes.
  • Tropical year - This is the year measured with respect to the seasons. It is defined so that the vernal equinox is around March 21.
  • Zenith - This is the point on the celestial sphere directly above your head.


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Last updated: 04/11/2005

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