Dictionary of Astronomy

If you have a word you are unsure of and want it added to this dictionary please ask.



Defect in an image formed by a lens or mirror, e.g. coma

Aberration of light

Shift in the apparent position of stars due to the finite speed of light

Absolute Magnitude

A measure of the true brightness of an object assuming a distance from the observer of 10 parsecs, also see Magnitude


A refracting telescope with a lens (usually a doublet) that tries to correct for chromatic aberrations.


A refracting telescope with a lens (usually a triplet) that tries to correct for chromatic aberrations, corrects better than an Achromat, also known as an ‘APO’.

Apparent Magnitude

A measure of the brightness of an object as seen from the Earth, also see Magnitude


Abbreviation for Altitude


A coordinate used alongside azimuth in the ALT/AZ coordinate system, to mark the position of an object in the sky, the Altitude or ALT as it is also known, is an angular measure of how high an object is in the sky at your location. The ALT/Az system is a local coordinate system and the positions of objects in the sky change with time and location.


The diameter of the primary optical element in a telescope


Abbreviation for Azimuth


A coordinate used alongside altitude in the ALT/AZ coordinate system, to mark the position of an object in the sky, the Azimuth or AZ as it is also known, is an angular measure from North (or sometimes south) eastwards to an object in the sky at your location. The ALT/Az system is a local coordinate system and the positions of objects in the sky change with time and location.


Baade’s Window

A gap between “curtains” of dust clouds which lets us see close to the galactiic center of the Milkyway and see stars on the far side of galactic bulge.

Blazhko Effect

The periodic variation of RR Lyrae type stars isn’t as stable as you might think, the period and amplitude varies over a cycle of about 40 ish days around the extablished period for the star in question, the cause of the effect is not known but there are two popular hypothesis one to do with the resonance of the stars interior and the other to do with the magnetic field of the star when the axis of the magnetic field is out of line with it’s rotational axis. The effect is named after Sergey Blazhko who discovered it.

Blue Shift

Light from an object traveling towards you is ‘compressed’ this pushes the spectral lines towards the blue end of the spectrum.



A telescope that uses mirrors and lenses to focus the light

Chromatic Aberration

This is an aberration in an optical system that results in light from different wavelengths being focused at slightly different points, so it is impossible for all the light to be in focus at the same time, typically this results in a blue or purple haze around bright objects and a loss of contrast.


The lining up of the optical elements in a telescope along with the tube and focuser, essential to get the best out of any telescope, refractors are usually collimated in the factory and don’t need nightly adjustment


An optical aberration that makes stars have little tails directed away from the centre of the field of view


A line of sight phenomena when two or more solar system bodies are visible in the same area of sky.



Abbreviation for Declination


A coordinate used alongside Right Ascension in the equatorial coordinate system, to mark the position of an object in the sky, the declination or DEC as it is usually known is an angular measure similar to that of Latitude, however whereas latitude starts at the Earth’ equator, DEC starts at the celestial Equator.


Secondary mirror used to divert the light from the primary out to the side of a Newtonian telescope


The exact point when the Moon (or Mercury or Venus) are at quarter phase (50% illuminated as seen from the Earth)

Drake Equation

An equation that predicts the number of observable alien civilisations in the Milkyway.

Doublet Lens

A lens made from two separate lenses, usually to combat chromatic aberration.



The orbital plane of the Earth around the Sun, or you can think of it as the apparent path of the Sun in the sky over a year.

Extrinsic Variables

Stars that vary due to external influences, for example binary stars eclipsing each other.



The bright regions on the Sun’s photosphere observed in white light

Fermi Paradox

The contradiction between the estimates of the probability of the existence of alien civilizations see Drake equation – and the lack of evidence for these.

Field of View

The area of sky can see, to see the full Moon you need a Field of View of half a degree


Dark areas on the solar disk when viewed in hydrogen alpha, occurring in the upper chromosphere, if they extend beyond the solar limb they become bright areas called solar prominences.


A coloured glass used to block specific wavelengths from the sensor (or eye) of the telescope.
V (Visual) Filters are used to approximate the visual view when using a CCD and are a common filter for photometry ensuring different observers have comparable observations.


Fast Low-Ionization Emission Regions. Emission regions moving outwards at supersonic speeds. Their formation is not easily explained by any models of stellar evolution.

Focal Length

The distance the focus is away from the primary objective

Focal ratio

Simply the focal length divided by the aperture of a telescope, usual expressed as FN, where N is the number of times larger the focal length is compared to the aperture. When ‘N’ is small this is termed ‘Fast’ when N is large this is termed’ Slow’



German equatorial mount, a type of telescope mount.



Intrinsic Variables

Stars that vary due to the internal nature of the star, for example a star that is pulsating.



The point in an orbit where you become nervous about having possibly made the wrong decision. Well it made me laugh.




Leptogenesis is the term used to describe the (hypothetical) physical processes that resulted in the universe being made up of matter rather than an even mix of matter and anti matter, ‘Lepto’ referring to particles called leptons, most famous of which is the electron.

Light Year

The distance light can travel in one year, equal to about 10 trillion kilometres, the nearest star (after the Sun) is about 4.2 light years



The ‘brightness’ scale used in Astronomy, the lower the figure the brighter, it is a logarithmic scale, so a difference of 1 magnitude is about 2.5 times, 5 magnitudes difference is 100 times thus a difference of 8 magnitudes would be (5+1+1+1) (100*2.5*2.5*2.5= 1560 times brighter/fainter)


An imaginary line in the sky passing north south

Meteor Train

A meteor train appears as a faint nebulous streak of light left behind, along the track of a meteor, but AFTER the meteor itself has extinguished, about a quarter of meteors leave a momentary train and trains lasting over a second are quite rare .

Meteor Trail

The moving streak of light a meteor



A type of reflecting telescope



Axial tilt of a body, for the planets and the Sun in relation to its orbital plane.


The main mirror or lens in an optical system

Observed – Calculated (O-C)

This is generally a plot of an observed measurement minus the calculated measurement, by doing this you plot the residuals and can quickly identify any changes, used commonly for the measurement of eclipsing binary stars.


The passing of one object behind another, for example an occultation of Saturn by the Moon, the object behind is said to be occulted, see also transit.


Optical Tube Assembly, a telescope is made from a mounting system and an optical tube assembly.



A measure of distance, based on the parallax of the object using the Earth as a base line, 1 parsec is equal to 3.2615 light years.


The measurement of an objects intrinsic brightness


Bright area in solar chromosphere seen in Hydrogen alpha


Generally referring to the main ‘light gathering’ optical element in a telescope




The point in the sky where meteor showers appear to originate, this is not to say that a meteor will start at this position, but if you plotted the observed meteors from a specific shower and extended their trail backwards they would all converge onto the radiant

Red Shift

Light from distant galaxies is stretched as space between the galaxy and you expands this results in the spectral lines appearing to be shifted towards the red end of the spectrum


A telescope with a mirror that focuses the light


A telescope with a lens that focuses the light


Measured in arc seconds, this is the finest detail you can see, or image, it is generally limited by aperture and atmospheric conditions.

Right Ascension

A coordinate used alongside declination in the equatorial coordinate system, to mark the position of an object in the sky, the right ascension or RA as it is usually known is an angular measure similar to that of longitude, however whereas longitude starts at Greenwich England, RA starts at the position of the vernal equinox. Rather than using degrees, as longitude does, it uses hours, minutes and seconds, where 1 hour = 15 degrees.


Abbreviation for Right Ascension


A telescope with a lens that focuses the light


Schröter’s Effect

This is the effect where by the theoretical and observed times for Venus to be at dichotomy differ by a day or two.


Generally referring to a mirror in a telescope used to change the direction and or point of focus in a telescope


Stands for Sudden Ionospheric Disturbance, a change in the ionosphere due to energetic matter from the Sun interacting with it. see our SID page



The passing of one object in front of another, for example the transit of Venus in front of the Sun, the object in front is said to transit the other object, see also occultation


A lens made of 3 components used to make Achromatic Refractors



Abbreviation for Universal Time

Universal Time

Closely related to Greenwich Mean Time (GMT) UT is measure of time used for all civil time keeping, for us astronomers in Crayford we just need to remember to remove an hour if we are in British summer time



Very Low Frequency, name given to the 3-30kHz range of the electromagnetic spectrum, VLF radio signals are used to indirectly monitor solar activity see more here






Point in the sky directly overhead.

Zenithal Hourly Rate

The expected number of meteors that would be observed if the radiant is at the Observers Zenith.


Abbreviation for Zenithal Hourly Rate