The Binocular Sky


Glossary of Astronomical Terms

This glossary is intended to grow as time permits. If you have any requests for inclusion, suggestions for improvement or clarity of explanation, or corrections, please . In the mean time, you may find what you need in the tutorials on my other web site, where many of the terms below are expanded upon in greater detail and context.

Where a term is a combination of words, it will normally be indexed under the first word of the term. For example, Apparent Magnitude is indexed as Apparent Magnitude, not Magnitude, Apparent.



Aberration: An optical effect which degrades an image.

Absolute Magnitude: The apparent magnitude that an object would possess it if was placed at a distance of 10 parsecs from the observer. In this way, absolute magnitude provides a direct comparison of the brightness of stars.

Achromatic: Literally "no colour". A lens combination in which chromatic aberration is corrected by bringing two colours to the same focus.

Altitude: The angle of a body above or below the plane of the horizon; negative altitudes are below the horizon.

Albedo: The proportion of incident light which a body reflects in all directions. The albedo of Earth is 0.36, that of the Moon is 0.07 and that of Uranus is 0.93. The true albedo may vary over the surface of the object so, for practical purposes, the mean albedo is used.

Anomaly: The angle at the Sun between a planet and its perihelion.

Aperture: The unobstructed diameter of the objective lens.

Aphelion: The position in a heliocentric orbit at which the orbiting object is at its greatest distance from the Sun.

Apochromatic: A lens combination in which chromatic aberration is corrected by bringing three colours to the same focus. Some manufacturers use the term to describe achromatic doublets whose false colour is approximately equivalent to that of an apochromatic triplet lens.

Apogee: The position in a geocentric orbit at which the orbiting object is at its greatest distance from Earth.

Apparent Magnitude: The brightness of a body, as it appears to the observer, measured on a standard magnitude scale. It is a function of the luminosity and distance of the object, and the transparency of the medium through which it is observed.

Arcminute: One sixtieth of a degree of arc (a circle is divided into 360 °).

Arcsecond: The second division of a degree of arc. One sixtieth of an arcminute. (1/3600th of a degree.)

Argument of the Pericentre: The angle, measured around the orbital plane, between the ascending node and the pericentre.

Ascending Node: The position in the orbit of a planet (or the Moon) where it crosses the plane of the ecliptic, moving northward.

Astigmatism: An optical aberration resulting from unequal magnification across different diameters.

Astronomical Twilight: When the centre of the Sun is between 12° and 18° below the horizon; faint stars become visible.

Astronomical Unit (AU): The mean distance from the Earth to the Sun, i.e. 149,597,870 km or 499.005 light seconds.

Azimuth: The angular distance around the horizon, usually measured from north, through east, of the great circle passing through the object and the zenith.


Catadioptric: A telescope whose optics, not including the eyepiece, consists of both lenses and mirrors. The most common examples of these are the Schmidt-Cassegrain telescopes, whose "lens" is an aspheric corrector plate, and the Maksutov-Cassegrain telescopes, whose "lens" is a deeply curved meniscus.

Celestial Co-ordinates: A system by which the position of a body on the celestial sphere is plotted with reference to a reference plane and a reference direction. For more detail, see the tutorial on positional astronomy. The four systems in use are Ecliptic Co-ordinates, Equatorial Co-ordinates, Galactic Co-ordinates, and Horizon Co-ordinates.

Celestial Equator: The projection of the Earth's equator upon the celestial sphere. It is the reference plane for measurement of the declination in the system of equatorial co-ordinates.

Celestial Poles: The two points where Earth's axis of rotation is projected onto the celestial sphere.

Celestial Sphere: The projection of space and the objects therein onto an imaginary sphere surrounding the Earth and centred on the observer.

Chromatic Aberration: An aberration of refractive optical systems in which light is dispersed into its component colours, resulting in false colour in the image.

Circumpolar: An object that does not set from its observer's latitude.

Civil Twilight: When the centre of the Sun is less than 6° below the horizon; normal daylight activities are possible.

Collimation: The bringing of the optical components of an optical system into correct alignment. In a binocular, the optical axes in each tube must be aligned with each other and with the axis of the hinge that allows the interpupillary distance (IPD) to be adjusted. If they are only aligned with each other and not the hinge, it is called Conditional Alignment and will change if the IPD is changed.

Coma: (i) The matter surrounding the nucleus of a comet – it results from the evaporation of the nucleus. (ii) An optical aberration in which stellar images are fan-shaped, similar to comets.

Conjunction: There are at least three definitions of conjunction. Bodies are said to be in conjunction when they have the same ecliptic longitude (this is the strict definition) or when they have the same Right Ascension or when they are at their closest. Planets are said to be "at conjunction" when they are in conjunction with the Sun. (See diagram.) For extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.

Culmination: An object culminates when it reaches greatest and least altitudes (upper culmination and lower culmination respectively). For non circumpolar objects, the lower culmination is below the horizon. Most objects (the Moon sometimes being a notable exception) culminate when they reach the observer's meridian.


Declination: The angle of an object above or below the celestial equator. It is part of the system of equatorial co-ordinates.

Descending Node: The position in the orbit of a planet (or the Moon) where it crosses the plane of the ecliptic, moving southward.

Dichotomy: When the phase is exactly 50%.

Dioptre: (also diopter) A measure of the refractive power of a lens. As a measurement, it is the reciprocal of the focal length of the lens in metres. e.g. if a lens has a focal length of 20cm, =0.2m, its dioptre will be 1/0.2 = 5 m-1

Dioptre Adjustment: An adjustment, usually on the right eyepiece of a centre-focus binocular, that enables the observer to compensate for a difference in focus between his/her eyes.

Direct motion: Another term for prograde motion.


Eccentricity: The eccentricity of an orbit is a measure of its departure from a circle. Elliptical orbits have an eccentricity >0 and <1, parabolic paths have an eccentricity =1, and hyperbolic paths have an eccentricity >1.

Eclipse: An alignment of two bodies with the observer such that either the nearer body prevents the light from the further body from reaching the observer (strictly speaking, these are occultations), e.g. solar eclipse or eclipsing binary stars, or when one body passes through the shadow of another, e.g. lunar eclipse, eclipses of Jovian satellites.

Ecliptic: The apparent path the Sun on the celestial sphere. It intersects the celestial equator at the equinoxes. It is so named because, when the Moon is on the ecliptic, solar and lunar eclipses can occur.

Ecliptic Co-ordinates: A system of celestial co-ordinates that uses the ecliptic as the reference plane and the First Point of Aries as the reference direction. The co-ordinates are given as ecliptic latitude (β) and ecliptic longitude (λ). (These are also called celestial latitude and celestial longitude.)

Elongation: The angular distance between the Sun and any other solar system body, or between a satellite and its parent planet. The greatest elongation of an inferior planet is its maximum angular distance from the Sun; at this time the planet sets (greatest elongation east) or rises (greatest elongation west) at the greatest time from sunset or sunrise. (See diagram.) For extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.

Epoch: (i) The date at which a the co-ordinates on a star chart will be correct with respect to precession. (ii) The date of reference in astronomical calculations.

Equation of Time: The correction which must be applied to solar time in order to obtain mean solar time. See my tutorial on Time.

Equatorial Co-ordinates: A system of celestial co-ordinates that uses the celestial equator as the reference plane and the First Point of Aries as the reference direction. The co-ordinates are given as Right Ascension (RA) and Declination (Dec).

Equatorial Mount: A mounting in which one of two mutually perpendicular axes is aligned with Earth's axis of rotation, thus permitting an object to be tracked by rotating this axis so that it counteracts Earth's rotation. It is unsuited to binoculars because the eyepieces will not be kept horizontal.

Equinox: Literally "equal night". it refers to the time of year when day and night are of equal length. (i) The positions where the centre of the Sun crosses the celestial equator. (ii) The dates when the declination of the Sun is zero (i.e. when it is on the celestial equator).

Exit pupil: The position of the image of the objective lens (or primary mirror) formed by the eyepiece. It is the smallest disc through which all the collected light passes and is therefore the best position for the eye's pupil.

Extinction: Loss of light from an object as a consequence of absorption or scattering by an intervening medium. An example is the atmospheric extinction of light from stars near the horizon.

Eye relief: The distance from the eye lens of the eyepiece to the exit pupil. Spectacle wearers require sufficient eye relief to enable them to place the eye at the exit pupil.

Eye ring: An alternative name for the exit pupil.


Faculae: Unusually bright spots on the Sun's surface.

Finder: A small telescope, ideally of wide field of view, that is fixed to the main instrument in order to facilitate the finding of objects.

First Point of Aries (FPA): The Vernal Equinox point, i.e. that where the centre of the Sun, moving northwards, crosses the equator. It is the reference direction for the equatorial system of co-ordinates.

Focal length: The distance from the centre of a lens or mirror to its point of focus.

Focal plane: The plane (usually this is actually the surface of a sphere of large radius) where the image is formed by the main optics of the telescope. The eyepiece examines this image.

Focal ratio: The ratio of the focal length to the aperture.

Focuser: The part of the binocular which varies the optical distance between the objective lens and the eyepiece. This is usually achieved in one of two ways: Centre Focus, in which a rotating wheel on the hinge focuses both eyepieces simultaneously, either by moving the eyepieces or by moving an internal lens. Independent Focus, in which each eyepiece incorporates a helical focuser and is focused by being rotated. More info and images here.


Galactic Co-ordinates: The system of celestial co-ordinates in which the galactic plane as the reference plane and the galactic centre as the reference direction. The positions are given in galactic latitude and galactic longitude.

Galilean Moons: The four Jovian moons first observed by Galileo ( Io, Europa, Ganymede and Callisto). They are observable with small amateur telescopes.

Geocentric: Earth-centred.

Geostationary Orbit: The orbit of a satellite which is both geosynchronous and in the equatorial plane. The satellite will appear to remain in a fixed position in relation to the observer.

Geosynchronous Orbit : The orbit of a satellite in which the orbital period of the satellite is equal to Earth's period of rotation. If the orbit is in the equatorial plane, the satellite will be geostationary; if the orbit is inclined to the equatorial plane the satellite will appear to trace a lemniscate in the sky.

Great circle: A circle formed on the surface of a sphere which is formed by the intersection of a plane which passes through the centre of a sphere. A great circle path is the shortest distance on a spherical surface between two points.


Helical Focuser: A focuser that operates by rotating the eyepiece housing or the focuser body itself. It is found on the right eyepiece of centre-focus binoculars and on both eyepieces of individual focus (IF) binoculars. More info and images here.

Heliocentric: Sun-centred.

Horizon Co-ordinates: The system of celestial co-ordinates in which the observer's horizon is the reference plane and the north point is the reference direction. The positions are given in altitude and azimuth.

Hour Angle: The angle, measured westwards around the celestial equator, between the observer's meridian and the hour circle of an object.

Hour Circle: (i) A great circle passing through an object and the celestial poles. (ii) The setting circle on the polar axis of an equatorial mount.


Inclination: (i) The angle between the ecliptic and the orbital plane of a planet. (ii) The angle between the orbital plane of a satellite and the equatorial plane of the parent body.

Individual Focus (IF): Binoculars in which each side is focused independently of the other, normally with a helical focuser on each eyepiece. More info and images here.

Inferior Conjunction: The conjunction of Mercury or Venus when they lie between Earth and the Sun.

Inferior Planets: Planets (i.e. Mercury and Venus) whose orbits lie between Earth's orbit and the Sun.

Integrated Magnitude: The magnitude which would apply if all the light energy from an extended object was coming from a point source.

Inter-pupillary Distance (IPD): The separation of the centres of the eyepieces (i.e. optical axes) of a binocular. It should correspond to the separation of the centres of the pupils of the observer's eyes.


Jovian: Pertaining to the planet Jupiter.


Kepler Elements: Another term for orbital elements.


Light Year: The distance travelled by light in one year. 9.4607 × 1012 km or 63,240 AU or 0.3066 parsecs.

Limb: The edge of the disc of a celestial body.

Limiting Magnitude: The magnitude of the faintest star visible. It may relate to the unaided eye or to an instrument. If it is not qualified, it can be taken to mean the Limited Naked Eye magnitude at the Zenith (LNEM).

Line of nodes: The imaginary line passing through the ascending and descending nodes of an orbit. It is the line of intersection of the orbital plane with the reference plane.

Longitude of the Ascending Node: The angular distance, measured around the reference plane, between the FPA and the ascending node of an orbit.

Luminosity: The amount of energy radiated into space per second by a star. The bolometric luminosity is the total amount of radiation at all frequencies; sometimes luminosity is given for a specific band of frequencies (e.g. the visual band).


Magnification: The increase in the angle subtended by an object. See my tutorial on telescope function.

Magnitude: The brightness of a celestial body on a numerical scale. See also absolute magnitude, apparent magnitude, bolometric magnitude and integrated magnitude.

Mean Anomaly: The anomaly which would exist if a planet orbited at a uniform speed in a circular orbit.

Mean Solar Time: Time based on an imaginary "mean Sun". See my tutorial on Time.

Meridian: The great circle passing through the celestial poles and the observer's zenith.

Meteor: The incandescent ionised trail of a meteoroid as it enters Earth's atmosphere.

Meteorite: A meteoroid that reaches Earth's surface.

Meteoroid: A fragment of matter which may turn into a meteor if it enters Earth's atmosphere or a meteorite if it strikes Earth.

Minor Planet: Another term for an asteroid.


Nadir: The point on the celestial sphere directly below the observer. Opposite of zenith.

Nautical Twilight: When the centre of the Sun is between 6° and 12° below the horizon; the marine horizon becomes invisible.

Near-Earth Asteroid (NEA): An asteroid whose orbit brings it close to Earth's orbit.

Near-Earth Object (NEO): A comet or asteroid whose orbit brings it close to Earth's orbit. The criterion is a perihelion distance < 1.3 AU.


Objective Lens (or Object Glass): The lens in a binocular tube or refractor that is closest to the object under observation.

Obliquity of the Ecliptic: The angle between the plane of the ecliptic and that of the celestial equator.

Occultation: An alignment of two bodies with the observer such that the nearer body prevents the light from the further body from reaching the observer. The nearer body is said to occult the further body. A solar eclipse is an example of an occultation.

Opposition: The position of a planet such that Earth lies between the planet and the Sun. Planets at opposition are closest to Earth at opposition and thus opposition offers the best opportunity for observation. For extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.

Orbital Elements: The six numerical values that completely define the orbit of one body about another of known mass. They are the semi-major axis (a), the eccentricity (e), the inclination to the reference plane (i), the mean anomaly (M), the argument of the pericentre (ω), and the longitude of the ascending node (&Omega). The elements vary with time as a consequence of perturbations of other bodies, so their epoch is important. For comets and asteroids, the perihelion conditions are often of interest, so the date of perihelion (T) and perihelion distance (q) are usually used instead of M and a. (At T, M=0; q = a(1-e) )

Osculating Orbit: The orbit that a body would follow if the only gravitational force acting on it was that of the primary body, i.e. if its motion was not perturbed by the presence of other bodies.


Parsec: The distance at which a star would have a parallax of one arcsecond. (3.2616 light years, 206,265 astronomical units, 30.857 x 10^12 km).

Penumbra: Literally "next to the umbra". (i) The shadow that results when only part of the bright object is occulted; e.g. an observer will see a partial eclipse when he is in the penumbra of the shadow of the moon. (ii) The lighter area surrounding a sunspot.

Perigee: The position in a geocentric orbit at which the orbiting object is at its least distance from Earth.

Perihelion: The position in a heliocentric orbit at which the orbiting object is at its least distance from the Sun.

Phase: The percentage illumination, from the observer's perspective, of an object (normally planet or Moon).

Planisphere: The projection of a sphere (or part thereof) onto a plane. It commonly refers to a simple device which consists of a pair of concentric discs, one of which has part of the celestial sphere projected onto it, the other of which has a window representing the horizon. Scales about the perimeters of the discs allow it to be set to show the sky at specific times and dates, enabling its use as a simple and convenient aid to location of objects.

Primary body: The body that is being orbited. E.g. the Sun is the primary of the orbits of the planets and comets. With respect to multiple star systems, it is the most massive star.

Primary mirror: In a reflecting telescope or binocular, the mirror that collects the light and focuses it to the focal plane.

Prime Meridian: The polar great semi-circle adopted as the reference direction for measurement of longitude. Earth's prime meridian is the Greenwich meridian.

Prograde: The apparent eastward motion of a planet with respect to the stars.

Proper motion: The apparent motion of a star with respect to its surroundings.


Quadrature: The position of a body (Moon or planet) such that its elongation is 90° or 270°; i.e. the Sun-Earth-body angle is 90°. (See diagram.)


Radiant: The position in the sky from which a meteor shower appears to radiate.

Reflector: A telescope whose optics, apart from the eyepiece, consist of mirrors.

Refractor: A telescope whose optics consist entirely of lenses.

Resolution: A measure of the degree of detail visible in an image. It is normally measured in arcseconds.

Reticle: A system of lines and/or concentric circles at the focal plane of a telescope or finder, used for positioning or guiding the telescope or binocular. It is usually incorporated into an eyepiece and may be illuminated in order to render the lines visible against a dark background sky. In unit-power reflex finders, it is the pattern of rings projected onto the sky.

Retrograde: Apparent westward movement off a planet with respect to the stars.

Right Ascension (RA): The angle, measured eastward on the celestial equator, between the First Point of Aries and the hour circle through the object.


Scintillation: The twinkling of stars, resulting from atmospheric disturbance.

Secondary: Abbreviation for secondary mirror. Small mirror that directs the light from the primary mirror to the eyepiece.

Sidereal Time: The hour angle of the First Point of Aries. It is time measured with respect to the stars. See my tutorial on Time.

Solar Time: Time measured with respect to the Sun. See my tutorial on Time.

Solstice: Literally "sun still". It refers to the apparent standstill of sunrise and sunset points at midsummer and midwinter. (i) The most southerly and northerly declinations of the Sun. (ii) The times at which the Sun attains its greatest and least declinations.

Spherical Aberration: An optical aberration in which light from different parts of a mirror or lens is brought to different foci.

Superior Conjunction: The conjunction of Venus and Mercury when they are more distant than the Sun. (See diagram.)

Superior Planets: Those planets whose orbits lie outside Earth's orbit.

Syzygy: The alignment of any three celestial bodies. Planets are at syzygy when they are at opposition or conjunction (aligned with Sun and Earth).


Terminator: The boundary of the illuminated part of the disc of a planet or moon.

Topocentric: Referred to a position on the surface of the Earth (cf geocentric, which is referred to the centre of the Earth.)

Transit: (i) The passage of Mercury or Venus across the disc of the Sun (ii) The passage of a planet's moon across the disc of the parent planet (iii) The passage of a planetary feature (such as Jupiter's Great Red Spot) across the central meridian of the planet. (iv) The passage of an object across the observer's meridian (see culmination). In the latter case, for extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.

Twilight: The period of decreasing sky brightness after sunset, or of increasing sky brightness before sunrise. There are three definitions of twilight: Civil Twilight, Nautical Twilight, and Astronomical Twilight. Twilight lasts longer in higher latitudes. See my tutorial on Twilight.


Umbra: (i) The shadow that results when a bright object is completely occulted. A total eclipse of the Sun occurs when the observer is in the Moon's umbra. (ii) The dark inner region of a sunspot.


Vernal equinox: (i) The equinox (on or near March 20) when the Sun is at its ascending node. (ii) The First Point of Aries.


Zenith: The point on the meridian directly above an observer.

Zenithal Hourly Rate (ZHR): The theoretical hourly rate of meteors which would be observed at the peak of a shower, by an experienced observer, with the radiant at the zenith, under skies with a limiting naked eye magnitude of 6.5.

Zodiacal light: A faint conical glow occurring above the horizon along the ecliptic during twilight, best seen when the ecliptic is at a steep angle to the horizon.