Glossary of Astronomical Terms

Alt-Azimuth: A type of mount which allows a telescope tube to be moved horizontally (by rotation in azimuth or compass direction) and vertically (by rotation in altitude or elevation). To follow a star the telescope must be adjusted simultaneously in both axes.

Aperture: The effective diameter of the primary mirror or lens of a telescope.

Aphelion: The point in an planet’s orbit when it is farthest from the Sun.

Apogee: The point in the Moon’s or planet’s orbit when it is farthest from the Earth.

Asterism: A pattern of stars that is not an official constellation but appear within a constellation. Two example’s are the “Big Dipper” which is a portion of the constellation Ursa Major and the three prominent stars which form “Orion’s Belt” within the constellation of Orion.

Aurora: The glow in the Earth’s ionosphere caused by the interaction between the Earth’s magnetic field and charged particles from the Sun. It is know as Northern Lights or Aurora Borealis in the Northern Hemisphere and the Aurora Australis in the Southern Hemisphere.

Averted Vision: A technique of looking slightly to the side of a faint object to increase its visibility through a telescopic eyepiece. By looking slightly off to the side its light falls on the part of the retina most sensitive to light.

Binary Star: A system of two stars orbiting around a common center of mass due to their mutual gravity. Binary stars are twins in the sense that they formed together out of the same interstellar cloud.

Black Hole:
An object thought to be the result of a massive star’s implosion in which its gravity has become so dense and strong that it prevents light from escaping.

Blue Moon: The second full moon in a calendar month or the third full moon in a season containing four.

Circumpolar: Stars that appear close enough to one of the celestial poles that they seem to circle around the pole above the horizon. These stars are visible year-round.

Comet: An icy object in independent orbit about the Sun that is smaller than a planet, usually having a highly elliptical orbit extending out beyond Jupiter.

Conjunction: When two bodies appear close together in the sky usually within 15 degrees or less.

Constellation: An grouping of stars which form a pattern.

Corona: The outer layer and hottest part of the Sun’s atmosphere.

Dark Nebula: A dense cloud of interstellar matter whose dust particles obscure the light from stars beyond it and give the cloud the appearance of a region devoid of stars.

Declination: A system for measuring the altitude of a celestial object, expressed in degrees north or south of the celestial equator. Angles are positive if a point is North of the celestial equator, and negative if South. It is used in conjunction with Right Ascension to locate celestial objects.

Deep Sky Object:
A term for nebulae, galaxies and star clusters beyond our own solar system.

Diffuse Nebula: An irregularly shaped cloud of interstellar gas or dust.

Earth shine: Light reflected from the Earth’s atmosphere onto the dark part of the New Moon.

Eclipse: Occultation of one celestial body by another which passes between it and the observer.

Ecliptic: The visible path of the Sun and planets as seen against the stars. The plane of the Earth’s equator is inclined at 23.5 degrees to its orbit and the ecliptic is inclined to the celestial equator by the same angle. The ecliptic intersects the celestial equator at the two equinoxes.

Elongation: The angular distance between the Sun and the planets expressed in degrees. The term greatest elongation is applied to the inner planets and is the maximum elongation from the Sun. At greatest elongation a planet will appear at 50 percent phase.

Ephemeris: A list or table of the positions of a celestial object in an orderly sequence for a number of dates.

German Equatorial: A telescope mount designed so that the two axes which support it are aligned—one to the polar axis and the other to the Earth’s equator. Once an object is centered in the telescope’s field of view, only the polar axis needs to be adjusted to keep the object in view.

Equinox: This is the time when the Sun crosses the celestial equator. There are two equinoxes: Vernal (Spring), around March 21st and Autumnal (Autumn) around September 23rd. On these dates, day and night are equal.

Field of View (FOV):
The portion or area of sky that can be seen within a telescope/eyepiece.

Focal Ratio: The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number; f/# = F/D. Also defines the cone angle of the beam. Small focal ratios e.g. f/# = F/D = 1 are said to be “fast” and represent a very large cone angle. Large focal ratios e.g. f/# = 35 are said to be “slow” and indicate a very small cone angle.

Full Moon: The Moon when it lies directly opposite of the Sun. The Moon is full two weeks after New Moon. The full moon rises at sunset and sets at sunrise.

Galaxy: Vast star systems containing thousands of billions of stars, dust and gas, held together by gravity. There are three main classes of galaxies: elliptical, spiral and barred, named after their appearance. The Milky Way is a spiral galaxy.

Geosynchronous Orbit:
Refers to when an artificial satellite’s orbital velocity is matched to the rotation of the Earth—a geosynchronous satellite would appear to be stationary relative to the Earth. Also know as a geostationary orbit.

Gibbous: The phase in which the Moon is more than half lit but less than fully lit.

Globular Cluster: A star cluster that packs hundreds of thousands of stars into a region only about a hundred light-years across. An example of a bright globular cluster is M13.

Grazing Occultation: When one celestial object is partly or intermittently hidden as it appears to move along the uneven edge of a closer object, usually the Moon.

Inferior Conjunction: When Mercury or Venus are directly between the Sun and Earth.

Inferior Planets: The planets (Mercury and Venus) that orbit between the Earth and the Sun. Also known as the inner planets.

Light Year: The distance traveled by light in one year, equal to 186,000 miles per second.

Magnitude: The degree of brightness of a celestial object designated on a numerical scale. The brightest star has magnitude -1.4 and the faintest star visible to the naked eye has magnitude of 5 or 6. Limiting magnitude refers to the faintest object visible without telescopic aid.

Messier Object (M Object): One of the 110 deep sky objects in the catalogue compiled by Charles Messier. Messier’s are comprised of nebulae, galaxies and star clusters.

Meteor: Also known as a shooting star or falling star, it is a bright streak of light in the sky caused by a meteoroid as it burns up in the Earth’s atmosphere.

Meteorite: A rock of extraterrestrial origin that survives its trip through the Earth’s atmosphere and lands on the ground.

Meteoroid: A rocky or metallic object of extraterrestrial origin that is smaller than an asteroid that would become a meteor if it entered the Earth’s atmosphere.

Meteor Shower: An increased number of meteors appearing to radiate from a single area within a constellation at certain times of the year when the Earth crosses the debris trail of comets.

Milky Way: The name of our own spiral galaxy and the band of light from the combined glow of stars and galaxies that lie along the galaxy’s equatorial plane.

Minor Planets: Another term for asteroids.

Moon: A naturally occurring satellite or relatively large body orbiting a planet.

Naked Eye (Unaided Eye): A term used to describe observing without the aid of optical instruments.

Nebula: A term used to describe celestial objects which have a fuzzy or cloudy appearance. Nebula is Latin for cloud.

New Moon: The Moon when it lies in the same direction as the Sun and the beginning of a cycle of lunar phases. The New Moon rises and sets with the Sun.

Newtonian Telescope: A type of reflecting telescope with a parabolic primary mirror and a small secondary mirror angled at 45 degrees to deflect the focus of the primary to a position outside the tube near the top of the telescope. Newtonian telescopes were developed by Sir Isaac Newton.

Night Vision: The enhanced ability to see objects in the dark. Astronomers use red-lensed flashlights and red observatory lights to preserve their night vision.

Nova: An existing star which suddenly increases its brightness by more than 10 magnitudes and then slowly fades.

Objective: The primary mirror of a reflecting telescope or the primary lens of a refractor.

Occultation: When one celestial body passes in front of and obscures another celestial object.

Open Cluster:
A group of young stars possibly bound together by gravity.

Opposition: A position of an outer planet when it appears opposite the Sun (inner planets cannot come into opposition).

The path of a celestial body around another due to the influence of gravity.

The outer shadow cast during an eclipse. It is also the lighter area surrounding the central region of a sunspot. Penumbra is Latin for dim light.

Perigee: The point in the Moon’s or planet’s orbit when it is closest to Earth.

Perihelion: The point in an planet’s orbit when it is closest to the Sun.

Planisphere: An observing aid for locating stars and constellations. It consists of two discs—one with the entire night sky and the other with an opening through which the lower disc shows a star map of the sky. The second disc is set according to the date and time and shows what the night sky looks like at that moment.

A second-magnitude star in the constellation Ursa Minor that lies near the direction in the sky toward the North Pole. Also known as the North Star.

A cloud or plume of hot, luminous gas in the Sun’s corona.

Quasar: An incredibly powerful source of light and radio wave.

Quiet Sun: Refers to the Sun when it is at the lowest portion of its 11-year cycle.

The part of the sky from which a particular meteor stream appears to come from. Meteor showers are usually named after the constellation in which the radiant originates.

Red Dwarf:
Red dwarfs are much fainter, cooler and smaller than the Sun. They are the most common type of star in the galaxy accounting for 70 percent of all stars.

Red Giant: A highly luminous but relatively cool star that has reached a late stage in its life. Such stars are in a more advanced state of evolution and are running out of nuclear fuel and have become less dense.

Resolution: The amount of detail visible in a telescopic image. Low resolution shows only large features, high resolution shows many small details.

Retrograde: The movement of a planet when it appears to “travel” backwards.

Right Ascension (RA): Used together with declination, it is one of the coordinates used to reference celestial objects and is equivalent to a longitude reference on the Earth. There are 24 hours of right ascension within 360 degrees, so one hour is equivalent to 15 degrees.

Schmidt-Cassegrain telescope:
A telescope with a spherical primary mirror and a thin refractive corrector plate with a complex, non-spherical shape.

Seeing: The degree of sharpness of astronomical images. Turbulence in Earth’s atmosphere can cause stars and planets to twinkle—such an occurrence is referred to as “poor seeing.”

Sky glow: The light pollution from excessive and misdirected artificial outdoor lighting. Sky glow is seen as an orange dome of light over towns and cities and is visible for miles.

Solar Cycle: The 11-year variation in sunspot activity. More sunspots are seen at the solar maxima with a quiet Sun occurring during the minima.

Solar Flare: A sudden burst of energy on the Sun’s surface lasting from minutes to hours.

Solar Wind: A stream of charged particles emitted from the Sun which travel into space along lines of magnetic flux.

Solstice: This is the time when the Sun reaches its most northerly or southerly point. It marks the beginning of Summer and Winter in the Northern Hemisphere and the opposite in the Southern Hemisphere. Summer solstice occurs on June 21st and winter solstice on December 22nd.

Star Cluster:
A loose association of stars within the the Milky Way. Examples are the Pleiades (Seven Sisters) Hyades clusters.

A cooler region of the Sun’s photosphere which appears as a dark spot on the Sun’s disc. It is caused by concentrations of magnetic flux occurring in groups or clusters. The number of sunspots varies according to the Sun’s 11 year cycle.

Superior Conjunction:
This is when Mercury or Venus are behind the Sun.

Superior Planets: These are the planets beyond the Earth’s orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto). Also known as the outer planets.

An exploding star.

Terminator: The boundary between day and night of the Moon’s or a planet’s disc.

Transit: The visible journey of Mercury or Venus across the Sun’s disc or of a planet’s moon across the disc of that planet.

Transparency: The degree to which celestial light is able to pass through Earth’s atmosphere.

Twilight: The time preceding sunrise and following sunset when the sky is partially illuminated. Civil twilight occurs when the central point of the Sun’s disk is between 90°50′ and 96°, nautical twilight from 96° to 102°, and astronomical twilight from 102° to 108°.

Umbra: It is the shadowed area defining a total eclipse or the dark central region of a sunspot. Umbra is Latin for shade.

Variable Star: Any star whose brightness or magnitude varies with time. The variations can be caused by eclipses, dust and other phenomena. Variations can also be irregular or periodic.

Waning Crescent: The phase of the Moon between third quarter and new moon. Waning means declining or fading.

Waning Gibbous: The phase of the Moon between New Moon and Last Quarter.

Waxing Crescent: The phase of the Moon between New Moon and First Quarter. Waxing means increasing.

Waxing Gibbous: The phase of the Moon between First Quarter and Full.

White Dwarf: A faint, extremely dense dying star that has used up its nuclear fuel and is slowly fading from view. A typical white dwarf has 60 percent of the Sun’s mass but is little larger than the Earth.

Worm Hole: A hypothetical bridge to another universe through the space time continuum created by a black hole.

Zenith: The point on a celestial body directly above an observer or the highest point in the sky reached by a celestial body.

Zodiac: The twelve constellations (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces) that formed an ancient calendar of the Sun’s progress in the sky during one Earth-year. Also, it is the visible path followed by the Sun, Moon and most planets, lying within 10 degrees of the celestial equator.

Zodiacal Light: A faint glow caused by scattered sunlight reflected off interplanetary dust. The glow extends away from the Sun along the ecliptic, visible in the western sky shortly after sunset or in the eastern sky shortly before sunrise.

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