Astronomy Definitions

Glossary of Astronomical Terms

Absolute Magnitude
A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light years) away from Earth. On this scale, the Sun has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it is so close.
Altitude
The angular distance from the observer's horizon, usually taken to be that horizon that is unobstructed by natural or artificial features (such as mountains or buildings), measured directly up from the horizon toward the zenith; positive numbers indicate values of altitude above the horizon, and negative numbers indicate below the horizon --- with negative numbers usually being used in terms of how far below the horizon the sun is situated at a given time [for example, the boundary between civil twilight and nautical twilight is when the sun is at altitude -6 degrees].
Aperture
The size of the primary optical surface of an astronomical instrument (telescope), usually given in inches, centimeters, or meters. In the case of a reflecting telescope, the aperture usually refers to the size of the main mirror; in the case of a refracting telescope (of which binoculars are one example), the aperture refers to the size of the primary lens (which in binoculars is usually given in millimeters).
Aphelion
For an object orbiting the sun, the point (distance and time) where/when the object is furthest from the sun in its elliptical orbit.
Arc minute(s)
There are 60 minutes (denoted as 60') of arc in 1 degree. In the sky, with an unobstructed horizon (as on the ocean), one can see about 180 degrees of sky at once, and there are 90 degrees from the true horizon to the zenith. The full moon is about 30' (30 arc minutes) across, or half a degree.
Arc second(s)
There are 60 seconds (denoted 60") of arc in one minute of arc.
Astronomical Unit (AU)
Approximately equal to the mean earth-sun distance, which is about 150,000,000 km or 93,000,000 miles. Formally, the AU is actually slightly less than the earth's mean distance from the sun (semi-major axis) because it is the radius of a circular orbit of negligible mass (and unperturbed by other planets) that revolves about the sun in a specific period of time.
AU
See Astronomical Unit.
Azimuth
Angular distance measured clockwise around the observer's horizon in units of degrees; astronomers usually take north to be 0 degrees, east to be 90 degrees, south to be 180 degrees, and west to be 270 degrees.
CCD (Charge-Coupled Device)
Charge-coupled device, a very sensitive electronic device that is revolutionizing astronomy in the 1990s. CCD cameras are composed of silicon chips that are sensitive to light, changing detected photons of light into electronic signals that can then be used to make images of astronomical objects or to analyze how much light is being received from such objects. CCDs require computers for reduction of data, so the expense can be much greater than for, say, photography --- but CCDs can detect much fainter objects than can photographs. Unfiltered CCDs tend to be more red-sensitive than the human eye.
Celestial Equator.
An imaginary line that divides the celestial sphere into a northern and southern hemisphere.
Celestial Sphere
An imaginary sphere of great (or infinite) radius that is centered on the earth and is used for practical purposes in astronomical observing. Since stars (other than our own sun!) are very distant from us, they make up a background that is essentially unchanging from year to year; of course, over a period of years, the closer stars will move very slightly and factors such as precession cause a change in the appearance of the stars in our skies over many years. But we create a map grid on the celestial sphere for identifying, referring to, and locating objects in the sky; some of these map grids include equatorial coordinates (right ascension and declination), ecliptic coordinates (ecliptic longitude and latitude), and galactic coordinates (galactic longitude and latitude) --- which refer to the earth's rotation, the earth's revolution about the sun, and the Milky Way galaxy's plane, respectively.
Cepheid Variable
This is a variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter Cepheids will have a longer period.
Circumpolar
A celestial object that never sets but always stays above the horizon. This depends on the location of the observer. The further South one travels (in the northern nemisphere) the fewer stars or objects will be circumpolar. Polaris, the North Star, is circumpolar in most of the northern hemisphere.
Coma (cometary)
A comet's atmosphere (composed of dust and/or various gases) surrounding its nucleus. The coma is rather tenuous (except very close to the nucleus), and stars can be occasionally easily seen through it, shining from behind. And yet, the coma is usually thick enough that it masks our view of the true nucleus of the comet, as seen from the earth. As a comet's nucleus is usually quite small, it is not able to retain its coma for long periods of time, and the coma material gradually drifts away into space (helped out by the solar wind). Much coma material is thrown back into what we see as the comet's tail. But all coma material originates in the comet's nucleus, and solar sublimation due to heating causes gases to move outward, often in jets, taking dust material with them to form the coma and tail.
Coma (telescopic)
An aberration where point sources (stars) at the center of the image are focused to a point but typically appears as "comet-like" radial smudges that get worse towards the edges of the image.
Comet
A celestial body orbiting the sun (though some may be ejected from the solar system by planetary perturbations) that displays (at least during a portion of its orbit) some diffuseness and/or a "tail" of debris that points generally in the anti-solar direction. Both the diffuseness (generally called a coma) and the tail are composed of gas and/or dust of various atomic or molecular compositions, as is ascertained by spectroscopy. The coma and tail material come from a much smaller nucleus that is usually invisible due to the bright surrounding coma activity. Close-up pictures of a cometary nucleus did not occur until spacecraft fly-bys of Halley's comet in 1986.
Conjunction
An event that occurs when two or more celestial objects appear close close together in the sky.
Constellation
A grouping of stars that make an imaginary picture in the sky. The International Astronomical Union designated 88 constellations in 1922. The majority of constellations visible from the northern hemisphere are named after the original 48 constellations in the Almagest, an astronomy work written by the ancient Greek astronomer Ptolemy sometime in the 2nd century, A.D.
Crescent
The phase of a body that is less than one-half illuminated.
Declination
One element of the astronomical coordinate system on the sky that is used by astronomers. Declination, which can be thought of as latitude on the earth projected onto the sky, is usually denoted by the lower-case Greek letter delta and is measured north (+) and south (-) of the celestial equator in degrees, minutes, and seconds of arc. The celestial equator is defined as being at declination zero (0) degrees; the north and south celestial poles are defined as being at +90 and -90 degrees, respectively. When specifying a comet's location on the sky, one must state the right ascension and declination (with equinox), along with date and time (since a comet moves with respect to the background stars).
Degree
A unit used in the measurement of angles, heavily used particularly in astronomy. Due to ancient Babylonian mathematics, we still divide a circle into 360 even units of arc and call each of these units one degree. The entire sky, therefore, spans 360 degrees. Up to about 180 degrees of sky is visible from any given point on earth with an unobstructed horizon (as measured from, say, east to west, or north to south). The degree is used to make measurements of distance, or position (as with declination) in astronomy. In turn, a degree is composed of 60 minutes of arc, and also of 3600 seconds of arc.
Dwarf Planet
A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. Pluto is considered to be a dwarf planet.
Eclipse
The total or partial blocking of one celestial body by another.
Ecliptic
The apparent path of the sun against the sky background (celestial sphere); formally, the mean plane of the earth's orbit about the sun. This is the only place in the sky where solar and lunar eclipses occur.
Elongation
Angular distance of a celestial object from the sun in the sky; more simply, how far above the horizon an inferior planet is above the horizon.
Equatorial Mount
A mount for instruments that follows the rotation of the sky (celestial sphere) by having one rotational axis parallel to the Earth's axis of rotation.
Equinox
Literally meaning "equal night (as day)." Either of the two points (vernal, autumnal) on the celestial sphere where the ecliptic (which is the apparent path of the sun on the sky) intersects the celestial equator. Due to precession, this point moves over time, so positions of stars in catalogues and on atlases are usually referred to a "mean equator and equinox" of a specified standard epoch.
Eyepiece
The lens at the viewing end of a telescope (or binoculars). The eyepiece is responsible for enlarging the image captured by the instrument. Eyepieces are available in different focal lengths, which, based on the focal length of the telescopic system, yield differing amounts of magnification. The magnification is usually calculated by dividing the telescope focal length by the eyepiece focal length.
Faculae
Bright patches that are visible on the surface of the Sun (also known as the photosphere).
Finder (or finderscope)
A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location.
Fireball
An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some may be accompanied by a sonic boom.
Galaxy
A large grouping of stars, usually in the billions. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes billions of years to reach the Earth.
Galilean Moons
The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius. If not for the light of Jupiter so nearby,the light of these moons would be visible to the naked eye under dark, moonless skies. They are visible in most small binoculars, and in nearly any small telescope.
Gibbous
More than half but less than fully illuminated. Used of the moon or a planet.
Globular Cluster
A spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.
Heliocentric
Referring to the sun. A heliocentric orbit is one based on the Sun as one of the two foci of the (elliptical) orbit (or as the center of a circular orbit).
Inferior Conjunction
A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.
Julian Date
The interval of time in days (and fraction of a day) since Greenwich noon on Jan. 1, 4713 BC. The JD is always half a day off from Universal Time, because the current definition of JD was introduced when the astronomical day was defined to start at noon (prior to 1925) instead of midnight. Thus, 1995 Oct. 10.0 UT = JD 2450000.5.
km
kilometer = 0.6 mile.
Libration
An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time.
Light Pollution
The emission of stray light or glare from lighting fixtures in manners that counter the purpose of the light (which is to light what is below); also known as the waste of money and energy in the form of electric light, usually meant in the form of outdoor night lighting. Such light trespass causes severe safety problems for motorists, pedestrians, and cyclists at night from lighting that shines onto streets and highways and sidewalks from poorly-designed or poorly-mounted lighting. Such glare also imposes on privacy, by shining brightly into bedroom windows at night and into backyards where adults and children are trying to observe the night sky (also known as "light trespass"). While most people have accepted such bad, glare lighting without question and assumed that nothing could be done about it, dedicated groups of volunteers around the world are now showing that effective laws and guidelines can be instated at the local and regional levels of government (and in planning and engineering offices), which mean that proper outdoor night lighting can be a norm so that everybody benefits --- auto drivers, sleeping residents, government budgets, and skygazers alike. Laws mandating full-cutoff light fixtures are already in place in states such as Maine and Connecticut and are pending elsewhere.
Light Year
An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.
Limb
The outer edge or border of a planet or other celestial body.
Local Group
A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.
Luminosity
The amount of electromagnetic energy a celestial body (such as a star) radiates per unit of time.
Lunar Eclipse
A phenomenon that occurs when the Moon passes into the shadow of the Earth. A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow. In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.
Lunar Month
The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.
Magnitude
The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, Sirius, shines at visual magnitude -1.5. Jupiter can get about as bright as visual magnitude -3 and Venus as bright as -4. The full moon is near magnitude -13, and the sun near mag -26. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness. In the case of comets, we speak of a magnitude that is "integrated" over an observed coma diameter of several arc minutes; this is called the comet's "total (visual) magnitude", and is usually denoted by the variable m1. Thus, a 7th-magnitude comet is much harder to see than a 7th-magnitude star -- the latter having all its light in a pinpoint, and the former having the same amount of light spread out over a large area (imagine defocussing a 7th-magnitude star to the size of a diffuse comet). Typically, however, when comets become very bright, their apparent coma sizes shrink so that the majority of visible light is in a small, intense core of the comet's head (and the comet may appear starlike with a tail emanating from the comet's head).
Mare (pl. - maria)
A term used to describe a large, circular plain. The word mare means "sea". On the Moon, the maria are the smooth, dark-colored areas.
Meridian
An imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the horizon, and through the nadir. It is perpendicular to the local horizon. Because it is fixed to the local horizon, stars will appear to drift past the local meridian as the earth spins.
Meteor
A small particle of rock or dust that burns away in the Earth's atmosphere. Meteors are also referred to as shooting stars. Meteor showers are generally thought to be produced by the debris left by comets as the latter orbit the sun.
Meteor Shower
An event where a large number of meteors enter the Earth's atmosphere from the same direction in space at nearly the same time. Most meteor showers take place when the Earth passes through the debris left behind by a comet.
Meteorite
An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth's surface. Meteors become meteorites if they reach the ground.
Meteoroid
A small, rocky object in orbit around the Sun, smaller than an asteroid.
Nebula
A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material of which stars are made.
Occultation
An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon, though this term is most often applied when a larger object like the Moon or a planet obscures a star.
Open Cluster
A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.
Opposition
The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.
Orbit
The path of one object about another (used here for an object orbiting the sun).
Parallax
The apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object (as from two different observing sites on earth). An example of parallax would be to hold your thumb out at arm's length while viewing a distant object, then alternately closing one eye, then the other.
Penumbra
The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.
Perigee
The point where (and when) an object's orbit about the earth in which it is closest to the earth; only applicable to objects orbiting the earth (not to objects orbiting the sun).
Perihelion
The point where (and when) an object orbiting the sun is closest to the sun.
Perturbations
Gravitational influences ("tugging" and "pulling") of one astronomical body on another. Comets are strongly perturbed by the gravitational forces of the major planets, particularly by the largest planet, Jupiter. These perturbations must be allowed for in orbit computations, and they lead to what are known as "osculating elements" (which means that the orbital element numbers change from day to day and month to month due to continued perturbations by the major planets, so that an epoch is necessarily stated to denote the particular date that the elements are valid.
Phase
The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.
Photon
A particle of light composed of a minute quantity of electromagnetic energy.
Planet
A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.
Planetary Nebula
A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes. Planetary nebulas have nothing to do with planets.
Quadrature
A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.
Radiant
A point in the sky from which meteors in a meteor shower seem to originate.
Red Giant
A stage in the evolution of a star when the fuel begins to exhaust and the star expands to about fifty times its normal size. The temperature cools, which gives the star a reddish appearance.
Reflector
A telescope that uses as its primary optical element a mirror. Nearly all large telescopes in use today by amateur and professional astronomers are reflecting telescopes.
Refractor
A telescope that uses as its primary optical element a lens. Binoculars are a type of refractor. In general, refractors are much more expensive to build and buy than are reflectors.
Retrograde Motion
The phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. This motion is caused when the Earth's faster revolution around the Sun overtakes a body in its orbit compared to the background stars.
Right Ascension (R.A. or RA)
One element of the astronomical coordinate system on the sky, which can be though of as longitude on the earth projected onto the sky. Right ascension is usually denoted by the lower-case Greek letter alpha and is measured eastward in hours, minutes, and seconds of time from the vernal equinox. There are 24 hours of right ascension, though the 24-hour line is always taken as 0 hours. More rarely, one sometimes sees right ascension in degrees, in which case there are 360 degrees of right ascension to make a complete circuit of the sky.
Satellite
A natural or artificial body in orbit around a planet.
Solar Eclipse
A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is close enough to completely block the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is not able to completely block the light. This results in a ring of light around the Moon.
Solstice
The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the Summer and Winter seasons.
Star
A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.
Star Cluster
A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.
Sunspots
Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field.
Superior Conjunction
A conjunction that occurs when a superior planet passes behind the Sun and is on the opposite side of the Sun from the Earth.
Superior Planet
A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and Venus. These two planets are inferior planets.
Supernova
A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life. Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions.
Telescope
An instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography. A Telescope can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves.
Terminator
The boundary between the light side and the dark side of a planet or other body.
Total (visual) magnitude
Total, integrated magnitude of a comet's head (meaning coma + nuclear condensation). This can be estimated visually, as the comet's "total visual magnitude". The variable m1, usually found in comet ephemerides, is used to denote the total (often predicted) magnitude. See also definition for "Magnitude", above.
Transit
The passage of a celestial body across an observer's meridian; also the passage of a celestial body across the disk of a larger one.
Umbra
The area of total darkness in the shadow caused by an eclipse.
Universal Time (UT, or UTC)
A measure of time used by astronomers; UT conforms (within a close approximation) to the mean daily (apparent) motion of the sun. UT is determined from observations of the diurnal (daily) motions of the stars for an observer on the earth. Coordinated Universal Time (UTC) is that used for broadcast time signals (available via shortwave radio, for example), and it is within a second of UT.
Variable Star
A star that fluctuates in brightness. These include eclipsing binaries, regular and semi-regular stars, and eruptive stars.
Zenith
The point directly overhead in the sky from an observer's location.