Astronomical Glossary

Fred Espenak

absolute magnitude - The apparent magnitude a star would have if it were observed from a standard distance of 10 parsecs (32.616 light years); the absolute magnitude (M) is calculated from the apparent magnitude (m) and distance (p) in parsecs as follows: M = m + 5 (log10(p)-1).

anomalistic month - The time required for the Moon to make one revolution around its orbit with respect to the perigee; the length of the mean anomalistic month as calculated for the year 2000 is 27.55455 days (27d 13h 18m 33s); the actual duration can vary by several days due to the gravitational perturbations of the Sun on the Moon's eccentric orbit.

aphelion - The point along a planetary orbit that is farthest from the Sun; Earth's mean distance at aphelion is 152,097,701 km; Earth's true distance at aphelion varies from 152,083,140 km to 152,104,533 km because of gravitational perturbations of the Sun, Moon and planets; See: Earth at Perihelion and Aphelion: 2001 to 2100.

apogee - The point along the Moon's orbit that is farthest from Earth; the Moon's mean distance at apogee is 363,396 km; the Moon's true distance at apogee varies from 404,042 to 406,725 km because of gravitational perturbations of the Sun and Earth; See: Moon at Perigee and Apogee: 2001 to 2100.

apparent magnitude - The magnitude or brightness of a star as seen from Earth; magnitudes measure brightness on a logarithmic scale where a difference of 5 magnitudes is a factor of 100 times in brightness; the difference in brightness 'x' between two stars of magnitudes 'a' and 'b' is: x = 2.512(a-b)

appulse - The closest approach of one celestial object to another, as seen by an observer on Earth. An appulse usually refers to the close approach of two planets, or to the Moon and a bright star or planet. Although an appulse is related to a conjunction their definitions differ. While an appulse occurs when the separation between two bodies is at its minimum, a conjunction occurs at the instant when the two bodies have the same right ascension or the same ecliptic longitude.

asterism - A prominent pattern or group of stars, typically having a popular name but smaller than (and not officially recognized as) a constellation; an example of an asterism is the Pleiades (M45) in Taurus.

Astronomical Unit - The mean distance of Earth from the Sun and equal to about 92,955,807 miles (149,597,871 km).

constellation - in ancient astronomy, a pattern formed by prominenet stars in the night sky associated with a cultural or mythological person or object; the Greek astronomer Ptolemy identified 48 constellations in his Almagest (2nd century); in modern astronomy, one of 88 internationally defined areas of the celestial sphere (see: 88 Constellations List).

chromosphere - A thin, 2,000 kilometer thick layer lying above the Sun's photosphere and below the corona; the name literally means "sphere of colour" and it was first observed as a red glow during total eclipses; the chromosphere glows red due to a transition in the hydrogen atom when an electron drops from its third to second lowest energy level; the temperature increases dramatically from 4,400 K to 25,000 K between the bottom and top of the chromosphere.

coronal mass ejection - A massive burst of solar wind and magnetic fields rising from the solar corona and released into space; coronal mass ejections (CMEs) originate from active regions on Sun's surface and are often associated with solar flares; near solar maxima the Sun produces about three CMEs every day, whereas near solar minima there is about one CME every five days.

coma - A nebulous envelope of ice and dust surrounding the nucleus of a comet; it forms as a comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate to form the hazy coma; this feature gives a comet a "fuzzy" appearance that distinguishes it from stars.

conjunction - An alignment of two celestial objects in which they share the same right ascension or the same ecliptical longitude, as seen by an observer on Earth. A conjunction usually involves two planets, or to the Moon (or Sun) and a bright star or planet.

Coordinated Universal Time (UTC) - The primary time standard by which the world regulates clocks and time; UTC is one of several closely related successors to Greenwich Mean Time (GMT); for most common purposes, UTC is synonymous with GMT, although GMT is no longer precisely defined by the scientific community; while Universal Time (UT1) is based on the rotation of Earth in relation to distant celestial objects (stars and quasars), UTC is based on International Atomic Time and is adjusted to remain within 1 second of Universal Time (UT1) through the occasional addition of a leap second.

declination - The latitude of a point on the celestial sphere using the equatorial coordinate system; declination divides the sky into 180 degrees (90 degrees north (+) and south (-) of the celestial equator).

diffuse nebula - An interstellar cloud of dust and gas consisting mostly of hydrogen and helium, but other ionized gasses may be present; diffuse nebulae are often associated with star forming regions in which the force of gravity collapses the cloud into clumps and knots, and densities eventually grow high enough to initiate nuclear fusion in the cores of new stars; some of the remaining dust and gas are believed to form planets and solar systems; newly formed open clusters are often found still embedded within diffuse nebulae.

Diffuse nebulae (sometimes called bright nebulae) cover extended areas and have no well-defined boundaries and are sub-divided into either reflection or emmission nebulae; reflection nebulae shine by light reflected from nearby stars; emmission nebulae contain ionized gasses that glow at specific spectral frequencies characteristic of the elements contained within (e.g. - ionized hydrogen dominates in many emmission nebulae which glow red due to hydrogen's strongest spectral line); dark nebulae do not emit or reflect light, but are visible in silhouette against a brighter background of stars or diffuse nebulae; examples of diffuse nebulae include the M8 in Sagittarius and the Great Nebula (M42) in Orion.

draconic month - the average interval between two successive passes of the moon through the ascending node of its orbit and is equal to 27.212220 days (27 d 5 h 5 min 35.8 s); the moon's orbit slowly rotates westward on its axis and precesses over 360 ° in a period of 18.6 years.

Earthshine - reflected earthlight visible on the Moon's night side when the Moon is in the evening or morning crescent phase; Earthshine is also known as the Moon's ashen glow or as the old Moon in the new Moon's arms; for examples, see Earthshine.

ecliptic - The apparent path of the Sun on the celestial sphere as seen from Earth; the plane of Earth's orbit around the Sun as seen projected onto the sky from Earth; the Moon and planets all appear within several degrees of the ecliptic.

ephemeris - A table of values that gives the positions of astronomical objects in the sky over a range of times; the positions of celestial objects (Sun, Moon, planets, etc.) are given in right ascension (celestial longitude) and declination (celestial latitude); the plural is ephemerides.

equinox - The moment in time at which the vernal point, celestial equator, and other elements are used in the definition of a celestial coordinate system; Equinox J2000 is the current standard equinox taken at 2000 January 1 at 12:00:00 Terrestrial Time (TT).

galaxy - An enormous gravitaionally bound stellar system, containing tens to hundreds of millions of stars, and huge clouds of dust and gas; the diameter of a galaxy can range from 2000 to more than 200,000 light years.

Galaxies are divided into three basic categories:

  1. spiral - disk-shaped, with a spherical nucleus of old stars from opposite sides of which arms, containing younger stars, spiral outwards around the nucleus; in a barred spiral the arms originate at the ends of a bar-shaped nucleus; examples of spiral galaxies include the Andromeda Galaxy (M31), and the Triangulum Galaxy (M33).
  2. elliptical - ranging from spherical to strongly elongated elliptical in shape with no spiral arms and consisting of old stars; ellipticals are the most common type of galaxies; examples of elliptical galaxies include M87 and M89 both in Virgo.
  3. irregular - no well defined shape and often chaotic in appearance; examples of irregular galaxies include the Large Magellanic Cloud in Dorado and the Small Magellanic Cloud in Tucana.

geocentric - As seen fron the center of Earth; the geocentric coordinates of a planet is the position of the planet as seen from Earth's center.

globular cluster - A tight, gravitaionally bound collection of stars having a strong spherical symmetry; star density rapidly increases towards the center of the cluster; the stars in an globular cluster form together from the same interstellar cloud so they all have similar ages, distances, and initial elemental compositions (although the masses, temperatures and the stage of stellar evolution can vary considerably from star to star within the cluster); the typical globular is 100 light years in diameter and contains tens of thousands of stars; globular clusters are more than 10 billion years in age making them among the oldest objects in the Milky Way; examples of open clusters include the M13 in Hercules and M22 in Sagittarius.

Greenwich Mean Time (GMT) - A time system originally referring to mean solar time at the Royal Observatory (Greenwich, London), which later became adopted as a global time standard; it is essentially the same as Coordinated Universal Time (UTC) which is a standard astronomical time system (astronomers no longer use the term "Greenwich Mean Time"); for more information, see Greenwich Mean Time (Wikipedia).

Greatest Elongation - The point in an inferior planet's orbit (either Mercury or Venus) where the angular separation between the planet and the Sun (as seen from Earth) reaches a maximum; at greatest eastern elongation, the planet is east of the Sun and appears in the evening sky; at greatest western elongation, the planet is west of the Sun and appears in the morning sky; greatest elongation for Mercury ranges from 18° and 28°; greatest elongation for Venus ranges from 45° and 47° the elongation value varies because of the elliptical orbits of the planets.

Gregorian Calendar - The civil calendar based on 12 months of 365 days, and a leap year of 366 days every 4 years; the Gregorian is a modification of the Julian Calendar - normally, all years divisible by 4 are leap years, but years divisible by 100 are NOT leap years, while years divisible by 400 ARE leap years; the Gregorian Calendar first came into use in 1582 with the Gregorian Calendar Reform (Wikipedia), and currently the internationally accepted civil calendar; for more information, see Gregorian Calendar (Wikipedia).

horizontal parallax - The apparent shift in the Moon's position when viewer by two observers who see the Moon directly in the zenith and on the horizon, respectively; it can also be defined as the angle subtended at the distance of the Moon by the radius of the Earth; also referred to as lunar parallax or lunar horizontal parallax for more information, see lunar parallax (Wikipedia).

Julian Calendar - The civil calendar based on 12 months of 365 days, and a leap year of 366 days every 4 years; the Julian Calendar was used from 46 BCE until the Gregorian Calendar reform in 1582; for more information, see Julian Calendar (Wikipedia).

Julian Day Number - The number of days and fractions elapsed since noon Universal Time on January 1, 4713 BCE; the Julian date is a convenient way of calculating dates in astronomical predictions; the Calendar Date Converter can be used to convert between Gregorian Calendar date, Julian Calendar date and Julian Day Number.

Julian year - A unit of time defined as exactly 365.25 days.

light year - The distance light travels in a vacuum in a period of 1 Julian year; a unit of distance equal to about 5.9 trillion miles (9.5 trillion kilometers).

magnitude - The a measure of the brightness of a celestial body (usually as seen by an observer on Earth); in ancient astronomy, the naked eye stars were divided into 6 categories or magnitudes of brightness with 1st magnitude being the brightest and 6th the dimmest; modern astronomy defines that magnitude 1 is exactly 100 times brighter than magnitude 6; Sirius is the brightest naked eye star at magnitude -1.46.

Milky Way - The galaxy in which our solar system resides; it is a spiral galaxy containing approximately 100 billion stars, has a diameter of 100,000 light years and a thickness of 10,000 light years; visible to the naked eye as a broad, diffuse band running across the sky through some of the brightest constellations (e.g. - Taurus, Orion, Carina, Crux, Scorpius, Sagittarius, Cygnus, Cassiopeia)

open cluster - A loose collection of stars that are gravitaionally interacting; the stars in an open cluster form together from the same interstellar cloud so they all have similar ages, distances, and initial elemental compositions (although the masses, temperatures and the stage of stellar evolution can vary considerably from star to star within the cluster); examples of open clusters include the Pleiades (M45) in Taurus and M41 in Canis Major.

parsec - A unit of distance equal to about 3.25 light years or 19 trillion miles (31 trillion kilometers); a parsec is defined as the distance at which a celestial object exhibits a parallax shift of 1 arc-second when viewed from two different positions separated by the perpendicular distance of 1 Astronomical Unit.

perihelion - The point along a planetary orbit that is closest to the Sun; Earth's mean distance at perihelion is 147,098,074 km; Earth's true distance at perihelion varies from 147,091,358 km to 147,112,452 km because of gravitational perturbations of the Sun, Moon and planets; See: Earth at Perihelion and Aphelion: 2001 to 2100.

perigee - The point along the Moon's orbit that is closest to Earth; the Moon's mean distance at perigee is 363,396 km; the Moon's true distance at perigee varies from 356,355 to 370,399 km because of gravitational perturbations of the Sun and Earth; See: Moon at Perigee and Apogee: 2001 to 2100.

photosphere - A thin layer in the Sun where gas changes from opaque to transparent, thus giving the appearance of a surface; the name literally means "sphere of light" and it forms the visible disk of the Sun seen in white light; the temperature in the photosphere is between 4500 and 6000 K, with an effective temperature of 5777 K.

planetary nebula - An expanding shell of ionized gas surrounding a star late in its life; most stars between 0.8 and 10 solar masses eventually exhaust their hydrogen fuel, expand enormously to become cool red giants, undergo heleium fusion, become unstable and eject their outer atmospheres in the form of a planetary nebula; the lifetime of a planetary nebula is quite short (10,000 years) compared to the lifetime of the star (billions of years) producing it; the typical planetary nebula is about one light year in diameter; there are about 3000 known planetary nebulae in the Milky Way galaxy; examples of planetary nebulae include the M27 in Vulpecula and M57 in Lyra.

right ascension - The longitude of a point on the celestial sphere using the equatorial coordinate system; right ascension divides the sky into 24 sections called hours (1 hour = 15 degrees).

prominence - A large, bright cloud of glowing gas extending outward from the Sun's surface, often in a loop shape; prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona; they are composed of a hot plasma, similar in composition to that of the chromosphere, and typically form over timescales of a day; a stable prominences may persist in the corona for several months; some prominences break apart and give rise to coronal mass ejections.

sidereal time - A time scale that is based on Earth's rotation measured relative to the fixed stars; the sidereal time at a geographic location is equal to the right ascension crossing the local meridian at any given instant.

solar flare - An enormous explosion on the Sun's surface resulting in the ejection of electrons, ions and atoms through the solar corona and into space; a solar flare produces a sudden brightening as large as 1/6 of the total energy output of the Sun each second; they are usually followed by a colossal coronal mass ejection or CME.

solar corona - The outer atmosphere of the Sun extending millions of kilometers into space, most easily seen during a total solar eclipse; the corona is composed of a super heated plasma with temperatures in excess of 2 million degrees K; light from the corona three primary sources: sunlight scattering off free electrons (K-corona), sunlight reflected off dust particles (F-corona), narrow spectral emission lines produced by ions (E-corona).

spectral type - A classification used to describe the temperature and state of ionization in the atmosphere of a star; most stars are classified into one of the following spectral types (from hottest to coolest): O, B, A, F, G, K, and M; see stellar classification (Wikipedia) for more information.

spicule - A dynamic jet of gas occurring in the Sun's chromosphere; spicules are typically 500 km in diameter and last 10-15 minutes; they are usually associated with regions of high magnetic flux.

stellar luminosity - The intrinsic brightness of a star relative to the Sun; for example, a star with a stellar luminosity of 25*L(sol) is 25 times brighter than the Sun.

stellar mass - The intrinsic mass of a star relative to the Sun; for example, a star with a stellar mass of 10*M(sol) is 10 times more massive than the Sun

stellar radius - The physical radius of a star relative to the Sun; for example, a star with a stellar radius of 50*R(sol) has a radius (and diameter) 50 times greater than the Sun.

supernova - The cataslysmic explosion of a highly evolved star; the star's luminosity increases by as much as 20 magnitudes and most of the star's mass is blown away at very high velocity, sometimes leaving behind an extremely dense core; the sudden burst of radiation often outshines the host galaxy, before fading from view over several weeks or months.

Supernovae are divided into two categories:

  1. Type Ia - a binary star system in which a carbon-oxygen white dwarf accrets matter from the companion star; the core of the white dwarf eventually reaches critical density and trigers uncontrolled fusion in a cataslysmic explosion.
  2. Type II - a massive, highly evolved star that exhausts its nuclear fuel and undergoes gravitational collapse, thereby producing a cataslysmic explosion.

supernova remnant - An expanding shell of gas ejected at high speeds by a supernova explosion; the remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way; it is visible as a diffuse gaseous nebula usually with a shell-like structure and may resemble a "bubble"; an example of a supernova remnant is the Crab Nebula (M1) in Taurus.

starscape - A nighttime landscape photograph that features a recognizable object or foreground against backdrop of stars and the Milky Way. This type of photography has been popularized by World At Night (TWAN).

synodic month - The orbital period of the Moon with respect to the Sun; the period of the Moon's phases has a mean duration of 29.530589 days (29 d 12 h 44 min 2.9 s); due to gravitaional perturbations in the Moon's orbit, the synodic month varies by +/- 6 hours from one month to the next; see Phases of the Moon Photo Gallery.

Terrestrial Time (TT) - The modern astronomical time standard defined by the International Astronomical Union, used for time-measurements of astronomical observations made from the surface of Earth; Terrestrial Time is a uniform time scale free of the irregularities of mean solar time; the "Astronomical Almanac" uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth; for more information, see Terrestrial Time (Wikipedia).

Universal Time (UT1) - A time scale based on the rotation of Earth in relation to distant celestial objects (stars and quasars); UT1 is a non-uniform time scale because Earth's rotational period is slowly increasing due to tidal interaction with the Moon; the modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich; for the casual user, UT1 (sometimes referred to simply as UT) is essentially the same as GMT and is often used interchangably; there are several versions of UT including Coordinated Universal Time (UTC); for more information, see Universal Time (Wikipedia).

zodiac - The 12 constellations lying along the Sun's annual path across the sky; the zodiacal constellations consist of Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces. According to the modern constellation borders, the ecliptic also passes through the constellation Ophiuchus making it the thirteenth Zodiacal constellation.

zodiacal light - A faint, diffuse, cone-shaped glow seen during morning or evening twilight that appears to extend up from the vicinity of the Sun along the ecliptic; it is best seen in spring and autumn when the zodiac makes a steep angle to the horizon; caused by sunlight scattered from dust in orbit about the Sun, the zodiacal light is so faint that it can only be seen from very dark skies.