Glossary

University of Alberta observatory domes

Home
Up

"Thank you so much for visiting our class on Friday! The kids loved it...they thought it was pretty cool to meet a "real" Astronomer! Thanks again, Janine"

Updated March 14, 2011

Search for:

Altitude: Angular elevation of a point in the sky, measured in degrees above the horizon (horizon = 0, the zenith 90. Often used in conjunction with azimuth to pinpoint the celestial position of an object seen from a specific location at a specified moment in time. 

Aphelion: The point in a body's orbit which is furthest from the Sun.

Apollo asteroid: A minor planet whose orbit passes within that of Earth, but whose average distance from the Sun is greater than 1.0. astronomical unit.

Asterism: A group of relatively bright stars forming an easily-recognized pattern. Not constellations as such, asterisms often form part of constellations (such as the Big Dipper in the larger constellation of Ursa Major, the Great Bear), or encompass stars from more than one constellation (such as the Summer Triangle which consists of the brightest stars from three different constellations, Cygnus the Swan, Lyra the Harp, and Aquila the Eagle).

Asteroid: a.k.a. minor planet. Rocky body much smaller than a planet. There are currently over 10,000 known asteroids, 95% of which are situated between the orbits of Mars and Jupiter with an orbital period of 3-6 years. The other 5% of asteroids have much different orbits, some of which cross Earth's orbit, the so-called Apollo and Aten asteroids. Included in this number are the Potentially Hazardous Asteroids.

Astronomical unit (A.U.): A measure of distance defined as the average distance between Earth and the Sun. 1 AU is equal to 149,597,870 km, or 499.005 light-seconds.

Aten asteroid: A minor planet whose orbit passes within that of Earth, but whose average distance from the Sun is less than 1.0. astronomical unit.

Azimuth: Angular distance in the horizontal plane, measured clockwise from north. (North = 0, East = 90, South = 180, West = 270). Often used in conjunction with altitude to pinpoint the celestial position of an object seen from a specific location at a specified moment in time. 

Bolide: an exploding or fragmenting meteor or fireball.

Carbonaceous chondrite: A fragile class of meteorite containing organic compounds of carbon, hydrogen, oxygen and nitrogen. Bombardment by such bodies may have seeded Earth with the amino acids and other compounds necessary for the origin of life. 

Celestial sphere: The sky envisioned as a (hemi)sphere at infinite focus. This is how it actually appears, although in reality all observable objects are at widely varying distances.

Circumpolar: A circle of sky which is permanently above the observer's horizon, with stars that never set. (Literally, "around the pole".) This is a function of the observer's latitude; e.g. in Edmonton, with a latitude of 54 N., the North Star, Polaris, appears permanently fixed 54 above the northern horizon; all stars within 54 of Polaris do a daily wheel around it without ever setting. There is a commensurate 54 circle of the southern sky which never rises. 

Comet: One of a large number of minor bodies of the solar system which nonetheless is of great interest and importance. Although individually very tiny, comets are so numerous that their total mass is thought to approximately equal that of the major planets combined. There are two vast reservoirs of comets, the so-called Kuiper Belt outside the orbit of Neptune, and the Oort Cloud, a sphere of cometary bodies extending halfway to the nearest star. Of greatest interest are those comets which have been pulled toward the Sun through gravitational tugs of the giant planets or even other stars. As it approaches the Sun, a comet's nucleus of ice, dust, amino acids, and other primordial material sublimates (evaporates directly from solid to gaseous state) material from its surface layer. From the tiny (1-10 km) nucleus can form an enormous temporary atmosphere, called the coma or head of the comet, on the order of 100,000 km across, and a tail of ions and dust ejected from the comet nucleus and streaming away from the Sun for millions of km. Although exceedingly tenuous, comets are highly reflective and are sights of rare beauty. (See also: Periodic comet)

Constellation: A specific area of the sky, generally characterized by imaginary figures or patterns formed by the brightest stars therein. Since 1930 the sky has been divided into 88 formal constellations by the International Astronomical Union. Although each can be envisioned as an area on the celestial sphere, a constellation is actually a three-dimensional volume of space that in addition to its stars, contains (temporary) foreground objects like the Sun, Moon, and planets, to distant background objects like galaxies and quasars. There is a great deal of cultural myth, legend, and lore about the constellations.

Declination: One of two coordinates (see also: right ascension) used to define the position of an object on the sky. Declination (Dec) is the angular distance of the object north (+) or south (-) off the celestial equator. Equivalent to terrestrial latitude. 

Diurnal motion: The apparent motion of a star or point on the sky caused by the daily rotation of Earth.

Duration: The length of time a meteor shower is at strength or greater. Ranges from a fraction of a day (Leonids) to several days (Perseids, Orionids). The interval between the earliest and latest stragglers in a shower can be much greater.

Ecliptic: The reference plane of the solar system, defined by the Sun and Earth; as seen from Earth, the centre of the Sun is exactly on the ecliptic. The major planets and the Moon can always be found within a few degrees of this plane. Smaller bodies of the solar system are much less restricted to this zone, especially comets and their related meteoroid streams which can be at virtually any angle, and either prograde or retrograde.

Fall ellipse: The location of a number of meteorite fragments from a single fall will spread out over an area in the shape of an ellipse. Generally the most massive fragments will travel the furthest and collect at one end of the ellipse, the least massive at the other.

Fireball: An extremely bright meteor, generally defined as exceeding the magnitude of Venus (mag. -4).

Forward scatter: The reflection (re-radiation) of a radio signal off the ionization train of a meteor from a transmitter to a separate receiving station.

Inclination: The angle formed between the orbital path of a solar system body and the ecliptic plane. Planets move in prograde orbits and generally conform to the ecliptic, with low inclinations of a few degrees. Comets and meteoroid streams are much less confined, and can intersect the Earth-Sun plane at any angle. A perfect retrograde orbit would have an inclination of 180, while one intersecting perpendicularly (at right angles) from the north or south would be considered to be 90 or 270 respectively.

Ion: An atom that has lost or (less commonly) gained one or more electrons and which therefore contains an electric charge.

Ionization: The process by which electrons are removed or added to atoms, thereby turning them into ions.

Ionization train: A persistent glow along the path of a meteor or fireball caused by the ionization of the upper atmosphere caused by the collision of a meteoroid.

Kuiper Belt: A large loose ring of comets and asteroids beyond the orbit of Neptune, which is the main supplier of short period comets. Postulated by Gerard Kuiper in 1951, the first Kuiper Belt (aka 'Trans-Neptunian') object was observed in 1992. Today there are tens of thousands of known KBOs. Some astronomers believe Pluto should be more correctly classified as the first and largest of these.

Light pollution: The degradation of the night sky caused by the combined glow of artificial light, particularly light which is spilled upwards due to poor design. In urban locations light pollution can reduce the limiting magnitude by two or more full magnitudes, reducing the number of stars visible by a factor of 10 and completely obscuring diffuse light such as that cast by our own galaxy, the Milky Way.

Light year: A measure of distance defined as the distance light, traveling at the speed of 299,792,458 metres per second, travels in one year, 9.46 trillion kilometres. Because interstellar distances are so vast, a much larger measuring stick is required.

Limiting magnitude: A measure of observing conditions (sky transparency) by identifying the faintest stars visible.

Long-period comet: A comet with an orbital period of greater than 200 years.

Main asteroid belt: A ring of minor planets loosely collected in an irregular ring between Mars and Jupiter. The source of Potentially Hazardous Asteroids and Apollo Asteroids.

Magnitude: Scale used by astronomers as a measure of the brightness of astronomical objects, based on a system developed by the Greek astronomer Hipparchus, in which he considered the brightest stars to be of the first magnitude, and the dimmest the eye could see to be of the sixth. This system has been refined using a logarithmic scale, where 5.0 magnitudes equals a factor of 100; a difference of a single magnitude is therefore the fifth root of 100, or 2.512. Stars and other objects can be measured to accuracies of .01 magnitude using modern photometric techniques. Counter intuitively, because magnitudes of lower numbers are of greater brightness, the most brilliant objects of all carry negative magnitudes: Sun = mag -27; full Moon -12.5; half Moon -10; Venus -4; Jupiter -2. The brightest stars are approximately magnitude 0. All can be used as comparison objects when estimating the brightness of a meteor or fireball.

Major shower: A meteor shower with a Zenith Hourly Rate (ZHR) of greater than 10 per hour.

Meteor: The light phenomenon which results from the entry into the Earth's atmosphere of a solid particle from space (a meteoroid). Popularly referred to as shooting stars or falling stars.

Meteor shower: A regular (typically annual) event in which the orbit of Earth intersects that of a meteoroid stream, causing a number of meteors with approximately parallel trajectories which appear to emanate from the same radiant.

Meteor storm: A high intensity meteor shower, generally accepted where observed rates of meteors exceed 1,000 per hour. Storm conditions are rare and typically of short duration.

Meteorics: The study of meteors.

Meteorite: An object of extraterrestrial origin which survives passage through Earth's atmosphere as a bolide or fireball without being completely vaporized, so that one or more pieces reach the ground intact.

Meteoritics: The study of meteorites.

Meteoroid: A solid object moving in interplanetary space, much smaller than an asteroid but much larger than an atom or molecule.

Meteoroid stream: A stream of solid particles released from a parent body, typically a comet or asteroid, so that they share common orbital characteristics. A fully developed meteoroid stream will have a reasonably equal distribution of particles throughout its orbit, which can be millions of km across, and is best visualized as the broad wake of a comet. In some cases the parent body will have completely disappeared, or have had its orbit changed from that of the stream through a gravitational encounter with a giant planet such as Jupiter.

Meteoroid swarm: A clump of meteoroids which have been relatively freshly released and therefore remain densely packed and fairly close to the parent body. They are responsible for meteor storms.

Micrometeorite: A particle of interplanetary dust too small to burn up in Earth's atmosphere as a meteor, which rains down gently to Earth's surface. Earth accumulates thousands of tonnes of matter through this process annually. 

Minor planet: See asteroid.

Minor shower: A meteor shower with a Zenith Hourly Rate (ZHR) of fewer than 10 per hour.

Observed rate: A raw count of actual meteors observed by a single observer per unit period (typically per hour). For scientific comparison purposes, an observed rate is converted to a Zenith Hourly Rate by factoring out local variables such as sky conditions (limiting magnitude), cloud cover and altitude of the radiant. Since these variables have a deleterious effect, observed rates are almost invariably lower, often by a factor of two or more, than predicted rates which are expressed in terms of ZHR.

Oort cloud: A vast, roughly spherical reservoir of comets at the outer limit of the solar system. Postulated by Jan Oort in 1950, the Oort Cloud is thought to be the source of the long period comets.

Orbit: The path through space of any object - planet, moon, artificial satellite, comet - or group of objects - planetary rings, meteoroid streams - that is moving in the gravitational field of another object. An object orbiting under the influence of a single mass must move in a circle, an ellipse, a parabola or a hyperbola (the "conic sections"). Most objects in the solar system have elliptical (egg-shaped) orbits; those of the planets are highly circularized, those of comets and meteoroid streams highly elliptical (cigar-shaped).

Overdense trail: One of two types of meteor train detectable by radio using the forward scatter method. Reradiated signals from overdense trails (greater than 2 x 1014 electrons per metre) have higher amplitude and longer duration than underdense trails, but destructive interference due to reflection from different parts of the trail can produce fluctuations in the signal.

Parallax: The apparent movement of an object across the sky when seen from two different points, which can then be used to calculate the distance to the object by triangulation.

Path: The projection of the line of motion of a meteor against the background of the celestial sphere, as seen by the observer. Observers at different locations can see the same meteor project different apparent paths; accurate measurement thereof can allow the meteor's actual position in three-dimensional space to be determined using triangulation.

Peak: The time of greatest intensity of a meteor shower or meteor storm. The peak of a shower can be predicted with a reasonably high degree of precision.

Perihelion: The point in a body's orbit which is closest to the Sun.

Periodic comet: A "captured" comet which makes repeated passages through the inner solar system. Those with an orbital period of less than 200 years are considered short-period comets. Because comets are small, volatile, and unstable, they typically survive only about 100 perihelion passes. The 150 or so known short-period comets represent only about 4% of all known comets, but are responsible for almost all meteor showers due to the repetitious nature of their orbits. The best known periodic comet is Comet Halley, with a period of 76 years; the shortest period is that of Comet Encke, with a period of 3.3 years.

Persistent Train: A glow along the path of a meteor which persists for greater than one second. A fireball can leave a train with a duration of several minutes.

Photographic meteor detection: The capture of meteors on photographic film. This enables the path of the meteor to be determined with a high degree of accuracy.

Planetesimal: In the formative stages of the solar system, a clump of matter massive enough to attract one another gravitationally rather than meeting simply by chance.

Plasma: A gas containing free ions and electrons, and therefore capable of conducting electric currents.

Potentially Hazardous Asteroids: PHAs are currently defined based on parameters that measure the asteroid's potential to make threatening close approaches to the Earth. Specifically, all asteroids with an Earth Minimum Orbit Intersection Distance of 0.05 astronomical unit or less and are larger than about 150 m (estimated based on their reflectivity) in diameter. This "potential" to make close Earth approaches does not mean a PHA will impact the Earth. It only means there is a possibility for such a threat. There are currently some 450 known PHAs, a number which is growing monthly.

Prograde: An orbit or rotation that is oriented in the same direction as that of Earth and other planets. 

Radiant: The direction in the sky from which a meteor enters the atmosphere and from which shower meteors appear to radiate. Meteor shower names are generally derived from the name of the constellation in which the radiant resides. The radiant is a function of the angle of intersection of the Earth's orbit and that of the meteoroid stream.

Radio astronomy: Study of the Universe using radio waves.

Radio meteor detection: The detection of meteors using radio wavelengths and equipment. The primary method available to amateurs is forward-scatter observations, in which signals from a distant transmitter (an FM radio station beyond the horizon works well) are reflected by a meteor's ionization train to enable a brief reception of the signal at a remote receiver. A second method uses radar observations, also known as radio-echo or back-scatter, carried out by professional astronomers, which enable more precise measurement of radiants and the orbits of meteoroid streams. With this technique radio waves are reflected back to the transmitter.

Radio observatory: Any system which makes astronomical observations at radio wavelengths, generally requiring an antenna, receiver, and observer/data recorder.

Retrograde: An orbit or rotation that is oriented in the opposite direction of that of Earth and the other planets. 

Right ascension: One of two coordinates (see also: declination) used to define the position of an object on the sky. Right ascension (R.A.) is the angular distance of the object eastward from the vernal equinox, measured not in degrees but in terms of time: hours minutes and seconds. (1 hour = 15 arc degrees.) Equivalent to terrestrial longitude, with the vernal equinox playing the role of the Greenwich meridian.

Short-period comet: A comet with an orbital period of less than 200 years.

Sporadic meteor: An isolated meteor not associated with a shower.

Telescopic meteor detection: A method of visual observation using a telescope, or more commonly binoculars, to enable the detection of fainter meteors. The drawback is the great restriction of the observer's field of view.

Trail: The short-term luminous glow left in the path of a meteor, a.k.a. a wake. Trails lasting more than one second are referred to as trains.

Train: A glow along the path of a meteor which persists for greater than one second, a.k.a. a persistent train. A fireball can leave a train with a duration of several minutes.

Trajectory: The line of motion of a meteor relative to Earth, considered in three dimensions.

Triangulation: The process of determining the height and distance of a target by measuring the difference in its angular position as seen from two or more sites of known (or measurable) separation. Accurate measurements to the target can then be made using trigonometry.

Underdense trail: One of two types of meteor train detectable by radio using the forward scatter method. Reradiated signals from underdense trails (fewer than 2 x 1014 electrons per metre) rise above the receiver noise almost instantaneously and then decay exponentially. The duration of many meteor bursts is about a second or less.

Universal Time (UT): The standard clock for astronomical purposes, synonymous for all practical purposes with Greenwich Mean Time (GMT). Many astronomical events are listed in UT, from which an adjustment must be made for local time. For example, Mountain Standard Time is 7 hours earlier than UT, Mountain Daylight Time 6 hours earlier. Therefore 1h UT on Nov. 13 = 1800 (6 p.m.) MST on November 12. 

Velocity: A measure of both the speed with which an object is moving and its direction.

Visual meteor detection: The monitoring of meteor activity by an observer or group of observers using the unaided eye. While less precise than other (electronic) methods, it is the easiest to carry out. Large numbers of recorded observations provide statistically significant results.

Wake: See trail.

Zenith: The point in the sky directly over the observer's head, and 90 from the horizon in every direction.

Zenith hourly rate (ZHR): An estimated number of shower meteors a single observer would see under virtually perfect conditions: zero clouds, a limiting magnitude of 6.5, and the radiant at the zenith. As the last of these never occurs in practice, and would be instantaneous in any event, the ZHR is always higher than the observed rate of meteors, typically by a factor of two or more depending on other restrictions such as moonlight, clouds and/or haze, and light pollution.

 

Copyright 1999-2015 by Sky Scan, Edmonton, Alberta, Canada. 

We gratefully acknowledge the financial support of the 

Edmonton Centre of the Royal Astronomical Society of Canada, Department of Physics (University of Alberta)

and the

Natural Sciences and Engineering Research Council of Canada

Home    Site Map   Search   FAQ    Links   About Sky Scan   Webmaster  Contact Us