Astronomical Lexicon

Absolute Magnitude
The brightness a star would appear to have if viewed from a standard distance (10 parsecs, or 32.6 light-years). It is therefore a measure of a star's luminosity.
A maneuver where a spacecraft skims the upper atmosphere of a planet, utilizing the friction between the atmosphere and the spacecraft to slow the spacecraft's velocity, thereby decreasing the spacecraft's orbital period about the planet.
AMateur SATellite - a satellite devoted to amateur radio communications - also known as the OSCAR series of satellites.
Annular Solar Eclipse
A special case of a solar eclipse where the moon does not totally obscure the face of the sun leaving an annulus or ring of the sun to shine around the edges of the moon.
The point on a planet's or comet's orbit at the greatest distance from the Sun.
The point on a satellite's orbit at the greatest distance from its primary.
Apparent Magnitude
The brightness of a star as seen from the earth, a value dependent on both its absolute magnitude and its distance.
An alternative name for a minor planet.
A group of stars which form a familiar shape. The Big Dipper is more properly an asterism which is a part of the constellation of Ursa Major.
Astronomical Unit
An Astronomical Unit is approximately equal to 93,000,000 miles or the mean distance from the sun to the earth.
Autumnal Equinox
The point at which a planet crosses the celestial equator. When the Sun rises directly in the East and sets directly in the West. Day and Night are of equal length. The sun reaches this point in September. In the Northern hemisphere, days will be shorter than nights until March.
Black hole
The remnants of an exploded star greater than 3 solar masses, or more accurately, the region around a "dead" star so dense and gravitationally attractive that light cannot escape from it. Usually detected from its X-ray emissions as matter passes through the accretion disc surrounding the dead star.
A bolide is a meteoroid that explodes or vaporizes at the end of its path through the Earth's atmosphere.
A ball of ices composed of water, ammonia, methane, carbon dioxide, and other substances, also mixed with rocks and dust. As a comet approaches the sun, the comet heats up and the ices melt. The solar wind pushes particles away forming a glowing tail pointing away from the sun.
The point on a planet's orbit at which the Sun is directly between the earth and the planet.
One of the 88 defined regions of the celestial sphere.
Declination (Dec.)
The angular distance of a celestial body north (+) or south (-) of the celestial equator.
Deep Space Network
NASA's Deep Space Network - or DSN - is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe.
Passage of the moon wholly or partly across the sun, or the passage of a satellite wholly or partly through its primary's shadow.
The apparent path of the sun around the celestial sphere, approximately marking the plane of the solar system.
The condition of an inferior planet (Mercury, Venus) when it is at its greatest angular distance from the sun.
The point at which a planet crosses the celestial equator. When the Sun rises directly in the East and sets directly in the West. Day and Night are of equal length. The sun reaches these points in March and September.
Field of View
The area seen through the eyepiece of a telescope or pair of binoculars. As magnification increases, field of view decreases.
Largest assemblages of stars in the universe, from millions of stars to thousands of billions of stars in size. Our galaxy is known as the "Milky Way" galaxy.
GPS Environmental & Earth Science Information System - GENESIS is a suite of data services that brings together the new Earth science products generated by spaceborne Global Positioning System (GPS) receivers on a diversity of new US and international missions.
Globular Cluster
A roughly spherical association of stars apparently of similar age. Usually very old, and comprising tens of thousands to millions of stars. Such clusters occur mainly in the halo of the galaxy and consist of stars formed at an early stage in the galactic condensation process.
Gravity Assist
A process whereby a satellite increases its acceleration by approaching a planet at a grazing angle thereby allowing the planet's gravity to pull the satellite towards it but not capture it in an orbit. The satellite "slingshots" around the planet and increases its speed without using any fuel.
International Association for Astronomical Studies
Inferior Conjunction
An inferior conjunction occurs when an inferior planet is between the earth and the sun.
Inferior Planet
A planet whose orbit is smaller than that of the earth.
The International Scientific Optical Network is a group of observatories in ten countries who have organized to detect, monitor and track objects in space.
Jet Propulsion Laboratory
The Jet Propulsion Laboratory, managed by the California Institute of Technology, is NASA's lead center for robotic exploration of the solar system.
Lagrange Point
On a line connecting the Earth and the Moon there are three points where a small body may exist in an unstable equilibrium if it has a velocity giving it the same period as the Moon. In these points, the sum of the gravitational forces from the Earth and the Moon supplies just the centripetal force necessary for a circular orbit with the period of the moon. These points are called Lagrange points.
An arbitrary measure of distance, taken as the distance traveled by light in one terrestrial year. It is equal to 5,880,000,000,000 miles.
An acronym for Lincoln Laboratory Near Earth Asteroid Research - A robotic telescope whose primary task is to discover and catalog objects in near earth orbits.
An acronym for The Lowell Observatory Near-Earth Object Search - LONEOS is a system designed to find Earth-crossing asteroids (ECAs) and comets (ECCs), collectively known as near-Earth objects (NEOs). These objects can occasionally collide with Earth sometimes with devastating consequences. Finding large NEOs is the first step in averting a collision.
The classification of a star's real (absolute) or apparent brightness.
The meridian is an imaginary line extending from north to south directly over the observers position on earth. An object is said to transit the meridian whe it crosses this imaginary lin heading toward the western horizon.
A meteor is a flash of light in the sky produced by a fragment of rock or metal from space as it hurtles into our atmosphere, at a speed of about 25 miles per second, and is heated to incandescence by friction.
A meteorite is a fragment of rock or metal that has not disintegrated or vaporized during entry and has fallen to Earth.
A fragment of rock or metal still in space.
Meteor Shower
Many meteoroids are known to be orbiting the Sun in streams which follow the orbits of comets. If such a comet orbit crosses, the orbit of the Earth, whenever Earth passes that point in its own orbit, the number of meteors will increase. This is a meteor shower, during which all the meteors seem to come from one direction in the sky, radiating outward from a region called the radiant.
Mars Global Surveyor
Launch: November 7, 1996 - This orbiter has studied the entire Martian surface, atmosphere and interior, and has returned more data about the red planet than all other Mars missions combined.
The Russian space station
MIR Amateur Radio Experiment
Malin Space Science Systems
Malin Space Science Systems (MSSS) was established in 1990 to design, develop, and operate instruments to fly on unmanned spacecraft.
Near Earth Asteroid Tracking (NEAT) - The NEAT discovery team at the NASA/Jet Propulsion Laboratory has a cooperative agreement with the U.S. Air Force to use a GEODSS telescope to discovery near-Earth Objects.
Concentrations of gas and dust which form into dense clouds.There are numerous types of nebulae (e.g. dark, emission, reflection, planetary, galactic, supernova remnants).
The glow found around a star or nebula caused by the ionization of the surrounding space.
New General Catalog - Issued in 1888 and reprinted in 1953 by the Royal Astronomical Society, London. This lists over 7,000 nebulae and star clusters, giving brief notes on each and their exact positions in the sky.
A star that suddenly becomes thousands of times brighter and then gradually fades to its original intensity.
An object, usually a moon, planet, star, or asteroid, passes in front of another stellar object thus covering (or occulting) the object.
Oort Cloud
The Oort Cloud is a vast cloud of gas, dust and icy bodies (such as comets) that encircle the solar system. This cloud surrounds the solar system at distance of 93 trillion miles (100,000 times the distance from the Earth to the Sun).
Open Cluster
A loose galactic cluster of stars, containing at most a few hundred stars.
The condition of the moon or a planet when opposite the sun in the sky as seen from the earth. When the earth is between the planet and the sun.
The path followed through space by a celestial body.
The Panoramic Survey Telescope & Rapid Response System -- PANSTARRS is an innovative design for a wide-field imaging facility developed at the University of Hawaii's Institute for Astronomy.
The apparent displacement of a body against its background when seen from different stations.
A distance of about 3.26 light-years, at which the earth's orbit with a diameter of 186,000,000 miles, would subtend an angle of 1 second of arc.
The partly illuminated outer region of the shadow cast by a solid body from a light source of appreciable diameter.
The point on a satellite's orbit at which it is closest to its primary.
Perhelic Opposition
The point at which two concentric orbits non-circular are at their closest.
The point on a planet's or comet's orbit at which it is closest to the Sun.
The remnants from an exploded star which appears to emit a pulse of energy or radiation as it rotates. Not as massive as a black hole.
A configuration in which two celestial bodies (as the moon and the sun) have an angular separation of 90 degrees as seen from the earth.
Long suspected to be the most distant objects in the universe. Proto-galaxies.
The point in the sky from which a meteor shower appears to come from.
Retrograde Motion
Real or apparent motion of a planet, comet, or satellite in the opposite sense to that usual in the solar system. It can also be applied to binary stars. "One of the more interesting results of parallax appears in the Retrograde Motion of the planets. Mars shows the largest retrograde motion due to its smaller distance from Earth. The Earth moves in its orbit faster than Mars and therefore overtakes Mars about every 26 months. This causes the apparent motion of Mars across the background stars to actually change direction for a short period of time. The apparent motion of Mars is normally eastward due to its orbit about the Sun. But, while the Earth passes Mars due to its smaller and faster orbit, Mars appears to progress westward for a couple of months. Once the Earth overtakes Mars in its orbit, the motion of Mars returns to the normal eastward motion." (from "TheSky - Astronomy Software")
Right Ascension (R.A.)
The celestial equivalent of longitude, measured eastward from the spring or vernal equinox.
Space Amateur Radio Experiment
Seeing is quality of observing conditions which is directly affected by climactic conditions in our atmosphere. When it is cloudy, windy, and the stars are twinkling, the seeing is poor. When the wind is calm, the skies are clear, and the stars appear steady, this is good seeing. The latter conditions are the conditions astronomers during which like to work.
Either of two times of the year when the sun has no apparent northward or southward motion.
Stationary Motion
Real or apparent motion of a planet, comet, or satellite where it appears to remain stationary within its orbit relative to the earth's motion.
Superior Conjunction
A superior conjunction occurs when an inferior planet is on the opposite side of the sun from earth.
An extremely bright nova that emits from ten million to a hundred million times as much light as the sun, estimated to occur in a galaxy about once every 600 years.
Total Lunar Eclipse
A lunar eclipse where the moon is totally obscured by the shadow (umbra) of the Earth.
Total Solar Eclipse
A solar eclipse where the moon totally obscures the face of the sun.
There are three meanings of the word. A star or planet transits when it crosses the meridian; a detail on a planet's disk transits when it is carried across the planet's meridian by its rotation; and a satellite (or its shadow) transits when it crosses in front iof its primary's disk (or stellar/interstellar object).
The region of the shadow, cast by a solid body, in which all direct light from the source is cut off.
Vernal Equinox
The point at which a planet crosses the celestial equator. When the Sun rises directly in the East and sets directly in the West. Day and Night are of equal length. The sun reaches this point in March. In the Northern hemisphere, days will be longer than nights until September.
The point on the celestial sphere directly above the observer.
A zone 18 degrees wide, centered along the ecliptic, inside which the major planets (except Pluto) and many minor planets are always to be found. The twelve constellations through which it passes are called the zodiacal constellations.

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