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The angular distance, usually measured in degrees, of an object above the horizon.
An appearance in astronomy, the period of observation of a planet, or comet.
The point where an object in orbit around the sun is farthest from it.
U.S. space program that included six piloted lunar landings between 1969 and 1972.
The point where an object in orbit around the Earth is farthest from it.
Regions of glowing gas in the upper atmosphere whose molecules are stimulated to emit light by collisions with streams of electrons. Known popularly as the northern lights.
A mostly rocky body less than 620 miles (1,000 kilometers) across that orbits the sun more accurately called a minor planet. Most asteroids orbit the sun between Mars and Jupiter. Asteroids are the source of most meteorites.
An ancient system of beliefs that attempts to explain or predict human actions by the position and interaction of the sun, moon, and planets. It is not a science.
Astronomical twilight
The time when the sun is 12 to 18 degrees below the horizon.
Astronomical unit (AU)
The average Earth-sun distance, equal to 149.60 million kilometers or 92.96 million miles.
A noticeable pattern of such as the Big Dipper or the Pleiades, that is part of a larger constellation.
An imaginary line passing through the center of a body, such as a planet, around which that body spins.
The angular distance, usually measured in degrees, of an object's direction along the horizon starting from north (0 or 360) through east (90), south (180), and west (270).
Binary star
A system containing two or more stars in orbit about one another.
Blue moon
A popular term denoting the second full moon to occur in a calendar month. Blue moons recur every 2.7 years on average with about 37 typically occurring each century. Through 2010 the blue moon months are July 2004, June 2007, and December 2009. Blue moons can occur twice in years where February, the shortest month, has no full moon this next happens in 2018.
A fireball that breaks up during its passage through the atmosphere.
U.S. mission to Saturn, launched in 1997 and, following successful flybys of Venus (1998, 1999), Earth (1999) and Jupiter (2001), due to arrive at Saturn in 2004. It will drop the Huygens atmospheric probe into the atmosphere of Saturn's largest moon, Titan, in early 2005.

Cassini-Huygens web site
Cassini Project information at NSSDC
In its scientific usage, the irregular motion or dynamics of physical systems. Chaotic systems show two defining characteristics -- periods of order interspersed with randomness and evolution that is extremely sensitive to initial conditions. Chaotic behavior is endemic to most, if not all, physical systems, including the atmosphere and solar system.
A global mapping mission to the moon launched in 1994 by the U. S. Department of Defense, with science support from NASA.

Clementine web site
Clementine information at NSSDC
A small body made of ice and rock that orbits the sun, usually much less than 62 miles (100 kilometers) across. As it nears the sun it usually brightens and develops a gaseous halo, or coma, and a tail of gas and dust. Most comets travel in very elongated orbits that keep them far from the inner solar system.

International Comet Quarterly comet information site
Comet Observation Home Page
The alignment of two celestial bodies that occurs when they share similar angles from the sun as measured along the ecliptic. This is also roughly when the bodies appear closest together in the sky.

Inferior conjunction: That point in the motions of the planets Mercury and Venus at which they pass between Earth and the sun.

Superior conjunction: That point in the motions of Mercury and Venus at which they appear in line with the sun on the far side of their orbits as viewed from Earth.

With the sun: That point in the motions of the superior planets at which they appear in line with the sun as viewed from Earth.

One of eighty-eight regions into which astronomers divide the sky, based mainly on earlier divisions formed by historical and mythological figures of Greek and Roman tradition.
The outermost atmosphere of the sun. It is normally hidden by the sun's glare, but can be seen during total solar eclipses or with the aid of coronagraphs. The corona is an extremely hot (up to two million Kelvins), tenuous (about a billion electrons per cubic centimeter), gas arrayed in nearly radial streamers and extending millions of miles from the solar surface. Its shape varies from spherical at the peak of the solar activity cycle to elliptical at the minimum.
A telescope in which an opaque disk blocks light from the solar surface it is thus able to reveal the faint solar corona and objects such as stars, planets and comets that lie very close to the sun. In effect, a coronagraph produces an artificial total solar eclipse.
Coronal hole
A large, tenuous, low temperature region in the solar corona from which extreme ultraviolet and x-ray emission is unusually low. Coronal holes are often found at the sun's poles and persist for about ten solar rotations. High speed (up to 1,000 kilometers per second) components of the solar wind are known to originate from coronal holes.
Coronal Mass Ejection (CME)
Eruptive events in the sun's corona that result in its most rapid and dramatic changes. CMEs can carry up to 10 billion tons of plasma traveling at speeds as high as 2,000 kilometers per second. Three or more may occur on a typical day during the sun's activity maximum, with this rate dropping to about one every ten or so days at solar minimum. They are commonly associated with eruptive prominences and long-lasting soft x-ray events, and sometimes with optical flares and hard x-ray events as well. The faster CMEs are associated with Type II and IV radio bursts.

CMEs resemble smoke rings in satellite coronagraph images. Those directed toward the Earth expand symmetrically around the sun and are termed " halo" events. Such CMEs can excite geomagnetic storms as they sweep past Earth. These storms have been linked to satellite communication failures and, in extreme cases, can induce electric currents that damage electric power transmission equipment.
Double star
Two stars that appear close to one another. They can be physically associated (a binary) or simply appear together from the point of view of an observer on Earth.
A blue-gray light seen during the moon's crescent phases on the portion not illuminated by the sun. The sunlit portion of the Earth is the source.

Earthshine Project
Eclipse, lunar
An event during which the moon enters into the shadow of the Earth as seen from some locations on Earth.

Penumbral lunar eclipse: A lunar eclipse in which the moon remains entirely in the penumbral shadow of the Earth.

Partial lunar eclipse: A lunar eclipse in which the moon partially enters the umbra of the Earth's shadow.

Total lunar eclipse: A lunar eclipse in which the moon passes completely into the umbra of the Earth's shadow.

Fred Espenak's Eclipse Home Page

Eclipse, solar
An event during which the moon passes in front of the sun as seen from some locations on Earth.

Partial solar eclipse: A solar eclipse in which the umbral shadow of the moon completely misses the Earth, but the penumbral shadow does not. At locations outside of the central track of total, annular, and hybrid eclipses the eclipse is commonly said to be partial since maximum obscuration of the sun never occurs.

Total solar eclipse: A solar eclipse in which the moon entirely covers the sun.

Annular eclipse: A solar eclipse in which the moon covers all but a thin ring, or annulus, of the sun.

Hybrid eclipse: A solar eclipse that would otherwise be classified as annular but for a small region along the eclipse's central track that experiences a short total eclipse.

Fred Espenak's Eclipse Home Page

The apparent yearly path of the sun through the sky. Since this apparent motion is actually a reflection of Earth's movement, the ecliptic also marks the plane of Earth's orbit. The moon and planets also roughly follow this path.
Electrophonic sounds
Sound produced through the conversion of radio energy at audible wavelengths by the vibration of objects near the observer, such as hair or eyeglasses. Meteors, lightning and the aurora can produce sound in this way.

Global Electrophonic Fireball Survey
Colin Keay's home page
The date of the year at which the sun's rays illuminate half the Earth, from pole to pole neither the north pole nor the south pole is angled into the sun. This phenomenon occurs on two days of the year, near March 21 and September 23. On these dates, the hours of daylight equal the hours of night (hence the name, meaning " equal night" ). The March equinox is considered the first day of spring in the northern hemisphere the September equinox the first day of fall.
The side of the moon always turned away from Earth.
An extremely bright meteor, usually one brighter than magnitude -4.
A vast collection of billions of stars, gas, and dust held together by the gravity of its members. The galaxy in which the sun resides is called the Milky Way.
U. S. space mission to study Jupiter's atmosphere, moons, and magnetosphere. The Galileo spacecraft arrived at Jupiter in 1995, dropped a probe into the planet's atmosphere, monitored jovian weather and undertook a series of close encounters with the four major moons Io, Europa, Ganymede and Callisto over the next six years, returning a total of over 14,000 images. It will be sent into the jovian atmosphere in 2003.

Galileo Project web site
Galileo information at NSSDC
Gas giants
The planets Jupiter, Saturn, Uranus, and Neptune.
Inferior conjunction
See conjunction.
The angular distance north or south from the Earth's equator measured in degrees. The equator is at 0 and the poles are at 90 N and 90 S.
The distance traveled through space by a beam of light in one year. Light travels at 186,282 miles (299,792 kilometers) per second, so a light-year is 5.88 trillion miles (9.50 trillion kilometers), or 63,240 times Earth's distance from the sun.
The angular distance east or west between the meridian of a particular place on Earth and that of Greenwich, England, expressed either in degrees or time.
One of two successful series of Soviet lunar missions. There were twenty-four missions to the moon in the Luna series. (The other series, Zond, had five lunar missions.) The first image of the farside of the moon was taken by the Soviet Luna 3 spacecraft in 1959.

Luna 3 information at NSSDC
Lunar Prospector
U. S. mission to the moon launched on January 6, 1998. Its instruments were designed to provide global maps and data sets of the moon's composition and magnetic and gravity fields from a low polar orbit. The mission lasted eighteen months and ended July 31, 1999 with a controlled crash landing into a crater at the moon's south pole.

Lunar Prospector web site
Lunar Prospector information at NSSDC
A measure of the relative brightness of stars and other celestial objects. The brighter the object the lower its assigned magnitude. This logarithmic scale is based on the ancient practice of noting that the brightest stars in the sky were of " first importance" or " first magnitude," the next brightest being " second magnitude," etc. In 1854 Norman Pogson formalized this scale and defined a difference of 5 magnitudes to be exactly a factor of 100 in brightness. The faintest naked-eye magnitude visible from a dark site is +6.5, Mars at its brightest is -2.9, Venus at its brightest is -4.7 and the full moon is -12.5.
Mariner 9
U. S. space mission to Mars, launched in 1971, achieved global imaging of the surface, including the first detailed views of the martian volcanoes, Valles Marineris, the polar caps, and the satellites Phobos and Deimos. The spacecraft gathered data on atmospheric composition, density, pressure and temperature and also on surface composition, temperature, gravity and topography of Mars.

Mariner 9 information at NSSDC
Mariner 8 and 9 overview at JPL
Mars Global Surveyor
U.S. mission to Mars launched in 1996. Its main instruments include a camera, laser altimeter, thermal emission spectrometer and magnetometer. Image resolution is several times better than any of those taken by the Viking Orbiter cameras, enabling features just a few meters across to be seen. An extended mission phase began in 2001.

Mars Global Surveyor web site
Mars Odyssey
U.S. mission to Mars launched in 2001. Its main instruments include gamma-ray and neutron spectrometers, a neutron detector and a thermal imaging camera. In February 2002, early results indicated large amounts of hydrogen, implying the presence of frozen water, in the martian soil.

Mars Odyssey web site
Mars Pathfinder
U.S. mission that on July 4, 1997, landed successfully on Mars, returning over 16,000 images during a four-month-long mission. An autonomous rover named Sojourner explored the area beyond the lander.

Mars Pathfinder web site
The streak of light caused by a solid body in orbit about the sun (a meteoroid) passing through the atmosphere also called a " shooting star." A meteorite is a meteoroid that strikes the surface of a planet or moon.
Meteor shower
The appearance of many within a few hours that seem to radiate from the same region of the sky. They occur when the Earth passes through the dusty debris near a orbit.
Milky Way
A faint band of light around the sky composed of vast numbers of stars too faint to see individually. Also, the name of the galaxy in which the sun resides.
A natural satellite orbiting a planet. Also, the name of Earth's natural satellite.
Nautical twilight
The time when the sun is 6 to 12 degrees below the horizon. The sky begins to brighten noticeably.
Near Earth Asteroid Rendezvous (NEAR)
U.S. space mission launched on February 17, 1996. Placed into orbit around asteroid 433 Eros on February 14, 2000, NEAR returned data on the body's bulk properties, composition, mineralogy, morphology, internal mass distribution and magnetic field. On February 12, 2001, it became the first spacecraft to land on an asteroid.

NEAR mission web site
The side of the moon that always faces Earth.
A cloud of gas and dust, sometimes glowing from the light of nearby stars and sometimes a dark patch that blocks starlight. New stars are born within a nebula.
A star which suddenly erupts, greatly increasing its brightness.
Of a comet, the solid ice-rock mixture at the center of a comet's gaseous head and tail. Of a spiral galaxy, the dense central portion made of older, redder stars. (plural: nuclei).
The point in a planet's orbit at which it appears opposite the sun in the sky. A planet at opposition is visible all night long. Because they orbit closer to the sun than Earth, Mercury and Venus never reach opposition.
The point where an object in orbit around the Earth is nearest to it.
The point where an object in orbit around the sun is nearest to it.
The cycle of varying shape in the sunlit portion of a planet or moon. The moon, Venus, and Mercury all show phases as seen from Earth.
The visible surface of the sun.
A body of substantial size held in orbit by the gravity of a star. A planet shines by reflecting the star's light.
Radio signals transmitted to and bounced back from an object. It stands for Radio DetectionAnd
The point in the sky from which shower meteors seem to appear.
Energy transmitted through space as waves or particles.
Retrograde motion
The apparent backward (westward) loop in a planet's motion across the sky. All planets display retrograde motion, but that of Mars is most striking.
A natural or artificial body in orbit around a planet.
A tremulous effect of starlight -- twinkling -- caused by its passage through our turbulent atmosphere. Planets usually don't exhibit this effect.
Sidereal period
The time taken by a planet to complete one revolution around the sun (or for the moon to complete an orbit around the Earth) as measured by reference to the background stars.
Solar wind
A stream of electrically charged particles (mainly electrons and ionized hydrogen) moving outward from the sun with velocities in the range 180-310 miles (300-500 kilometers) per second.
The date of the year at which either the Earth's north or south pole is angled most directly toward the sun. This occurs on two days of the year, near June 21 and December 21. The June solstice, the longest day of the year in the northern hemisphere, is considered the first day of summer there the sun makes its most northerly arc through the sky. The December solstice, marks the start of northern winter and is the shortest day of the year the sun then makes its most southern arc through the sky.
A hot, glowing sphere of gas, usually one that emits energy from nuclear reactions in its core. The sun is a star.
A magnetic disturbance on the sun. It is cooler than the surrounding area and, consequently, appears darker.
Superior conjunction
See conjunction.
An enormous stellar explosion that increases the brightness of a star by a factor of more than 100,000. Although the star itself is destroyed, a small portion of its central core may survive as a neutron star.
Synodic period
The average time between successive returns of a planet or the moon to the same apparent position relative to the sun -- for example, new moon to new moon, or opposition to opposition.
The edge of the sunlit portion of the moon or planets the line between day and night.
An amount of power equal to 1 x 1012 watts.
Terrestrial planets
Mercury, Venus, Earth, and Mars.
Periodic changes in the shape of a planet, moon, or star caused by the gravity of a body near it.
A dimly visible path left in the sky by the passage of a meteor.
The passage of a planet across the face of the sun. From Earth, only Mercury and Venus can transit.
See scintillation.
Variable star
A star that exhibits significant brightness changes.
U.S. mission to Mars, composed of two spacecraft, launched in 1975. Viking 1 and Viking 2 both consisted of an orbiter and a lander. Primary mission objectives were to obtain high-resolution images of the Martian surface (55,000 were returned by the orbiters, 1,400 from the landers), characterize the structure and composition of the atmosphere and surface and search for evidence of life.

National Air and Space Museum exhibit
Viking Project Information at NSSDC
U.S. mission consisting of two spacecraft launched in 1977 to explore Jupiter, Saturn, their moons, rings, and magnetic environments, each taking two years to reach Jupiter. Voyager 2 went on to explore Uranus and Neptune as well. Each spacecraft took two years to reach Jupiter. The last image was taken in 1989 and now both spacecraft are headed out of the solar system.

Voyager Project web site
Voyager information at NSSDC
Voyager information at USGS
White dwarf
A collapsed object formed from a star that has exhausted its nuclear fuel. The sun will one day become a white dwarf.
The point directly overhead, 90 above the horizon.