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Magnitude (astronomy), term used in astronomy to designate the brightness of a star . Magnitude, also called apparent magnitude, describes the brightness of a star as viewed from the earth . The term absolute magnitude refers to the brightness of a star as viewed from a standard distance of 10 parsecs, or about 32 light-years. The ancient Alexandrian astronomer Ptolemy originally divided all visible stars into six magnitudes: the brightest were called first magnitude, those barely visible to the naked eye were called sixth magnitude, and the other visible stars were assigned intermediate positions. After the introduction of the telescope in the 17th century, this system of magnitudes was used and extended to the fainter stars in different ways by different astronomers. In the 19th century a standard system was finally adopted under which a star of any given magnitude is 2.512 times as bright as a star of the next higher magnitude; thus, for example, a star of the second magnitude is 2...

Binary Star, two stars that are bound to each other by gravity and orbit about a common center of mass . Binary star systems are quite common and the pairing of stars appears to be random in most cases. Astronomers estimate that approximately one-fourth of the visible stars belong to a binary system. The time it takes for one star to orbit the other can range from hours to centuries depending on the distance between the two stars and their masses. Some binary pairs, called interacting binary systems, are so close that they exchange material. Binary stars are very useful to astronomers because they are the only stars of which astronomers can directly determine mass.

Orbit (astronomy and physics), path or trajectory of a body through space. A force of attraction or repulsion from a second body usually causes the path to be curved. A familiar type of orbit occurs when one body revolves around a second, strongly attracting body. In the solar system the force of gravity causes the moon to orbit about the earth and the planets to orbit about the sun, whereas in an atom electrical forces cause electrons to orbit about the nucleus. In astronomy, the orbits resulting from gravitational forces, which are discussed in this article, are the subject of the scientific field of celestial mechanics. An orbit has the shape of a conic section—a circle, ellipse, parabola, or hyperbola—with the central body at one focus of the curve. When a satellite traces out an orbit about the center of the earth, its most distant point is called the apogee and its closest point the perigee. The perigee or apogee height of the satellite above the earth's surface is often give...

Orbit ->> LAWS OF MOTION Early in the 17th century, the German astronomer and natural philosopher Johannes Kepler deduced three laws that first described the motions of the planets about the sun: (1) The orbit of a planet around the sun is an ellipse. (2) A straight line from the planet to the center of the sun sweeps out equal areas in equal time intervals as it goes around the orbit; the planet moves faster when closer to the sun and slower when distant. (3) The square of the period (in years) for one revolution about the sun equals the cube of the mean distance from the sun's center, measured in astronomical units . The physical causes of Kepler's three laws were later explained by the English mathematician and physicist Isaac Newton as consequences of Newton's laws of motion and of the inverse square law of gravity. Kepler's second law, in fact, expresses the conservation of angular momentum. Moreover, Kepler's third law, in generalized form, can be stat...

Astronomical Unit (AU), unit of distance used in the measurement of orbits and trajectories within the solar system . One AU is the average distance between the earth and the sun . Its value has been established as, roughly, 149,600,000 km (92,956,000 mi) by means of radar -ranging studies of nearby celestial objects such as Venus or passing asteroids; these studies have enabled astronomers to determine the scale of the solar system with great accuracy.

Nebula, in astronomy, a localized conglomerate of the gaseous and finely divided dust particles that are spread throughout interstellar space. Before the invention of the telescope, the term nebula (Latin, “cloud”) was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were called nebulas. Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of formation. Diffuse nebulas are extremely large structures, often many light-years wide, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighboring ...

Globular Cluster, compact, spherical group of stars , containing many thousands or even millions of stars. Astronomers have found more than 200 globular clusters in the Milky Way Galaxy (the earth’s galaxy). Most galaxies contain globular clusters and some galaxies contain thousands of such clusters. Most of the known globular clusters in the Milky Way move around the center of the galaxy in orbits that take them far outside the Milky Way. By finding the center of their orbits, the American astronomer Harlow Shapley of Harvard University located the Milky Way’s center in 1918. Globular clusters are the oldest structures associated with our galaxy. They contain only Population II stars—the oldest stars in the universe. All globular clusters in the Milky Way and the neighboring Andromeda Galaxy seem to be about the same age, suggesting they were created by conditions within galaxies while the galaxies were young. The diameters of globular clusters average about 50 light-years. The stars...

Galaxy, a massive ensemble of hundreds of millions of stars, all gravitationally interacting, and orbiting about a common center. Astronomers estimate that there are about 125 billion galaxies in the universe. All the stars visible to the unaided eye from Earth belong to Earth’s galaxy, the Milky Way . The Sun , with its associated planets, is just one star in this galaxy. Besides stars and planets, galaxies contain clusters of stars; atomic hydrogen gas; molecular hydrogen; complex molecules composed of hydrogen, nitrogen, carbon, and silicon, among others; and cosmic rays (see Interstellar Matter ). Galaxy M100 The spiral galaxy M100 is located between 35 million and 80 million light-years from earth. The Hubble Space Telescope captured this image of the core of M100 after repairs were made to the telescope in December 1993. Galaxies M86 and M84 The elliptical galaxies M86 (center) and M84 (right) are members of the Virgo cluster of galaxies, located about 50 million light-years awa...

Great Andromeda Spiral Galaxy, also known as M 31, large spiral galaxy in the constellation Andromeda , about 2.2 million light-years from Earth. The Great Andromeda Spiral Galaxy is the largest nearby galactic neighbor to the Milky Way Galaxy, Earth’s home galaxy. Because it is so near, it appears very bright, with a total magnitude (a measure of its brightness as seen from Earth) of 3.4, and is easily visible to the naked eye on a clear, dark night. Its immense diameter of about 200,000 light-years makes it appear five times larger than the full Moon in our sky. The Great Andromeda Spiral Galaxy is the most studied of external galaxies because astronomers can view features in it that they believe are also present in the Milky Way, but are made invisible by the Milky Way’s thick intervening clouds of dust. Astronomers study the Andromeda Galaxy’s spiral arms; the birth of stars, dense spherical groups of stars called globular clusters , as well as looser star groupings called open c...

Andromeda (astronomy), in astronomy, large constellation of the northern hemisphere situated just south of the constellation Cassiopeia and west of the constellation Perseus. Andromeda contains no stars of the first magnitude but is noted as the area of sky containing the Andromeda Galaxy, a member of the local group to which our own Milky Way belongs. At a distance of 2.2 million light-years, the Andromeda Galaxy is both the nearest spiral galaxy and the most distant object that can be seen with the naked eye. Before its nature was determined by means of powerful telescopes, it was erroneously believed to be a nebula, or cloud of interstellar matter. Through telescopes it is seen to have two small companion galaxies of elliptical form.

Open Cluster, or galactic cluster, group of associated stars that travel together through space. Astronomers have cataloged about 1200 open clusters in the earth’s galaxy, each containing from ten to many hundreds of stars (see Milky Way ). Astronomers have also discovered hundreds of open clusters in other galaxies. The stars in a galaxy usually orbit in a plane around a common center, and almost all open clusters lie close to the plane of the galaxy in which they’re located. Clusters range from about 5 light-years to about 70 light-years in diameter. Often a thin, misty light, caused by reflection of starlight off the cosmic dust and gas in the cluster, envelops an entire cluster. Open clusters are classified by the number of stars in them and by the degree to which the stars are concentrated toward the center of the cluster. All clusters in the same class are roughly the same size. The stars in an open cluster are usually relatively young, and the cluster is considered the same age...

Great Orion Nebula or Orion Nebula, or M 42, diffuse nebula , or cloud of gas and dust, located in the constellation Orion. With its brightest parts having an apparent magnitude (a measure of brightness as seen from the earth) of 4.0, the Great Orion Nebula can be seen with the unaided eye as part of Orion’s sword, which hangs from his belt. The Great Orion Nebula looks like a star when it is seen with the unaided eye, but even a small telescope reveals its cloudy nature. The entire Great Orion Nebula as seen from the earth is about 60 arc minutes wide, or about four times as wide as the full moon. The Great Orion Nebula is about 1500 light-years from the earth and it is about 30 light-years in diameter. Unlike some diffuse nebulae, which are visible only because they reflect the light of nearby stars, the Great Orion Nebula not only reflects light, but it also emits light. Emission nebulae like the Great Orion Nebula are so huge that they provide enough gas and dust to create new sta...

Orion (astronomy), constellation located on the celestial equator east of Taurus . It is an oblong configuration with three stars in line near its center. It is represented on pictorial charts as the figure of Orion, the hunter in Greek mythology, standing with uplifted club. Three bright stars represent his belt and three fainter stars aligned south of the belt represent his sword. Alpha (a) Orionis, or Betelgeuse , is located in the left corner of the oblong, corresponding to Orion's shoulder. Beta (β) Orionis, or Rigel , is diagonally opposite Betelgeuse. A nebula surrounding the three stars marking Orion's sword is one of the most conspicuous bright nebulas in the heavens. In Greek mythology, Orion is the handsome giant and mighty hunter, the son of Poseidon, god of the sea, and Euryale, the Gorgon. Orion fell in love with Merope, the daughter of Oenopion, king of Chios, and sought her in marriage. Oenopion, however, constantly deferred his consent to the marriage, and O...

Equator, in astronomy, the great circle in which the plane of the equator of the earth intersects the celestial sphere. The celestial equator is the line from which the declination of stars and planets is measured. See Ecliptic .

Magellanic Clouds, small, irregular galaxies that lie relatively near the Milky Way galaxy. The Large Magellanic Cloud (LMC), in the constellation Dorado (the Goldfish), and the Small Magellanic Cloud (SMC), in the constellation Tucana (the Toucan), are visible to the unaided eye in the southern hemisphere and as far north as 16° North latitude. They became known in Europe through descriptions made in 1521 by the Portuguese navigator Ferdinand Magellan, after whom they were named. The LMC lies about 150,000 light-years away from our sun, and the SMC lies about 173,000 light-years away. In the early 1980s, another galaxy, called the Mini Magellanic Cloud (MMC), was determined to lie about 20,000 light-years beyond the SMC, in the same line of sight. Apparently it was torn from the SMC by a near encounter with the LMC about 200 million years ago. A supernova was observed in the LMC in 1987.

Cassiopeia (astronomy), northern constellation , near the celestial pole. It is distinguished by a group of five stars, of second to fourth magnitude , in the form of a rough letter W. The brightest supernova on record appeared in the constellation in 1572 and was observed by the Danish astronomer Tycho Brahe . Brighter than the planet Venus, for about 16 months Cassiopeia was visible to the naked eye even at noon. It is named for the mythological Ethiopian queen Cassiopeia, the mother of Andromeda .

Transit, passage of one heavenly body over the disk of another, as of Mercury or Venus over the disk of the sun, or of a satellite over its primary. A transit of Mercury or Venus can take place only when either planet passes the sun at the time the sun is near one of the nodes of the planet; that is, when Venus or Mercury is in inferior conjunction, or is closer to earth than to the sun. The transit of Venus was first recorded in 1639. In 1679 the English astronomer Edmond Halley pointed out that these transits could be used to determine the distance of the sun. Usually two transits of Venus occur within 8 years of one another; then, after a lapse of 105 or 122 years, another two transits occur within 8 years. Transits of Mercury occur about 13 times in each century. The term transit also refers to an instrument for measuring the passage of an object past the local meridian (see Transit Instrument ). Likewise, the passage of an object past the local meridian is called a transit.

Mirfak, sometimes spelled Mirphak or Marfak, brightest star in the constellation Perseus, the Warrior. The star is also known as Algenib and is designated Alpha Persei. The name Mirfak is derived from the Arabic phrase Mirfaq al Thurayya, the “Elbow Nearest the Many Little Ones.” This refers to Mirfak’s position in Perseus near the Pleiades star cluster. Mirfak lies near the north celestial pole, a point in the sky about which the stars in the northern hemisphere appear to rotate. This appearance is actually due to the rotation of the earth. Observers who are north of latitude 40° north can see Mirfak all night long throughout the year, circling the north celestial pole. For this reason, Mirfak is called a north circumpolar star. Observers in the southern hemisphere can see Mirfak low in the northern sky from September through November. Stars that are visible to the unaided eye, such as Mirfak, belong to the earth’s home galaxy, the Milky Way , and tend to be very bright or relativel...

Pleiades (astronomy), loose cluster of 400 to 500 stars, about 415 light-years from the solar system in the direction of the constellation Taurus . The stars are about 1 light-year apart, on the average, and photographs show them to be surrounded by a nebulosity that shines by their reflected light . The cluster was named by the ancient Greeks after the “Seven Sisters” of mythology . Observers have claimed to be able to see with the naked eye as many as 12 of the stars in the cluster.

Taurus (astronomy) (Latin for “bull”), a constellation , represented pictorially by the forequarters of a bull. It is a zodiacal constellation—that is, a constellation located along the ecliptic, the apparent annual path of the sun across the sky (see Zodiac ). Taurus contains the two famous star groups known as the Hyades, which includes the brilliant red star Aldebaran, and the Pleiades . It also contains the Crab Nebula, associated with the spectacular supernova of the year ad1054.

Supernova, violent explosion that occurs when a large star uses up its supply of fuel, collapses under its own weight, and explodes. A shock wave from this catastrophic event expands into space, followed by a shell of material from the star’s atmosphere. The material blown off contains chemical elements created throughout the star’s lifetime. Debris from supernovas enriches the chemistry of interstellar space with material that becomes part of new stars and planets. See also Astronomy ; Interstellar Matter . Supernovas are rare phenomena—fewer than five supernovas in our Milky Way galaxy have been visible from Earth in the last 1,000 years. Some supernovas can be bright enough to see with the naked eye during the day. They may continue glowing for several weeks or even months after the explosion. Thick clouds of interstellar dust hide some supernovas, but astronomers can detect those by the radio waves that the supernova emits. See also Radio Astronomy. Supernovas occur in all galaxi...

Supernova ->> FORMATION OF A SUPERNOVA The mass of a star determines whether it will end its life in a supernova explosion. During the courses of their lifetimes, all stars convert hydrogen to helium in thermonuclear fusion reactions in their cores. Thermonuclear fusion reactions occur when the intense heat and gravitational force in a star’s nucleus force hydrogen atoms together. The atoms merge, or fuse together, creating helium atoms and releasing large amounts of energy in the form of electromagnetic radiation and heat. Massive stars have faster rates of fusion than smaller stars, so large stars may use up their fuel faster. After most of the hydrogen is used up, a star goes into a carbon-building phase, in which nuclear fusion turns the helium into carbon. After the helium is exhausted, most stars gradually cool until they no longer emit radiation. When a star eight or ten times more massive than the Sun exhausts its helium, however, the nuclear burning cycle is far from com...

Supernova ->> AFTER THE SUPERNOVA All supernova explosions produce clouds of debris and release huge amounts of energy, but Type I supernovas typically completely destroy their parent stars, while Type II explosions usually leave the stellar core behind. The stellar atmosphere of both types expands into space and appears as luminous clouds years, or even centuries, later. These clouds are called supernova remnants. The Crab Nebula is one of the most spectacular supernova remnants. The fate of the stellar core left behind by a Type II supernova depends on the mass of the original star. Normal atoms are made up of positively charged particles called protons, particles with no electric charge called neutrons, and much smaller, negatively charged particles called electrons. If the original star had a mass about ten times that of the Sun, the core collapses with such force that its protons and electrons combine to form neutrons. The resulting body is composed entirely of neutrons, so...

Supernova ->> STUDYING SUPERNOVAS Chinese astronomers recorded supernovas visible from Earth as far back as ad 185. Probably the most well-known ancient supernova is the one that created the Crab Nebula in 1054. From Chinese and Japanese records, astronomers estimate that it was about 20 times as bright as any other star in the night sky. It was visible even during the day for several weeks after it first appeared. The last time a supernova in the Milky Way galaxy became visible from Earth was October 1604. It was bright enough to be seen at night with the naked eye for more than a year. German mathematician Johannes Kepler made detailed observations of the supernova and carefully measured its position. Since then, astronomers have not seen any supernovas in the Milky Way. A number of supernovas have appeared in other galaxies, however. One of the most important supernovas of the 20th century, and the brightest in the sky of the northern hemisphere since 1937, burst into view on...

Star (astronomy), massive shining sphere of hot gas . Of all the stars in the universe , our Sun is the nearest to Earth and the most extensively studied. The stars visible to the naked eye all belong to the Milky Way Galaxy, the massive ensemble of stars that contains our solar system (the Sun and its nine planets ). About 5,000 stars can be seen with the naked eye, although not all of these stars are visible at any given time or from any given place. With a small telescope, hundreds of thousands of stars can be seen. The largest telescopes disclose millions of galaxies, which may each contain over 200 billion stars. Modern astronomers believe there are more than 1 x 1022 stars in the universe (this number is very large, a 1 followed by 22 zeros). The largest stars, if placed at the Sun's position, would easily engulf Earth, Mars, Jupiter, and Saturn. The smallest white dwarf stars are about the size of Earth, and neutron stars are less than about 20 km (about 10 mi) in diamet...

Planet, any major celestial body that orbits a star and does not emit visible light of its own but instead shines by reflected light. Smaller bodies that also orbit a star and are not satellites of a planet are called asteroids or planetoids. In the solar system, there are nine planets: Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune , and Pluto . Planets that orbit stars other than the Sun are collectively called extrasolar planets . Some extrasolar planets are nearly large enough to become stars themselves. Such borderline planets are called brown dwarfs . See also Planetary Science .

Meteor, in astronomy , small solid body known as a meteoroid that enters a planet's atmosphere from outer space and is raised to incandescence by the friction resulting from its rapid motion. Brilliant meteors, known as fireballs, occur singly and generally consist of a luminous head, followed by a cometlike train of light that may persist for several minutes; some, called bolides, have been seen to explode with a sound like thunder. Fainter meteors, called shooting or falling stars, usually occur singly and sporadically. At intervals, however, hundreds of such meteors occur simultaneously and appear to emanate from a fixed point. These swarms are called meteor showers and are named after the constellation in which they seem to have their point of origin. Some appear annually on the same days of each year and are called periodic showers; others occur infrequently at varying intervals. The periods of meteor showers generally coincide with those of certain comets . Most meteors a...

Meteoroid, solid body orbiting the sun, becoming a meteor, or shooting star, if it enters the earth's atmosphere. The vast majority of meteroids are the size of grains of dust, but they range upward in size without any definite limit. The largest can have masses of thousands of tons. See Asteroids ; Comets .

Asteroid, one of the many small or minor rocky planetoids that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter. SIZES AND ORBITS The largest representatives are 1 Ceres, with a diameter of about 1,003 km (about 623 mi), and 2 Pallas and 4 Vesta, with diameters of about 550 km (about 340 mi). The naming of asteroids is governed by the International Astronomical Union (IAU). After an astronomer observes a possible unknown asteroid, other astronomers confirm the discovery by observing the body over a period of several orbits and comparing the asteroid’s position and orbit to those of known asteroids. If the asteroid is indeed a newly discovered object, the IAU gives it a number according to its order of discovery, and the astronomer who discovered it chooses a name. Asteroids are usually referred to by both number and name. About 200 asteroids have diameters of more than 97 km (60 mi), and thousands of smaller ones exist...

Pallas, the second largest asteroid , the second to be discovered. It was first observed by the German astronomer Heinrich Olbers in 1802. It is about 480 km (about 300 mi) in diameter, and it revolves about the sun in 1684 days.

Vesta (astronomy), asteroid orbiting the Sun between the orbits of Mars and Jupiter at a mean distance of 2.36 astronomical units (about 353 million km/about 219.4 million mi) and having a diameter of approximately 385 km (about 240 mi). Vesta is the third largest asteroid and was the fourth asteroid to be discovered. German astronomer Heinrich Olbers found it in 1807. In 1999 the National Aeronautics and Space Administration (NASA) spacecraft Deep Space 1 made observations that suggested that asteroid 9969 Braille, which passes close to Earth, and Vesta are composed of similar materials. Scientists used the information to theorize about the relationship between Vesta and the many meteorites that have struck Earth over time.

Redshift, change, or shift, in the light radiated by an object, such as a star or galaxy, that indicates the object’s motion. Scientists have used redshifts to measure the velocities (speed and direction) of distant galaxies . Knowing the velocities of galaxies helps astronomers understand how the universe is changing. This knowledge allows scientists to interpret the distant past of the universe and to predict the universe’s distant future. Redshift only occurs when an object is moving. Another mechanism can also redden the light of astronomical objects, but it is not considered to be the same as redshift. Dust particles between stars are just the right size to scatter light with short wavelengths more than they scatter light with long wavelengths. As the light of a star passes through a cloud of dust on the light’s way to Earth, more of the long, red wavelengths get through the dust than the short, blue wavelengths do. This makes the star appear redder than it really is, but the lig...

Milky Way, the large, disk-shaped aggregation of stars, or galaxy, that includes the Sun and its solar system . In addition to the Sun, the Milky Way contains about 400 billion other stars. There are hundreds of billions of other galaxies in the universe, some of which are much larger and contain many more stars than the Milky Way. The Milky Way is visible at night, appearing as a faintly luminous band that stretches across the sky. The name Milky Way is derived from Greek mythology, in which the band of light was said to be milk from the breast of the goddess Hera. Its hazy appearance results from the combined light of stars too far away to be distinguished individually by the unaided eye. All of the individual stars that are distinct in the sky lie within the Milky Way Galaxy. From the middle northern latitudes, the Milky Way is best seen on clear, moonless, summer nights, when it appears as a luminous, irregular band circling the sky from the northeastern to the southeastern horiz...

Galaxies have three common shapes: elliptical, spiral, and irregular. Elliptical galaxies have an ovoid or globular shape and generally contain older stars. Spiral galaxies are disk-shaped with arms that curve around their edges, making these galaxies look like whirlpools. Spiral galaxies contain both old and young stars as well as numerous clouds of dust and gas from which new stars are born. Irregular galaxies have no regular structure. Astronomers believe that their structures were distorted by collisions with other galaxies. Astronomers classify the Milky Way as a large spiral or possibly a barred spiral galaxy, with several spiral arms coiling around a central bulge about 10,000 light-years thick. Stars in the central bulge are close together, while those in the arms are farther apart. The arms also contain clouds of interstellar dust and gas. The disk is about 100,000 light-years in diameter and is surrounded by a larger cloud of hydrogen gas. Surrounding this cloud in turn is ...

The Milky Way contains both the so-called type I stars, brilliant, blue stars; and type II stars, giant red stars. Blue stars tend to be younger because they burn furiously and use up all of their fuel within a few tens of millions of years. Red stars are usually older, and use their fuel at a slower rate that they can sustain for tens of billions of years. The central Milky Way and the halo are largely composed of the type II population. Most of this region is obscured behind dust clouds, which prevent visual observation. Astronomers have been able to detect light from this region at other wavelengths in the electromagnetic spectrum, however, using radio and infrared telescopes and satellites that detect X rays . Such studies indicate compact objects near the galactic center, probably a massive black hole . A black hole is an object so dense that nothing, not even light, can escape its intense gravity. The center of the galaxy is home to clouds of antimatter particles, which reveal t...

The Milky Way rotates around an axis joining the galactic poles. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise, and the spiral arms trail in the same direction. The period of rotation decreases with the distance from the center of the galactic system. In the neighborhood of the solar system the period of rotation is more than 200 million years. The speed of the solar system due to the galactic rotation is about 220 km/sec (about 140 mi/sec).

Perseus (astronomy), constellation in the northern hemisphere of the celestial sphere . Perseus is near the constellations Cassiopeia, Andromeda, Aries, and Taurus. The constellation represents Perseus, a mortal hero of Greek mythology. Perseus’s brightest star is the yellow super giant Mirfak . The constellation’s second brightest star, Algol , is the best known of all eclipsing binary stars . It loses and regains more than twice its normal brightness every three days. In 1901 light from an exploding star in the center of the constellation was detected by observers on the earth. The star increased in brightness more than 100,000 times, making it one of the brightest novas of recent centuries. Within three days it faded. Perseus is visible throughout all of the northern hemisphere and the northern two-thirds of the southern hemisphere. It reaches its highest point in the night sky in December.

Constellation (astronomy), in astronomy, any of 88 imagined groupings of bright stars that appear on the celestial sphere and that are named after religious or mythological figures, animals, or objects. The term also refers to the delimited areas on the celestial sphere that contain the named groups of stars. The oldest known drawings of constellations are motifs on seals, vases, and gaming boards from the Sumerians, indicating that constellations may have been developed as early as 4000 bc. The constellation Aquarius was named by the Sumerians after their god of heaven An, who pours the waters of immortality upon the earth. The division of the zodiac into 12 equal signs was known around 450 BC by the Babylonians. The northern constellations known today are little different from those known by the Chaldeans and the ancient Egyptians, Greeks, and Romans. Homer and Hesiod mentioned constellations, and the Greek poet Aratus of Soli (circa 315-c. 245 bc) gave a verse description of 44 con...

Zodiac, imaginary belt in the celestial sphere , extending about 8° on either side of the ecliptic , the apparent path of the Sun among the stars. The width of the zodiac was determined originally so as to include the orbits of the Sun and Moon and of the five planets (Mercury, Venus, Mars, Jupiter, and Saturn) that were known to the people of ancient times. The zodiac is divided into 12 sections of 30° each, which are called the signs of the zodiac. Starting with the vernal equinox and then proceeding eastward along the ecliptic, each of the divisions is named for the constellation situated within its limits in the 2nd century bc. The names of the zodiacal signs are Aries, the Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricorn, the Goat; Aquarius, the Water Bearer; and Pisces, the Fishes. Because of the precession of the equinoxes about the ecliptic, a 26,000-year ...

Celestial Sphere, imaginary sphere of the heavens, with the earth at its center. The sphere forms the basis for the coordinate systems used in assigning positions to objects observed in the sky. It is also used for designating time intervals and for navigation. The equatorial system of coordinates establishes a grid in the celestial sphere that is based on the earth's equator and poles, projected outwards to intersect with the sphere. Because the earth is moving around the sun, the appearance of celestial objects such as stars changes on the sphere from day to day. Thus one particular moment of the year is assigned as the time when the celestial grid is established. This moment is the vernal equinox, when the sun's disk passes directly above the equator and marks the beginning of spring in the northern hemisphere (see Ecliptic ). Celestial latitude is called declination, and celestial longitude is called right ascension in this equatorial system. Right ascension is measured fro...

Day, in chronology, period of time required for one rotation of a celestial body, especially the earth, on its axis. This period is shorter or longer depending on whether the sun or another star is used as a reference point; thus, the sidereal day—the time it takes for the earth to rotate once relative to a star not the sun—is 4 min shorter than the mean solar day. The solar day, measured by the interval between meridian passages of the sun, varies in length because of the variation in speed of the earth in its orbit. In consequence, the length of the solar day is averaged over the period of a year, and the mean solar day thus obtained is used for all civil and many astronomical purposes. Each type of day is divided into exactly 24 hr that vary in length proportionately to the respective type of day. The civil day now begins at midnight, local time. In ancient times, the Babylonian day began with sunrise and with sunset among the Athenians and Jews. The day is still often regarded as ...

Ecliptic, in astronomy, the apparent great-circle annual path of the sun in the celestial sphere , as seen from the earth . It is so named because eclipses occur only when the moon is on or near this path. The plane of this path, called the plane of the ecliptic, intersects the celestial equator (the projection of the earth's equator on the celestial sphere) at an angle of about 23°27’. This angle is known as the obliquity of the ecliptic and is approximately constant over a period of millions of years, although at present it is decreasing at the rate of 48 seconds of arc in each century and will decrease for several millenniums until it reaches 22°54’, after which it will again increase. The two points at which the ecliptic intersects the celestial equator are called nodes or equinoxes. The sun is at the vernal equinox about March 21 and at the autumnal equinox about September 23. Halfway on the ecliptic between the equinoxes are the summer and winter solstices. The sun arrives a...

Eclipse, in astronomy, the obscuring of one celestial body by another, particularly that of the sun or a planetary satellite. Two kinds of eclipses involve the earth: those of the moon, or lunar eclipses; and those of the sun, or solar eclipses . A lunar eclipse occurs when the earth is between the sun and the moon and its shadow darkens the moon. A solar eclipse occurs when the moon is between the sun and the earth and its shadow moves across the face of the earth. Transits and occultations are similar astronomical phenomena but are not as spectacular as eclipses because of the small size of these bodies as seen from earth. (see Transit ). Topics: Lunar Eclipses Solar Eclipses Frequency of Eclipses Observation of Eclipses

Eclipse ->>LUNAR ECLIPSES The earth, lit by the sun, casts a long, conical shadow in space. At any point within that cone the light of the sun is wholly obscured. Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra. The approximate mean length of the umbra is 1,379,200 km (857,000 mi); at a distance of 384,600 km (239,000 mi), the mean distance of the moon from the earth, it has a diameter of about 9170 km (about 5700 mi). A total lunar eclipse occurs when the moon passes completely into the umbra. If it moves directly through the center, it is obscured for about 2 hours. If it does not pass through the center, the period of totality is less and may last for only an instant if the moon travels through the very edge of the umbra. A partial lunar eclipse occurs when only a part of the moon enters the umbra and is obscured. The extent of a partial eclipse can range from near totality, when most of the moon is obscured, to a slight or ...

Black Hole, an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole is so strong that, if the body is large enough, nothing, including electromagnetic radiation , can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black. PROPERTIES The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert Einstein’s general theory of relativity . The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the Sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon, within which matter is forced to rotate with the black hole; in principl...