:: wikimiki.org ::

Inferior Planet

Inferior planet

The terms "inferior planet" and "superior planet" were coined by Copernicus to distinguish a planet's orbit's size in relation to the Earth's.
- "Inferior planet" is used in reference to Mercury and Venus, which are closer to the Sun than the Earth is.
- "Superior planet" is used in reference to Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, which are farther from the Sun than the Earth is. The terms are sometimes used more generally: for instance, the Earth is an inferior planet as seen from Mars. This classification should not be confused with the terms inner and outer planet, which designate those (rocky) planets which lie inside the asteroid belt and those (gaseous) that lie outside it, respectively. Category:Planets of the Solar System

Nicolaus Copernicus

Mikołaj Kopernik (February 19, 1473–May 24, 1543), more commonly known by the Latin form Nicolaus Copernicus, was a Polish astronomer, mathematician and economist, mainly remembered for developing a scientifically useful heliocentric (Sun-centered) theory of the solar system. His main occupations and services rendered were in Royal Prussia, as church canon, governor and administrator, economist, jurist, physician, astrologer and military leader (conducting defense against the Teutonic Order). Amid all his responsibilities, he treated astronomy as a hobby. His theory about the Sun as the center of the Universe, opposed to the traditional geocentric theory that placed Earth at the center, is considered one of the most important discoveries ever, and is the fundamental starting point of modern astronomy and modern science itself (it inaugurated a scientific revolution). His theory affected many aspects of human life, opening the door for young astronomers, scientists and scholars to take a skeptical attitude toward established dogma.

Biography

dogma Copernicus was born in 1473 at Toruń (Thorn) in Polish Royal Prussia. His father Nikolas, a citizen of Kraków (Cracow), then capital of Poland, had moved to Toruń in 1460 once the war with the Teutonic Knights was concluded, and had become a respected citizen of that city. Copernicus was ten when his father, a wealthy businessman and copper trader, died. Little is known of his mother, Barbara Watzenrode, who appears to have predeceased her husband. Copernicus' maternal uncle, Lucas Watzenrode, a church canon and later Prince-Bishop governor of Warmia, reared him and his three siblings after the death of Copernicus' father. Copernicus' brother Andreas became a canon at Frombork (Frauenburg). A sister, Barbara, became a Benedictine nun, and the other sister, Katharina, married a businessman and city councillor, Barthel Gertner. In 1491 Copernicus enrolled at the Jagiellonian University in Kraków, and here for the first time encountered astronomy, thanks to his teacher Albert Brudzewski. This science soon fascinated him, as shown by his books (later carried off as war booty by the Swedes during The Deluge, and now at the Uppsala University Library). After four years at Kraków, followed by a brief stay at Toruń, he went to Italy, where he studied law and medicine at the universities of Bologna and Padua. His bishop-uncle financed his education and wished for him to become a bishop as well. However, while studying canon and civil law at Ferrara, Copernicus met the famous astronomer, Domenico Maria Novara da Ferrara. Copernicus attended his lectures and became his disciple and assistant. The first observations that Copernicus made in 1497, together with Novara, are recorded in Copernicus' epochal book, De revolutionibus orbium coelestium. De revolutionibus orbium coelestium, by Bertel Thorvaldsen]] In 1497 Copernicus' uncle was ordained Bishop of Warmia, and Copernicus was named a canon at Frombork (Frauenburg) Cathedral, but he waited in Italy for the great Jubilee of 1500. Copernicus went to Rome, where he observed a lunar eclipse and gave some lectures in astronomy or mathematics. He would thus have visited Frombork only in 1501. As soon as he arrived, he requested and obtained permission to return to Italy to complete his studies at Padua (with Guarico and Fracastoro) and at Ferrara (with Bianchini), where in 1503 he received his doctorate in canon law. It has been supposed that it was in Padua that he encountered passages from Cicero and Plato about opinions of the ancients on the movement of the Earth, and formed the first intuition of his own future theory. His collection of observations and ideas pertinent to his theory began in 1504. Having left Italy at the end of his studies, he came to live and work at Frombork. Some time before his return to Warmia, he had received a position at the Collegiate Church of the Holy Cross in Wrocław (Breslau), Silesia, which he would resign a few years before his death. Through the rest of his life he made astronomical observations and calculations, but always in his spare time and never as a profession. Copernicus worked for years with the Prussian diet on monetary reform and published some studies about the value of money; as governor of Warmia, he administered taxes and dealt out justice. It was at this time (beginning in 1519, the year of Thomas Gresham's birth) that Copernicus came up with one of the earliest iterations of the theory now known as Gresham's Law. During these years he also traveled extensively on government business and as a diplomat, on behalf of the Prince-Bishop of Warmia. In 1514 he made his Commentariolus — a short handwritten text describing his ideas about the heliocentric hypothesis — available to friends. Thereafter he continued gathering evidence for a more detailed work. During the war between the Teutonic Order and the Kingdom of Poland (1519–1524) Copernicus successfully defended Allenstein (Olsztyn) at the head of royal troops besieged by the forces of Albert of Brandenburg. Albert of Brandenburg In 1533 Albert Widmanstadt delivered a series of lectures in Rome outlining Copernicus' theory. By 1536 Copernicus' work was already in definitive form, and some rumors about his theory had reached scientists all over Europe. From many parts of the continent, Copernicus received invitations to publish, but he feared persecution for his revolutionary work by the establishment. Cardinal Nicola Schoenberg of Capua wrote, asking him to communicate his ideas more widely and requesting a copy for himself; "Therefore, learned man, without wishing to be inopportune, I beg you most emphatically to communicate your discovery to the learned world, and to send me as soon as possible your theories about the Universe, together with the tables and whatever else you have pertaining to the subject." Some have suggested that this note may have made Copernicus leery of publication, while others have suggested that the Church wanted to ensure that his ideas were published. Copernicus was still completing his work (even if he was not convinced that he wanted to publish it) when in 1539 Georg Joachim Rheticus, a great mathematician from Wittenberg, arrived in Frombork. Philipp Melanchthon had arranged for Rheticus to visit several astronomers and study with them. Rheticus became a disciple of Copernicus' and stayed with him for two years, during which he wrote a book, Narratio prima, outlining the essence of the theory. In 1542, in Copernicus' name, Rheticus published a treatise on trigonometry (later included in the second book of De revolutionibus). Under strong pressure from Rheticus, and having seen that the first general reception of his work had not been unfavorable, Copernicus finally agreed to give the book to his close friend Tiedemann Giese, bishop of Chełmno (Kulm), to be delivered to Rheticus for printing at Nuremberg. Legend says that the first printed copy of De revolutionibus was placed in Copernicus' hands on the day he died, so that he could take farewell of his opus vitae. He supposedly woke from a stroke-induced coma, looked at his book, and died peacefully. Copernicus was buried in Frombork Cathedral. Archeologists searching for his remains had failed to locate them, though they had found interesting graves from various periods. On November 3, 2005, archeologists announced that in August they had recovered Copernicus' skull. (Please see "Grave," below.) See also: discussion of Copernicus' nationality.

The Copernican heliocentric system

Earlier theories

Much has been written about earlier heliocentric theories. Philolaus (4th century BC) was one of the first to hypothesize movement by the Earth, probably inspired by Pythagoras' theories about a spherical Globe. Aristarchus of Samos in the 3rd century BC had developed some theories of Heraclides Ponticus (speaking of a revolution by Earth on its axis) to propose what was, so far as is known, the first serious model of a heliocentric solar system. His work about a heliocentric system has not survived, so one may only speculate about what led him to his conclusions. It is notable that, according to Plutarch, a contemporary of Aristarchus accused him of impiety for "putting the Earth in motion." Indian mathematicians, astronomers and physicians, most notably Aryabhata and Bhaskara I, also anticipated Copernicus' discoveries, by about 1,000 years. The work of the 14th-century Arab astronomer Ibn al-Shatir contains findings similar to Copernicus', and it has been suggested that Copernicus might have been influenced by them. Copernicus cited Aristarchus and Philolaus in an early manuscript of his book which survives, stating: "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of Samos was of that opinion." For reasons unknown, he struck this passage before publication of his book. Inspiration came to Copernicus not from observation of the planets, but from reading two authors. In Cicero he found an account of the theory of Hicetas. Plutarch provided an account of the Pythagoreans Heraclides Ponticus, Philolaus, and Ecphantes. These authors had proposed a moving earth that revolved around a central sun. Copernicus did not attribute his inspiration to Aristarchus as is sometimes stated. When Copernicus' book was published, it contained a preface by the Lutheran theologian Andreas Osiander. This cleric stated that Copernicus wrote his heliocentric account of the earth's movement as a mere mathematical hypothesis, not as an account that contained truth or even probability. Copernicus' hypothesis contradicted the account of the sun's movement around the earth that appears in the Old Testament (Joshua 10:13). It has been argued that in developing the mathematics of heliocentrism Copernicus drew on, not just the Greek, but the Arabic tradition of mathethematics, especially the work of Nasir al-Din al-Tusi and Mu’ayyad al-Din al-‘Urdi.

The Ptolemaic system

The prevailing theory in Europe as Copernicus was writing was that created by Ptolemy in his Almagest, dating from about 150 A.D.. The Ptolemaic system drew on many previous theories that viewed Earth as a stationary center of the universe. Stars were embedded in a large outer sphere which rotated relatively rapidly, while the planets dwelt in smaller spheres between — a separate one for each planet. To account for apparent anomalies to this view, such as the retrograde motion observed in many planets, a system of epicycles was used, by which a planet rotated on a small axis while also rotating on a larger axis around the Earth. Some planets were assigned "major" epicycles (by which retrograde motion could be observed) and "minor" epicycles (which simply warped the overall rotation). A complementary theory to Ptolemy's employed homocentric spheres: the spheres within which the planets rotated, could themselves rotate somewhat. Also popular with astronomers were variations such as eccentrics — by which the rotational axis was offset and not completely at the center — or that added epicycles to epicycles. Ptolemy's unique contribution to this theory was the idea of an equant — a complicated addition which specified that, when measuring the rotation of the Sun, one sometimes used the central axis of the universe, but sometimes one set at a different location. This had an overall effect of making certain orbits "wobble," a fact that would greatly bother Copernicus (such wobbling rendered implausible the idea of material "spheres" in which the planets rotated). In the end, after all these complications, the astronomers could still not get observation and theory to match up exactly. In Copernicus' day, the most up-to-date version of the Ptolomaic system was that of Peurbach (1423-1461) and Regiomontanus (1436-1476).

Copernican theory

Copernicus' major theory was published in the book, De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres) in the year of his death, 1543, though he had arrived at his theory several decades earlier. 1543's Collegium Novum (New College) in Kraków.]] The book marks the beginning of the shift away from a geocentric (and anthropocentric) universe with the Earth at its center. Copernicus held that the Earth is another planet revolving around the fixed sun once a year, and turning on its axis once a day. He arrived at the correct order of the known planets and explained the precession of the equinoxes correctly by a slow change in the position of the Earth's rotational axis. He also gave a clear account of the cause of the seasons: that the Earth's axis is not perpendicular to the plane of its orbit. He added another motion to the Earth, by which the axis is kept pointed throughout the year at the same place in the heavens; since Galileo Galilei, it has been recognized that for the Earth not to point to the same place would have been a motion. Copernicus also replaced Ptolemy's equant circles with more epicycles. This is the main source of the statement that Copernicus' system had even more epicycles than Ptolemy's. With this change, Copernicus' system showed only uniform circular motions, correcting what he saw as the chief inelegance in Ptolemy's system. But while Copernicus put the Sun at the center of the celestial spheres, he did not put it at the exact center of the universe, but near it. Copernicus' system was not experimentally better than Ptolemy's model. Copernicus was aware of this and could not present any observational "proof" in his manuscript, relying instead on arguments about what would be a more complete and elegant system. From publication until about 1700, few astronomers were convinced by the Copernican system, though the book was relatively widely circulated (around 500 copies are known to still exist, which is a large number by the scientific standards of the time). Many astronomers, however, accepted some aspects of the theory at the expense of others, and his model did have a large influence on later scientists such as Galileo and Johannes Kepler, who adopted, championed and (especially in Kepler's case) sought to improve it. Galileo's observation of the phases of Venus produced the first observational evidence for Copernicus' theory. The Copernican system can be summarized in seven propositions, as Copernicus himself collected them in a Compendium of De revolutionibus that was found and published in 1878: #Orbits and celestial spheres do not have a unique, common, center. #The center of the Earth is not the center of the Universe, but only the center of the Earth's mass and of the lunar orbit. #All the planets move along orbits whose center is the Sun, therefore the Sun is the center of the World. (Copernicus was never certain whether the Sun moved or not, claiming that the center of the World is "in the Sun, or near it.") #The distance between the Earth and the Sun, compared with the distance between the Earth and the fixed stars, is very small. #The daytime motion of the Sun is only apparent, and represents the effect of a rotation that the Earth makes every 24 hours around its axis, always parallel to itself. #The Earth (together with its Moon, and just like the other planets) moves around the Sun, so the motions that the Sun seems to be making (its apparent motion during the daytime, and its annual motion through the Zodiac) are nothing else than effects of the Earth's actual motions. #These motions of the Earth and of the other planets around the Sun, can explain the stations, and all the particular characteristics of the planets' motions. Whether these propositions were "revolutionary" or "conservative" was a topic of debate in the late twentieth century. Thomas Kuhn argued that Copernicus only transferred "some properties to the sun many astronomical functions previously attributed to the earth." Other historians have since argued that Kuhn underestimated what was "revolutionary" about Copernicus' work, and emphasized the difficulty Copernicus would have had in putting forward a new astronomical theory relying alone on simplicity in geometry, given that he had no experimental evidence.

De revolutionibus orbium coelestium

Thomas Kuhn Main article: De revolutionibus orbium coelestium. Copernicus' major work, On the Revolutions of the Heavenly Spheres (1543), was the result of decades of labor. It opened with an originally anonymous preface by Andreas Osiander, a theologian friend of Copernicus, who urged that the theory did not necessarily have implications outside the limited realm of astronomy. Copernicus' actual book began with a letter from his (by then deceased) friend, the Archbishop of Capua, urging Copernicus to publish his theory. Then, in a lengthy introduction, Copernicus dedicated the book to Pope Paul III, explaining his ostensible motive in writing the book as relating to the inability of earlier astronomers to agree on an adequate theory of the planets, and noting that if his system increased the accuracy of astronomical predictions it would allow the Church to develop a more accurate calendar (calendar reform then being an important question and one of the major reasons for Church funding of astronomy). The work itself was then divided into six books. The first book comprises a general vision of the heliocentric theory, and a summarized exposition of his idea of the World. The second book is mainly theoretical and presents the principles of spherical astronomy and a list of stars (as a basis for the arguments developed in the subsequent books). The third book is mainly dedicated to the apparent motions of the Sun and to related phenomena. The fourth book gives a similar description of the Moon and its orbital motions. The fifth and sixth books comprise a concrete exposition of the new system.

Copernicus and Copernicanism

Copernicus' theory is of extraordinary importance in the history of human knowledge. Many authors suggest that only Euclid's geometry, Isaac Newton's physics and Charles Darwin's theory of evolution have exerted a comparable influence on human culture in general and on science in particular. Many meanings have been ascribed to Copernicus' theory, apart from its strictly scientific import. His work affected religion as well as science, dogma as well as freedom of scientific inquiry. Copernicus' rank as a scientist is often compared with that of Galileo. Copernicus' work contradicted then-accepted religious dogma: it could be inferred that there was no need of an entity (God) that granted a soul, power and life to the World and to human beings — science could explain everything that was attributed to Him. Copernicanism, however, also opened a way to immanence, the view that a divine force, or a divine being, pervades all things that exist — a view that has since been developed further in modern philosophy. Immanentism also leads to subjectivism: to the theory that it is perception that creates reality, that there is no underlying reality that exists independent of perception. Thus some argue that Copernicanism demolished the foundations of medieval science and metaphysics. A corollary of Copernicanism is that scientific law need not be congruent with appearance. This contrasts with Aristotle's system, which placed much more importance on the derivation of knowledge through the senses. Copernicus' concept marked a scientific revolution. Some, indeed, equate it with the initiation of "the scientific revolution" [http://www.anselm.edu/homepage/dbanach/timel.htm]. Immanuel Kant captured the symbolic character of Copernicus' revolution — its transcendent rationalism — postulating that it was human rationality that was the true interpreter of observed phenomena. More recent philosophers, too, have found continuing validity and philosophical meaning in Copernicanism.

Quotes

Goethe: :"Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus. The world had scarcely become known as round and complete in itself when it was asked to waive the tremendous privilege of being the center of the universe. Never, perhaps, was a greater demand made on mankind — for by this admission so many things vanished in mist and smoke! What became of our Eden, our world of innocence, piety and poetry; the testimony of the senses; the conviction of a poetic — religious faith? No wonder his contemporaries did not wish to let all this go and offered every possible resistance to a doctrine which in its converts authorized and demanded a freedom of view and greatness of thought so far unknown, indeed not even dreamed of." Copernicus: :"For I am not so enamored of my own opinions that I disregard what others may think of them. I am aware that a philosopher's ideas are not subject to the judgement of ordinary persons, because it is his endeavor to seek the truth in all things, to the extent permitted to human reason by God. Yet I hold that completely erroneous views should be shunned. Those who know that the consensus of many centuries has sanctioned the conception that the earth remains at rest in the middle of the heaven as its center would, I reflected, regard it as an insane pronouncement if I made the opposite assertion that the earth moves. :"For when a ship is floating calmly along, the sailors see its motion mirrored in everything outside, while on the other hand they suppose that they are stationary, together with everything on board. In the same way, the motion of the earth can unquestionably produce the impression that the entire universe is rotating. :"Therefore alongside the ancient hypotheses, which are no more probable, let us permit these new hypotheses also to become known, especially since they are admirable as well as simple and bring with them a huge treasure of very skillful observations. So far as hypotheses are concerned, let no one expect anything certain from astronomy, which cannot furnish it, lest he accept as the truth ideas conceived for another purpose, and depart from this study a greater fool than when he entered it. Farewell."

University

The university established in 1945 in Copernicus' home town, Toruń, is named Nicolaus Copernicus University.

Grave

In August 2005, a team of Polish archeologists led by Jerzy Gąssowski, head of an archaeology and anthropology institute in Pułtusk, in central Poland, discovered what they believe to be Copernicus' grave and remains, after scanning beneath the floor of Frombork Cathedral, on Poland's Baltic coast. The find came after a year of searching, and the discovery was announced only after further research, on November 3. Gąssowski said he was "[http://news.yahoo.com/s/ap/20051104/ap_on_re_eu/poland_copernicus;_ylt=AoyYrxLhVbs8RPdZHc.kINpxieAA;_ylu=X3oDMTA3MzV0MTdmBHNlYwM3NTM- almost 100 percent sure it is Copernicus]". Forensic experts used the skull to reconstruct a face that closely resembled the features — including a broken nose and a scar above the left eye — on a Copernicus self-portrait. The experts also determined that the skull had belonged to a man who had died about age 70 — Copernicus' age at the time of his death. The grave was in poor condition, and not all the remains were found. The archeologists hoped to find relatives of Copernicus in order to attempt DNA identification.

Reference

- Angus Armitage, The World of Copernicus, New York, Mentor Books, 1951.
- David C. Goodman and Colin A. Russell, eds. The Rise of Scientific Europe, 1500-1800 (Dunton Green, Sevenoaks, Kent: Hodder & Stoughton: The Open University, 1991).
- Thomas Kuhn, The Copernican Revolution: Planetary Astronomy in the Development of Western Thought (Cambridge, MA: Harvard University Press, 1957).

External links

-
- [http://www.frombork.art.pl/Ang01.htm Nicholaus Copernicus Museum in Frombork]
- [http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=10340 Find-A-Grave profile for Nicolaus Copernicus]
- Portraits of Copernicus:[http://serwisy.gazeta.pl/fotografie/5,35076,2999168.html?x=0 Copernicus' face reconstructed]; [http://www-groups.dcs.st-andrews.ac.uk/~history/PictDisplay/Copernicus.html Portrait]; [http://www.frombork.art.pl/Ang10.htm Nicolaus Copernicus]
- [http://www.hps.cam.ac.uk/starry/coperastrol.html Copernicus and Astrology] — A reliable website from Cambridge University describing Copernicus' astrological activities.
- [http://es.rice.edu/ES/humsoc/Galileo/Images/Astro/Conceptions/copernican_universe.gif The Copernican Universe from the De Revolutionibus]
- [http://digital.lib.lehigh.edu/planets/cop.php?num=F.1&exp=false&lang=lat&CISOPTR=0&limit=cop&view=full De Revolutionibus, 1543 first edition] — Full digital facsimile, Lehigh University.
- [http://www.bj.uj.edu.pl/bjmanus/revol/titlpg_e.html De Revolutionibus, autograph manuscript] — Full digital facsimile, Jagiellonian University.
- [http://www.hao.ucar.edu/public/education/sp/images/derevolutionibus.html The front page of the De Revolutionibus]
- [http://webexhibits.org/calendars/year-text-Copernicus.html The text of the De Revolutionibus]
- [http://www.flex.com/~jai/astrology/retrograde.html A java applet about Retrograde Motion]
- [http://www.bo.astro.it/dip/Museum/italiano/sto1_08.html Copernicus in Bologna] — in Italian
- "[http://www.npr.org/display_pages/features/feature_1746110.html Chasing Copernicus: The Book Nobody Read]". - Was One of the Greatest Scientific Works Really Ignored?. All Things Considered, NPR. March 4, 2004.
- [http://www.bede.org.uk/copernicus.htm Copernicus and his Revolutions] A detailed critique of the rhetoric of De Revolutionibus
- [http://www.copernicusfdn.org Copernicus Foundation Chicago]
- [http://www.uni.torun.pl/en/ Nicolaus Copernicus University in Toruń]
- [http://plato.stanford.edu/entries/copernicus/ Stanford Encyclopedia of Philosophy entry]
- [http://www.columbia.edu/~gas1/project/visions/case1/sci.1.html Article which discusses Copernicus's debt to the Arabic tradition]
- [http://news.bbc.co.uk/1/hi/world/europe/4405958.stm 'Body of Copernicus' identified] article on BBC, including image of Copernicus using facial reconstruction based on located skull. Copernicus, Nicolaus Copernicus, Nicolaus Category:History of astronomy Copernicus, Nicolaus Copernicus, Nicolaus Copernicus, Nicolaus Category:Roman Catholics Copernicus, Nicolaus ko:니콜라우스 코페르니쿠스 ja:ニコラウス・コペルニクス th:นิโคเลาส์ โคเปอร์นิคัส

Orbit

.]] :For other meanings of the term "orbit", see orbit (disambiguation) In physics, an orbit is the path that an object makes around another object while under the influence of a source of centripetal force, such as gravity.

History

Orbits were first analysed mathematically by Johannes Kepler who formulated his results in his laws of planetary motion. He found that the orbits of the planets in our solar system are elliptical, not circular (or epicyclic), as had previously been believed. Isaac Newton demonstrated that Kepler's laws were derivable from his theory of gravitation and that, in general, the orbits of bodies responding to the force of gravity were conic sections. Newton showed that a pair of bodies follow orbits of dimensions that are in inverse proportion to their masses about their common center of mass. Where one body is much more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body.

Planetary orbits

Within a planetary system, planets, asteroids, comets and space debris orbit the central star in elliptical orbits. Any comet in a parabolic or hyperbolic orbit about the central star is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. To date, no comet has been observed in our solar system with a distinctly hyperbolic orbit. Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about that planet. Due to mutual gravitational perturbations, the eccentricities of the orbits of the planets in our solar system vary over time. Pluto and Mercury have the most eccentric orbits. At the present epoch, Mars has the next largest eccentricity while the smallest eccentricities are those of the orbits of Venus and Neptune. As an object orbits another, the periapsis is that point at which the two objects are closest to each other and the apoapsis is that point at which they are the farthest from each other. In the elliptical orbit, the centre of mass of the orbiting-orbited system will sit at one focus of both orbits, with nothing present at the other focus. As a planet approaches periapsis, the planet will increase in velocity. As a planet approaches apoapsis, the planet will decrease in velocity. See also: Kepler's laws of planetary motion

Understanding orbits

There are a few common ways of understanding orbits.
- As the object moves sideways, it falls toward the orbited object. However it moves so quickly that the curvature of the orbited object will fall away beneath it.
- A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.
- As the object falls, it moves sideways fast enough (has enough tangential velocity) to miss the orbited object. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of the three one-dimensional coordinates oscillating around a gravitational center. As an illustration of the orbit around a planet (eg Earth), the much-used cannon model may prove useful (see image below). Imagine a cannon sitting on top of a (very) tall mountain, which fires a cannonball horizontally. The mountain needs to be very tall, so that the cannon will be above the Earth's atmosphere and we can ignore the effects of air friction on the cannon ball. 300px If the cannon fires its ball with a low initial velocity, the trajectory of the ball will curve downwards and hit the ground (A). As the firing velocity is increased, the cannonball will hit the ground further (B) and further (C) away from the cannon, because while the ball is still falling towards the ground, the ground is curving away from it (see first point, above). If the cannonball is fired with sufficient velocity, the ground will curve away from the ball at the same rate as the ball falls - it is now in orbit (D). The orbit may be circular like (D) or if the firing velocity is increased even more, the orbit may become more (E) and more (F) elliptical. At a certain even faster velocity (called the escape velocity) the motion changes from an elliptical orbit to a parabola.

Newton's laws of motion

For a system of only two bodies that are only influenced by their mutual gravity, their orbits can be exactly calculated by Newton's laws of motion and gravity. Briefly, the sum of the forces will equal the mass times its acceleration. Gravity is proportional to mass, and falls off proportionally to the square of distance. To calculate, it is convenient to describe the motion in a coordinate system that is centered on the heavier body, and we can say that the lighter body is in orbit around the heavier body. An unmoving body that's far from a large object has more energy than one that's close. This is because it can fall farther. This is called "potential energy" because it is not yet actual. With two bodies, an orbit is a flat curve. The orbit can be open (so the object never returns) or closed (returning), depending on the total kinetic + potential energy of the system. In the case of an open orbit, the speed at any position of the orbit is at least the escape velocity for that position, in the case of a closed orbit, always less. The path of a free-falling (orbiting) body is always a conic section. An open orbit has the shape of a hyperbola (or in the limiting case, a parabola); the bodies approach each other for a while, curve around each other around the time of their closest approach, and then separate again forever. This is often the case with comets that occasionally approach the Sun. A closed orbit has the shape of an ellipse (or in the limiting case, a circle). The point where the orbiting body is closest to Earth is the perigee, called periapsis (less properly, "perifocus" or "pericentron") when the orbit is around a body other than Earth. The point where the satellite is farthest from Earth is called apogee, apoapsis, or sometimes apifocus or apocentron. A line drawn from periapsis to apoapsis is the line-of-apsides. This is the major axis of the ellipse, the line through its longest part. Orbiting bodies in closed orbits repeat their path after a constant period of time. This motion is described by the empirical laws of Kepler, which can be mathematically derived from Newton's laws. These can be formulated as follows: # The orbit of a planet around the Sun is an ellipse, with the Sun in one of the focal points of the ellipse. Therefore the orbit lies in a plane, called the orbital plane. The point on the orbit closest to the attracting body is the periapsis. The point farthest from the attracting body is called the apoapsis. There are also specific terms for orbits around particular bodies; things orbiting the Sun have a perihelion and aphelion, things orbiting the Earth have a perigee and apogee, and things orbiting the Moon have a perilune and apolune (or, synonymously, periselene and aposelene). An orbit around any star, not just the Sun, has a periastron and an apastron # As the planet moves around its orbit during a fixed amount of time, the line from Sun to planet sweeps a constant area of the orbital plane, regardless of which part of its orbit the planet traces during that period of time. This means that the planet moves faster near its perihelion than near its aphelion, because at the smaller distance it needs to trace a greater arc to cover the same area. This law is usually stated as "equal areas in equal time." # For each planet, the ratio of the 3rd power of its semi-major axis to the 2nd power of its period is the same constant value for all planets. Except for special cases like Lagrangian points, no method is known to solve the equations of motion for a system with four or more bodies. The 2-body solutions were published by Newton in Principia in 1687. In 1912, K. F. Sundman developed a converging infinite series that solves the 3-body problem, however it converges too slowly to be of much use. Instead, orbits can be approximated with arbitrarily high accuracy. These approximations take two forms. One form takes the pure elliptic motion as a basis, and adds perturbation terms to account for the gravitational influence of multiple bodies. This is convenient for calculating the positions of astronomical bodies. The equations of motion of the moon, planets and other bodies are known with great accuracy, and are used to generate tables for celestial navigation. The differential equation form is used for scientific or mission-planning purposes. According to Newton's laws, the sum of all the forces will equal the mass times its acceleration (F = ma). Therefore accelerations can be expressed in terms of positions. The perturbation terms are much easier to describe in this form. Predicting subsequent positions and velocities from initial ones corresponds to solving an initial value problem. Numerical methods calculate the positions and velocities of the objects a tiny time in the future, then repeat this. However, tiny arithmetic errors from the limited accuracy of a computer's math accumulate, limiting the accuracy of this approach. Differential simulations with large numbers of objects perform the calculations in a hierarchical pairwise fashion between centers of mass. Using this scheme, galaxies, star clusters and other large objects have been simulated.

Analysis of orbital motion

(see also orbit equation and Kepler's first law) To analyse the motion of a body moving under the influence of a force which is always directed towards a fixed point, it is convenient to use polar coordinates with the origin coinciding with the centre of force. In such coordinates the radial and transverse components of the acceleration are, respectively: :

\frac - r\left( \frac \right)^2

and :

\frac\frac\left( r^2\frac \right)

. Since the force is always radial, the transverse acceleration is zero, and it follows that: :

\frac = hu^2

, where h is a constant of integration and we have introduced the auxiliary variable u defined as 1/r. If magnitude of the radial force is f(r) per unit mass of the orbiting body, then the elimination of the time variable from the radial component of the equation of motion yields: :

\frac + u = \frac

. In the case of an inverse square force law the right hand side of the equation becomes a constant and the equation is seen to be the harmonic equation (up to a shift of origin of the dependent variable). The equation of the orbit described by the particle is thus: :

r = \frac = \frac

, where φ and e are constants of integration and L is the Semi-latus rectum. This can be recognised as the equation of a conic section in polar coordinates.

Orbital parameters

See: Orbital elements For a general elliptic orbit, the relations between the axis, eccentricity, and least and largest distance are: :Semimajor axis = (periapsis + apoapsis)/2 = mean of the extreme radii :Periapsis = semimajor axis × (1 - eccentricity) = least distance :Apoapsis = semimajor axis × (1 + eccentricity) = largest distance Note that there are alternative definitions for a "mean radius" or "average distance": if you average the radius over time for one orbit (mean anomaly), or over the orbital angle as observed by the primary (true anomaly), then you get a different result. See here for details.

Orbital period

See: orbital period

Orbital decay

If some part of a body's orbit enters an atmosphere, its orbit can decay because of drag. At each periapsis, the object scrapes the air, losing energy. Each time, the orbit grows less eccentric (more circular) because the object loses kinetic energy precisely when that energy is at its maximum. Eventually, the orbit circularises and then the object spirals into the atmosphere. The bounds of an atmosphere vary wildly. During solar maxima, the Earth's atmosphere causes drag up to a hundred kilometres higher than during solar minimums. Some satellites with long conductive tethers can also decay because of electromagnetic drag from the Earth's magnetic field. Basically, the wire cuts the magnetic field, and acts as a generator. The wire moves electrons from the near vacuum on one end to the near-vacuum on the other end. The orbital energy is converted to heat in the wire. Another method of artificially influencing an orbit is through the use of solar sails or magnetic sails. These forms of propulsion require no propellant or energy input, and so can be used indefinitely. See statite for one such proposed use. Orbital decay can also occur due to tidal forces for objects below the synchronous orbit for the body they're orbiting. The gravity of the orbiting object raises tidal bulges in the primary, and since below the synchronous orbit the orbiting object is moving faster than the body's surface the bulges lag a short angle behind it. The gravity of the bulges is slightly off of the primary-satellite axis and thus has a component along the satellite's motion. The near bulge slows the object more than the far bulge speeds it up, and as a result the orbit decays. Conversely, the gravity of the satellite on the bulges applies torque on the primary and speeds up its rotation. Artificial satellites are too small to have an appreciable tidal effect on the planets they orbit, but several moons in the solar system are undergoing orbital decay by this mechanism. Mars' innermost moon Phobos is a prime example, and is expected to either impact Mars' surface or break up into a ring within 50 million years. Finally, orbits can decay via the emission of gravitational waves. This mechanism is extremely weak for most stellar objects, only becoming significant in cases where there is a combination of extreme mass and extreme acceleration, such as with black holes or neutron stars that are orbiting each other closely.

Earth orbits

See Earth orbit for more details.
- Low Earth orbit
- High Earth Orbit
- Intermediate circular orbit
- Geostationary orbit
- Geosynchronous orbit
- Geostationary transfer orbit
- Molniya orbit
- Polar orbit
- Polar Sun Synchronous Orbit (this is not a complete list).

Scaling in gravity

The gravitational constant G is defined to be:
- 6.6742 × 10⁻¹¹ N·m²/kg²
- 6.6742 × 10⁻¹¹ m³/(kg·s²)
- 6.6742 × 10⁻¹¹(kg/m³)^-1s^-2. Thus the constant has dimension density^-1 time^-2. This corresponds to the following properties. Scaling of distances (including sizes of bodies, while keeping the densities the same) gives similar orbits without scaling the time: if for example distances are halved, masses are divided by 8, gravitational forces by 16 and gravitational accelerations by 2. Hence orbital periods remain the same. Similarly, when an object is dropped from a tower, the time it takes to fall to the ground remains the same with a scale model of the tower on a scale model of the earth. When all densities are multiplied by four, orbits are the same, but with orbital velocities doubled. When all densities are multiplied by four, and all sizes are halved, orbits are similar, with the same orbital velocities. These properties are illustrated in the formula :

GT^2 \sigma = 3\pi \left( \frac \right)^3,

for an elliptical orbit with semi-major axis a, of a small body around a spherical body with radius r and average density σ, where T is the orbital period.

Role in the evolution of atomic theory

When atomic structure was first probed experimentally early in the twentieth century, an early picture of the atom portrayed it as a miniature solar system bound by the coulomb force rather than by gravity. This was inconsistent with electrodynamics and the model was progressively refined as quantum theory evolved, but there is a legacy of the picture in the term orbital for the wave function of an energetically bound electron state.

External links

- An on-line orbit plotter: http://www.bridgewater.edu/departments/physics/ISAW/PlanetOrbit.html
- [http://www.braeunig.us/space/orbmech.htm Orbital Mechanics] (Rocket and Space Technology) Category:Celestial mechanics Category:Solar System als:Umlaufbahn ja:軌道 (力学) simple:Orbit th:วงโคจร

Venus (Planet)

Venus, the second planet from the Sun, is named after the Roman goddess Venus. A terrestrial planet, it is sometimes called Earth's "sister planet", as the two are very similar in size and bulk composition. Although all planets' orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 1%. As Venus is closer to the Sun than the Earth, it always appears in roughly the same direction from Earth as the Sun (the greatest elongation is 47.8°), so on Earth it can usually only be seen a few hours before sunrise or a few hours after sunset. However, when at its brightest, Venus may be seen during the daytime, making it one of only two heavenly bodies that can be seen both day and night (the other being the Moon). It is sometimes referred to as the "Morning Star" or the "Evening Star", and when it is visible in dark skies it is by far the brightest star-like object in the sky. The cycle between one maximum elongation and the next lasts 584 days. After these 584 days Venus is visible in a position 72 degrees away from the previous one. Since 5
- 584 = 2920, which is equivalent to 8
- 365 Venus returns to the same point in the sky every 8 years (minus two leap days). This was known as the Sothis cycle in ancient Egypt, and was familiar to the Maya as well. Another association is with the Moon, because 2920 days equal almost exactly 99 lunations (29.5
- 99 = 2920.5). Venus has a very slow retrograde rotation, meaning that, unlike with most planets, the direction of rotation (around its axis) is the opposite of its orbital rotation (around the Sun). The very slow rotation means that the distinction between the Sidereal day (rotation relative to the stars) and the Solar day (relative to the Sun) is very significant. Solar day The pentagram has long been associated with the planet Venus and the worship of the goddess Venus, or her equivalent. It is most likely to have originated from the observations of prehistoric astronomers. When viewed from Earth, the successive conjunctions of Venus plot the points of a pentagram around the Sun every eight years, returning to its starting point after a forty year cycle. Venus was known to ancient Babylonians around 1600 BC, and to the Mayan civilization (the Mayans developed a religious calendar based on Venus's motion) and must have been known long before in prehistoric times, given that it is the third brightest object in the sky after the Sun and Moon. The Maasai people in Africa named the planet Kileken, and have a myth about it called "The Orphan Boy." The Morning Star was called the Bearer of Light ("phōsphoros" or "eōsphoros" in Greek and "Lucifer" in Latin, a term later used of the fallen angel cast out of heaven, see Isaiah 14:12). To the Jews it was known as Noga ("shining") and it was used in rabbinic literature as a symbol of beauty and purity Isaiah Its symbol is the sign also used in biology for the female sex, a stylized representation of the goddess Venus's hand mirror: a circle with a small cross underneath (Unicode: ♀). The Venus symbol also represents femininity, and in ancient alchemy stood for copper. Alchemists constructed the symbol from a circle (representing spirit) above a cross (representing matter). The association with sex and femininity is supposed to relate to the period of 266 days between the conjunction and maximum elongation of Venus, which corresponds more or less to the length of human pregnancy. The adjective Venusian is commonly used for Venus, but it is etymologically incorrect. The true adjective coming from Latin, Venereal, is avoided because of its modern association with sexually transmitted diseases. Some astronomers use Cytherean, which comes from Cythera. Other less common adjectives include Venerean, Venerian, and Veneran. The Chinese, Korean, Japanese and Vietnamese cultures refer to the planet as the metal star, 金星, based on the Five Elements.

Physical characteristics

Atmosphere

Venus has an atmosphere consisting mainly of carbon dioxide and a small amount of nitrogen, with a pressure at the surface about 90 times that of Earth (a pressure equivalent to a depth of 1 kilometer under Earth's oceans); its atmosphere is also roughly 90 times more massive than ours. This enormously CO₂-rich atmosphere results in a strong greenhouse effect that raises the surface temperature more than 400 °C (750 °F) above what it would be otherwise, causing temperatures at the surface to reach extremes as great as 500 °C (930 °F) in low elevation regions near the planet's equator. This makes Venus's surface hotter than Mercury's, even though Venus is nearly twice as distant from the Sun and only receives 25% of the solar irradiance (2613.9 W/m² in the upper atmosphere, and just 1071.1 W/m² at the surface). Owing to the thermal inertia and convection of its dense atmosphere, the temperature does not vary significantly between the night and day sides of Venus despite its extremely slow rotation of less than one rotation per Venusian year, meaning that, at the equator, Venus' surface rotates at a mere 6.5 km/h (4 mph). Upper atmosphere winds circling the planet approximately every 4 days help distribute the heat to other areas on the surface. The solar irradiance is so much lower at the surface of Venus because the planet's thick cloud cover reflects the majority of the sunlight back into space. This prevents most of the sunlight from ever heating the surface. Venus's bolometric albedo is approximately 60%, and its visual light albedo is even greater. Thus, despite being closer to the Sun than Earth, the surface of Venus is not as well heated and even less well lit by the Sun. In the absence of any greenhouse effect, the temperature at the surface of Venus would be quite similar to Earth. A common conceptual misunderstanding regarding Venus is the mistaken belief that its thick cloud cover traps heat, as the opposite is actually true. The cloud cover keeps the planet much cooler than it would be otherwise. The immense quantity of CO₂ in the atmosphere is what traps the heat by the greenhouse mechanism. There are strong 300 km/h (200 mph) winds at the cloud tops, but winds at the surface are very slow, no more than a few miles per hour. However, owing to the high density of the atmosphere at Venus's surface, even such slow winds exert a significant amount of force against obstructions. The clouds are mainly composed of sulfur dioxide and sulfuric acid droplets and cover the planet completely, obscuring any surface details from the human eye. The temperature at the tops of these clouds is approximately −45 °C (−50 °F). The mean surface temperature of Venus, as given by NASA, is 464 °C (864 °F). The minimal value of the temperature, listed in the table, refers to cloud tops —the surface temperature is never below 400 °C (750 °F). (This makes the surface temperature hot enough to melt lead.) The atmosphere also contains hydrogen sulfide (H₂S) and carbonyl sulfide (SCO). Hydrogen sulfide reacts with sulfur dioxide, which implies that some process must be creating these components. It is unclear how the carbonyl sulfide could be formed--it is often a sign of biological activity. Some have suggested that microbes exist in the clouds (which also contain droplets of water), and produce these components from water, carbon monoxide and sulfur dioxide. [http://www.newscientist.com/article/mg17523621.800.html New Scientist, Sept. 28, 2002, p. 16]

Surface features

sulfur dioxide Venus has slow retrograde rotation, meaning it rotates from east to west, instead of west to east as most of the other major planets do. (Pluto and Uranus also have retrograde rotation, though Uranus's axis, tilted at 97.86 degrees, almost lies in its orbital plane.) It is not known why Venus is different in this manner, although it may be the result of a collision with a very large asteroid at some time in the distant past. If the Sun could be seen from Venus' surface, it would appear to rise and set in a 116.75 day cycle (Venus' synodic rotation period), and a Venusian year would thus last 1.92 Venusian "days". In addition to this unusual retrograde rotation, the periods of Venus' rotation and of its orbit are synchronized in such a way that it always presents the same face toward Earth when the two planets are at their closest approach (5.001 Venusian days between each inferior conjunction). This may simply be a coincidence, but there is some speculation that this may be the result of tidal locking, with tidal forces affecting Venus' rotation whenever the planets get close enough together —although the tides raised by Earth on Venus are vanishingly small. Venus has two major continent-like highlands on its surface, rising over vast plains. The northern highland is named Ishtar Terra and has Venus's highest mountains, named the Maxwell Montes (roughly 2 km taller than Mount Everest) after James Clerk Maxwell, which surround the plateau Lakshmi Planum. Ishtar Terra is about the size of Australia. In the southern hemisphere is the larger Aphrodite Terra, about the size of South America. Between these highlands are a number of broad depressions, including Atalanta Planitia, Guinevere Planitia, and Lavinia Planitia. With only the exception of Maxwell Montes, all surface features on Venus are named after real or mythological females. Venus' thick atmosphere causes meteors to decelerate as they fall toward the surface, and even large meteors will strike the surface at too low a speed to form an impact crater if they have less than a certain threshold kinetic energy. Because of this, no impact crater smaller than about 3 km (2 mi) in diameter can form. Nearly 90% of Venus's surface appears to consist of recently (in the geological sense) solidified basaltic lava, with very few meteorite craters. The oldest features present on Venus seem to be only around 800 million years old, with most of the terrain being considerably younger (though still not less than several hundred million years for the most part). This suggests that Venus underwent a major resurfacing event in the not too distant geological past. The interior of Venus is probably similar to that of Earth: an iron core about 3000 km in radius, with a molten rocky mantle making up the majority of the planet. Recent results from the Magellan gravity data indicate that Venus's crust is stronger and thicker than had previously been assumed. It is theorized that Venus does not have mobile plate tectonics as Earth does, but instead undergoes massive volcanic upwellings at regular intervals that inundate its surface with fresh lava. Other recent findings suggest that Venus is still volcanically active in isolated geological hotspots. Venus's intrinsic magnetic field has been found very weak compared to other planets in the solar system. This may be due to its slow rotation being insufficient to drive an internal dynamo of liquid iron. As a result, solar wind strikes Venus's upper atmosphere without mediation. It is thought that Venus originally had as much water as Earth, but that under the Sun's assault water vapor in the upper atmosphere was split into hydrogen and oxygen, with the hydrogen escaping into space owing to its low molecular mass; the ratio of hydrogen to deuterium (a heavier isotope of hydrogen which doesn't escape as quickly) in Venus's atmosphere seems to support this theory. Molecular oxygen is thought to have combined with atoms in the crust (large amounts of oxygen, however, remain in the atmosphere in the form of carbon dioxide). Because of their dryness, Venus's rocks are much harder than Earth's, which leads to steeper mountains, cliffs and other features.

Venus' moon

Venus was once thought to possess a moon, named Neith after the chief goddess of Sais, Egypt (whose veil no mortal raised), first observed by Giovanni Domenico Cassini in 1672. German astronomers called the moon Kleinchen (literally "tiny"), and sporadic sightings by astronomers continued until 1892. These sightings have since been discredited, and are thought to have been either spurious internal reflections, mostly faint stars that happened to be in the right place at the right time, or maybe even asteroids passing by the planet. Venus is now known to be moonless.

Observations and explorations of Venus

Venus has been observed several times within the past 4000 years by a number of people, including the Greeks.

Appearance

Cultural references

:See also Venus in fiction Until it was penetrated by probes, Venus's opaque cloud layer gave science fiction writers free rein in imagining the planet's surface, and they frequently imagined it to be Earthlike. There are some religious sects who believe that Hell may be located on Venus. Its extremely high surface temperature and impenetrable cloud cover cause people to believe that the fires of Hell burn on the surface, obscured from our earthly view. Conversely, other sects consider Venus to be some form of paradise or an advanced secret base for angels/aliens to operate from.
- In Olaf Stapledon's epic Last and First Men (1930), Venus is an oceanic idyll where humans evolve the power of flight.
- In the mythology of Middle-earth (1937), by J. R. R. Tolkien, Venus is the Star of Eärendil. The star was created when Eärendil the Mariner was set in the sky on his ship, with a Silmaril bound to his brow. In fact, Tolkien chose the name directly from the ancient Old English word for the planet Venus.
- In H. P. Lovecraft's Cthulhu Mythos (1928–), there are mentions of the 'Lords of Venus', and conflicting indications that the Serpent People originated there.
- Edgar Rice Burroughs wrote a series of five books on Venus, featuring hero Carson Napier, who discovers that Venus (or Amtor, as it is known by the Venusians) is a world of sky-high trees, warring kingdoms and princesses in need of rescue. [http://www.tarzan.com/worlds/amtor.html]
- The H. P. Lovecraft and Kenneth Sterling short story 'In the Walls of Eryx' (1939), takes place on Venus, but is not considered part of the Cthulhu Mythos.
- The second book of the Space Trilogy (1938–1945) by C.S. Lewis, Perelandra 1943) takes place on Venus (called by the natives Perelandra), the site of a second garden of Eden.
- In the military science fiction classic Clash by Night (1943) by Henry Kuttner (writing as Lawrence O'Donnell) and C. L. Moore, underwater city-states hire mercenary companies and their battleships to fight their wars on the surface.
- Venus was the home planet of the Mekon, arch-enemy of the 1950s comic book hero Dan Dare.
- Many science-fiction movies and serials of the '50s and '60s, such as Abbott and Costello Go to Mars and Space Patrol, have used Venus' namesake goddess and her domain to contrive planetary populations of nubile women welcoming (or attacking) all-male astronaut crews.
- In the Noon Universe created by the Soviet science fiction writers Boris and Arkady Strugatsky, Venus is depicted as a extremely harsh planet covered by strange flora and fauna but also very rich in minerals and heavy metals. The novel The Land of Crimson Clouds (Strana Bagrovykh Tuch in the original) describes the first successful manned mission to Venus, although a full-scaled colonization of the planet was not initiated until much later (in 2119; see Noon: 22nd Century).
- Venus is the location of several Starfleet Academy training facilities and terraforming stations in the fictional Star Trek universe (1966–).
- In Jacqueline Susann's Yargo (1979), Venus is inhabited by bees that are as big as horses.
- Venus is briefly mentioned in Arthur C. Clarke's 3001: The Final Odyssey (1997).
- A presumably terraformed Venus was the setting of one episode of the anime Cowboy Bebop (1998). In the show, Venus was revealed to be an arid but habitable world. Much of the population lived in floating cities in the sky. In the cartoon Exosquad, terraformed Venus was portrayed as one of the three habitable planets in the solar system (the others being Earth and Mars).
- In the Japanese anime series, Bishoujo Senshi Sailor Moon (1992), Sailor Venus is a soldier representing the planet of the same name. In mythology, Venus is the Roman goddess of love (Aphrodite in Greek), therefore, Sailor Venus's attacks and weapons (e.g. Venus Love Me Chain and Venus Love and Beauty Shock) represent the idea of love and femininity. Her image colours are gold and orange--similar to the colour of the planet. Also, on her forehead is the planet's symbol.
- A more scientifically accurate depiction of the planet is offered in Ben Bova's novel Venus (2000, ISBN 031287216X)-

References

- Arnett, Bill (2005). [http://www.nineplanets.org/venus.html Venus]. Retrieved March 27, 2005.
- European Space Agency (2005). [http://www.esa.int/SPECIALS/Venus_Express/ Venus Express overview]. Retrieved March 27, 2005.
- Grayzeck, Ed (2004). [http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html Venus Fact Sheet]. NASA. Retrieved March 27, 2005.
- Grieger, Bjoern (2004). [http://www.space-vision.biz/product.venuslandscape.de.html Picture “Real Venus”]. Retrieved March 27, 2005.
- The Maya Astronomy Page (2002). [http://www.michielb.nl/maya/venus.html Venus]. Retrieved March 27, 2005.
- Mitchell, Don P. (2004). [http://www.mentallandscape.com/V_Venus.htm The Soviet Exploration of Venus]. Retrieved March 27, 2005.
- Rosenthal, David. (2003). [http://www.ridgecrest.ca.us/~n6tst/maya/newpage.html THE SOUTHERNMOST RISE OF VENUS AT UXMAL, 1997]. Retrieved March 27, 2005.
- Vienna University of Technology (2004). [http://www.vias.org/spacetrip/venus_dimensionalviews.html Venus Three-Dimensional Views]. Retrieved March 27, 2005.
- [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1996JBAA..106...16M]
- [http://www.ibiblio.org//e-notes/VRML/Globe/Globe.htm 3D VRML Venus globe]
- [http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html Venus Fact Sheet]

Pentagram

- http://www.mikecrowson.co.uk/pentagram.html
- http://www.symbols.com/encyclopedia/29/2914.html
- http://www.hyperflight.com/venus-five-pointed-star.htm
- [http://www.run4space.com/viewforum.php?f=8 Venus Forum]
- ko:금성 ms:Zuhrah ja:金星 simple:Venus (planet) th:ดาวศุกร์

Sun

:: For the astrological significance of the Sun, see Solar system in astrology. ::"Solar" redirects here; for the superhero by that name, see Solar (comics). The Sun (or Sol) is the star at the center of our Solar system. Earth orbits the Sun, as do many other bodies, including other planets, asteroids, meteoroids, comets and dust. Its heat and light support almost all life on Earth. The Sun is a ball of plasma with a mass of about 2 kg, which is somewhat higher than that of an average star. About 74% of its mass is hydrogen, with 25% helium and the rest made up of trace quantities of heavier elements. It is thought that the Sun is about 5 billion years old, and is about halfway through its main sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. In about 5 billion years time the Sun will become a white dwarf. Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 10⁶ K when its visible surface (the photosphere) has a temperature of just 6,000 K. Looking directly at the Sun can damage the retina and one's eyesight. See below for details.

General information

See below The Sun is classified as a main sequence star, which means it is in a state of "hydrostatic balance", neither contracting nor expanding, and is generating its energy through nuclear fusion of hydrogen nuclei into helium. The Sun has a spectral class of G2V, with the G2 meaning that its color is yellow and its spectrum contains spectral lines of ionized and neutral metals as well as very weak hydrogen lines [http://www.astro.uiuc.edu/~kaler/sow/spectra.html#classes], and the V signifying that it, like most stars, is a "dwarf" star on the main sequence[http://www.physics.uq.edu.au/people/ross/phys2080/spec/analyz.htm]. The Sun has a predicted main sequence lifetime of about 10 billion years. Its current age is thought to be about 4.5 billion years, a figure which is determined using computer models of stellar evolution, and nucleocosmochronology . The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic centre, completing one revolution in about 226 million years. The orbital speed is 217 km/s, equivalent to one light year every 1400 years, and one AU every 8 days. The astronomical symbol for the Sun is a circle with a point at its centre (Image:Sol.gif).

Structure

Image:Sol.gif The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means the polar diameter differs from the equatorial by about 10 km. This is because the centrifugal effect of the Sun's slow rotation is 18 million times weaker than its surface gravity (at the equator). Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass of the solar system, which is offset from the Sun's center mostly because of the large mass of Jupiter. The mass of the Sun is so comparatively great that the center of mass of the solar system is generally within the bounds of the Sun itself. The Sun does not have a definite boundary as rocky planets do, as the density of its gases drops off following an approximately exponential relationship with distance from the centre of the Sun. Nevertheless, the Sun has well defined interior structure, described below. The Sun's radius is measured from centre to the edges of the photosphere. The solar interior is not directly observable and the Sun itself is opaque to electromagnetic radiation. However, just as the study of the waves generated by earthquakes (seismology) can be used to study the interior structure of the Earth, helioseismology, the study of sound waves that travel through the Sun's interior, has also contributed greatly to our understanding of the Sun's structure . Computer modeling of the Sun is also used as a theoretical tool to investigate its deep layers.

Core

At the center of the Sun, where its density reaches up to 150,000 kg/m³ (150 times the density of water on Earth), thermonuclear reactions (nuclear fusion) convert hydrogen into helium, producing the energy that keeps the Sun in a state of equilibrium. About 8.9 protons (hydrogen nuclei) are converted to helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second or 383 yottawatts (9.15 tons of TNT per second). The core extends from the center of the Sun to about 0.2 solar radii, and is the only part of the Sun where an appreciable amount of heat is produced by fusion: the rest of the star is heated by energy that is transferred outward. All of the energy of the interior fusion must travel through the successive layers to the solar photosphere, before it escapes to space. The high-energy photons (gamma and X rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as constant absorption and re-emission at lower energies in the solar mantle (see below). Estimates of the "photon travel time" range from as much as 50 million years (Richard S. Lewis, The Illustrated Encyclopedia of the Universe, Harmony Books, New York, 1983, p. 65) to as little as 17,000 years [http://www.badastronomy.com/bitesize/solar_system/sun.html]. Upon reaching the surface after a final trip through the convective outer layer, the photons escape as visible light. Neutrinos are also released in the fusion reactions in the core, but unlike photons they very rarely interact with matter, and so almost all are able to escape the Sun immediately.

Radiation zone

From about 0.2 to about 0.7 solar radii, the material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone, there is no thermal convection: while the material grows cooler with altitude, this temperature gradient is slower than the adiabatic lapse rate and hence cannot drive convection. Heat is transferred by ions of hydrogen and helium emitting photons, which travel a brief distance before being re-absorbed by other ions. Because of this, it can take a photon nearly 1,000,000 years to reach the photosphere.

Convection zone

photosphere From about 0.7 solar radii to 1.0 solar radii, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone. The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a 'small-scale' dynamo that produces magnetic north and south poles all over the surface of the Sun.

Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere, sunlight is free to propagate into space and its energy escapes the Sun entirely. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 10²³/m³ (this is about 1% of the particle density of Earth's atmosphere at sea level). The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays.

Temperature minimum

The coolest layer of the Sun is the temperature minimum region about 500 km above the photosphere. It is about 4,000 K. It is the only part of the Sun cool enough to support simple molecules such as carbon monoxide and water; all other parts of the Sun are hot enough to break chemical bonds.

Chromosphere

Above the visible surface of the Sun is a thin layer, about 2,000 km thick, that is dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chromos, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun.

Corona

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 10¹¹/m³ (Earth's atmosphere near sea level has a particle density of about 2x10²⁵/m³). The temperature of the corona is several megakelvins.

Theoretical problems

Solar neutrino problem

megakelvin For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only a third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were constructed, including the Sudbury Neutrino Observatory and Kamiokande to try to measure the solar neutrino flux. It has recently been found that neutrinos have rest mass, and can therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth in a process known as neutrino oscillation . Thus, measurement and theory have been reconciled.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of about 6,000 K. Above it lies the solar corona with a temperature of one million kelvins. The high temperature of the corona suggests that it is heated by something other than the photosphere. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere. Two main mechanisms have been proposed to explain coronal heating: Wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other proposed mechanism is flare heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of solar flares and waves. , , , . Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfven waves have been found to dissipate or refract before reaching the corona (, ). In addition, Alfven waves do not easily dissipate in the corona . Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales , but this is still an open topic of investigation.

Faint young sun problem

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75 percent as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geologic record shows that the Earth has remained at a fairly constant temperature throughout its history. In fact, the young Earth was actually warmer than it is today. Some scientists have suggested that the young Earth's atmosphere contained much larger quantities of greenhouse gases such as carbon dioxide and/or ammonia than are present today . Others suggest that cosmic rays might strongly influence the Earth's climate, and that their flux was much higher in the early history of the solar system .

Magnetic field

cosmic ray's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [http://quake.stanford.edu/~wso/gifs/HCS.html]. (click to enlarge)]] All matter in the Sun is in the form of gas and plasma due to its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (28 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences. (See magnetic reconnection.) The solar activity cycle includes old magnetic fields being stripped off the Sun's surface starting from one pole and ending at the other. The magnetic field of the sun reverses once for each 11-year sunspot cycle. The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the largest structure in the Solar System, the Heliospheric current sheet. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's 10^-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10^-11 tesla. But satellite observations show that it is about 100 times greater at around 10^-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g. the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.

Position of the Sun through the year

The path of the Sun across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the analemma, and resembles a figure 8, aligned along the North/South direction. The most obvious variation in the Sun's apparent position through the year is a North/South swing over 47 degrees of angle, due to the 23.5 degree tilt of the Earth, but there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons on Earth.

Solar space missions

seasons using UV light from the He⁺ emission line at 30.4 nm. (Animation (980 kB MPEG))]] To obtain an uninterrupted view of the Sun, the European Space Agency and NASA cooperatively launched the Solar and Heliospheric Observatory (SOHO) on December 2, 1995. Originally a two-year mission, SOHO is now over ten years old (as of late 2005). It has proved so useful that a follow-on mission, the Solar Dynamics Observatory, is planned for launch in 2008. Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is much less well known. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. It returned to Earth in 2004 and is undergoing analysis, but it was damaged by crash-landing when its parachute failed to deploy on reentry to Earth's atmosphere.

History and future of the Sun

The Sun is thought to be a second-generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as iron, gold and uranium in the solar system: the most plausible ways that these elements could be produced are by endothermic nuclear reactions during a supernova or by transmutation via neutron absorption inside a massive first generation star. Our Sun does not have enough mass to explode as a supernova, and its mass is below the Chandrasekhar limit. Instead, in 4-5 billion years it will enter its red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches about 3 K. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. Following the red giant phase, giant thermal pulsations will cause the Sun to throw off its outer layers forming a planetary nebula. The Sun will then evolve into a white dwarf, slowly cooling over eons. This stellar evolution scenario is typical of low to medium mass stars.

Human understanding of the Sun

:see also sun worship sun worship mythology]] Mankind's most fundamental understanding of the Sun is as the luminous disk in the heavens whose presence above the horizon creates day, and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a deity or other supernatural phenomenon. One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peleponessus, and not the chariot of Helios. For teaching this heresy he was imprisoned by the authorities and sentenced to death (though later released through the intervention of Pericles). With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac. Thus, the Sun was considered by Greek astronomers to be one of the seven planets (Greek planetes "wanderer"), after which the seven days of the week are named in some languages.

The Sun as a power source

Sunlight — that is, light radiated from the surface of the Sun — is thought to be the main source of energy near the surface of Earth. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. It is about 1370 watts per square meter of area. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere, so that less power arrives at the surface — closer to 1000 watts per directly exposed square meter in clear conditions. This energy can be harnessed through several natural and synthetic processes. Photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or do other useful work. The energy stored in petroleum is thought to have been converted from sunlight by photosynthesis in the distant past.

Sun and eye damage

Sunlight is very bright, and looking directly at the Sun is painful to the eyes. Looking directly at the Sun when it is high in the sky causes temporary bleaching of the photosensitive pigments in the