Abū Ishāq Ibrāhīm al-Zarqālī
Abū Isḥāq Ibrāhīm ibn Yaḥyā al-Naqqāsh al-Zarqālī al-Tujibi. Although his name is conventionally given as al-Zarqālī, it is probable that the correct form was al-Zarqālluh. In Latin he was referred to as Arzachel or Arsechieles, a modified form of Arzachel, meaning'the engraver', he lived in Toledo, Al-Andalus before moving to Córdoba in his life. His works inspired a generation of Islamic astronomers in Al-Andalus, after being translated, were influential in Europe, his invention of the Saphaea proved popular and was used by navigators until the 16th century. The crater Arzachel on the Moon is named after him. Al-Zarqālī was born in a village near the outskirts of Toledo, the capital of the Taifa of Toledo, he was trained as a metalsmith and due to his burr skills he was nicknamed Al-Nekkach "the engraver of metals". His Latinized name,'Arzachel' is formed from the Arabic al-Zarqali al-Naqqash, meaning'the engraver', he was talented in Geometry and Astronomy. He is known to have taught and visited Córdoba on various occasions, his extensive experience and knowledge made him the foremost astronomer of his time.
Al-Zarqālī was an inventor, his works helped to put Toledo at the intellectual center of Al-Andalus. He is referred to in the works of Chaucer, as'Arsechieles'. In the year 1085 Toledo was taken by the Christian king of Castile Alfonso VI. Al-Zarqālī and his colleagues, such as Al‐Waqqashi of Toledo, had to flee, it is unknown whether the aged Al-Zarqālī died in a Moorish refugee camp. His works influenced Ibn Bajjah, Ibn Tufail, Ibn Rushd, Ibn al-Kammad, Ibn al‐Haim al‐Ishbili and Nur ad-Din al-Betrugi. In the 12th century, Gerard of Cremona translated al-Zarqali’s works into Latin, he referred to Al-Zarqali as an magician. Ragio Montanous wrote a book in the 15th century on the advantages of the Sahifah al-Zarqalia. In 1530, the German scholar Jacob Ziegler wrote a commentary on one of al-Zarqali’s works. In his "De Revolutionibus Orbium Coelestium", in the year 1530, Nicolaus Copernicus quotes the works of al-Zarqali and Al-Battani. Al-Zarqālī wrote two works on the construction of an instrument for computing the position of the planets using diagrams of the Ptolemaic model.
These works were translated into Spanish in the 13th century by order of King Alfonso X in a section of the Libros del Saber de Astronomia entitled the "Libros de las laminas de los vii planetas". He invented a perfected kind of astrolabe known as "the tablet of al-Zarqālī", famous in Europe under the name Saphaea. There is a record of an al-Zarqālī who built a water clock, capable of determining the hours of the day and night and indicating the days of the lunar months. According to a report found in al-Zuhrī's Kitāb al-Juʿrāfīyya, his name is given as Abū al-Qāsim bin ʿAbd al-Raḥmān known as al-Zarqālī, which has made some historians think that this is a different person. Al-Zarqali corrected geographical data from Al-Khwarizmi, he corrected Ptolemy’s estimate of the longitude of the Mediterranean sea from 62 degrees to the correct value of 42 degrees. In his treatise on the solar year, which survives only in a Hebrew translation, he was the first to demonstrate the motion of the solar apogee relative to the fixed background of the stars.
He measured its rate of motion as 12.04 seconds per year, remarkably close to the modern calculation of 11.77 seconds. Al-Zarqālī's model for the motion of the Sun, in which the center of the Sun's deferent moved on a small rotating circle to reproduce the observed motion of the solar apogee, was discussed in the thirteenth century by Bernard of Verdun and in the fifteenth century by Regiomontanus and Peurbach. In the sixteenth century Copernicus employed this model, modified to heliocentric form, in his De Revolutionibus Orbium Coelestium. Al-Zarqālī contributed to the famous Tables of Toledo, an adaptation of earlier astronomical data to the location of Toledo along with the addition of some new material. Al-Zarqālī was famous as well for his own Book of Tables. Many "books of tables" had been compiled, but his almanac contained tables which allowed one to find the days on which the Coptic, Roman and Persian months begin, other tables which give the position of planets at any given time, still others facilitating the prediction of solar and lunar eclipses.
He compiled an almanac that directly provided "the positions of the celestial bodies and need no further computation". The work provided the true daily positions of the sun for four Julian years from 1088 to 1092, the true positions of the five planets every 5 or 10 days over a period of 8 years for Venus, 79 years for Mars, so forth, as well as other related tables, his Zij and Almanac were translated into Latin by Gerard of Cremona in the 12th century, contributed to the rebirth of a mathematically based astronomy in Christian Europe and were incorporated into the Tables of Toledo in the 12th century and the Alfonsine tables in the 13th century. In designing an instrument to deal with Ptolemy's complex model for the planet Mercury, in which the center of the deferent moves on a secondary epicycle, al-Zarqālī noted that the path of the center of the primary epicycle is not a circle, as it is for the other planets. Instead it is oval and similar to the shape of a pignon; some writers have misinterpreted al-Zarqālī's description of an earth-cente
An astrolabe is an elaborate inclinometer used by astronomers and navigators to measure the altitude above the horizon of a celestial body, day or night. It can be used to identify stars or planets, to determine local latitude given local time, to survey, or to triangulate, it was used in classical antiquity, the Islamic Golden Age, the European Middle Ages and the Age of Discovery for all these purposes. The astrolabe's importance not only comes from the early development of astronomy, but is effective for determining latitude on land or calm seas. Although it is less reliable on the heaving deck of a ship in rough seas, the mariner's astrolabe was developed to solve that problem. OED gives the translation "star-taker" for the English word astrolabe and traces it through medieval Latin to the Greek word astrolabos, from astron "star" and lambanein "to take". In the medieval Islamic world the Arabic word "al-Asturlāb" was given various etymologies. In Arabic texts, the word is translated as "ākhdhu al-Nujuum", a direct translation of the Greek word.
Al-Biruni quotes and criticizes medieval scientist Hamzah al-Isfahani who stated: "asturlab is an arabization of this Persian phrase". In medieval Islamic sources, there is a folk etymology of the word as "lines of lab", where "Lab" refers to a certain son of Idris; this etymology is mentioned by a 10th-century scientist rejected by al-Khwarizmi. An early astrolabe was invented in the Hellenistic civilization by Apollonius of Perga between 220 and 150 BC attributed to Hipparchus; the astrolabe was a marriage of the planisphere and dioptra an analog calculator capable of working out several different kinds of problems in astronomy. Theon of Alexandria wrote a detailed treatise on the astrolabe, Lewis argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the Tetrabiblos; the invention of the plane astrolabe is sometimes wrongly attributed to Theon's daughter Hypatia, but it is, in fact, known to have been in use at least 500 years before Hypatia was born. The misattribution comes from a misinterpretation of a statement in a letter written by Hypatia's pupil Synesius, which mentions that Hypatia had taught him how to construct a plane astrolabe, but does not state anything about her having invented it herself.
Astrolabes continued in use in the Greek-speaking world throughout the Byzantine period. About 550 AD, Christian philosopher John Philoponus wrote a treatise on the astrolabe in Greek, the earliest extant treatise on the instrument. Mesopotamian bishop Severus Sebokht wrote a treatise on the astrolabe in the Syriac language in the mid-7th century. Sebokht refers to the astrolabe as being made of brass in the introduction of his treatise, indicating that metal astrolabes were known in the Christian East well before they were developed in the Islamic world or in the Latin West. Astrolabes were further developed in the medieval Islamic world, where Muslim astronomers introduced angular scales to the design, adding circles indicating azimuths on the horizon, it was used throughout the Muslim world, chiefly as an aid to navigation and as a way of finding the Qibla, the direction of Mecca. Eighth-century mathematician Muhammad al-Fazari is the first person credited with building the astrolabe in the Islamic world.
The mathematical background was established by Muslim astronomer Albatenius in his treatise Kitab az-Zij, translated into Latin by Plato Tiburtinus. The earliest surviving astrolabe is dated AH 315. In the Islamic world, astrolabes were used to find the times of sunrise and the rising of fixed stars, to help schedule morning prayers. In the 10th century, al-Sufi first described over 1,000 different uses of an astrolabe, in areas as diverse as astronomy, navigation, timekeeping, Salat, etc; the spherical astrolabe was a variation of both the astrolabe and the armillary sphere, invented during the Middle Ages by astronomers and inventors in the Islamic world. The earliest description of the spherical astrolabe dates back to Al-Nayrizi. In the 12th century, Sharaf al-Dīn al-Tūsī invented the linear astrolabe, sometimes called the "staff of al-Tusi", "a simple wooden rod with graduated markings but without sights, it was furnished with a plumb line and a double chord for making angular measurements and bore a perforated pointer".
The geared mechanical astrolabe was invented by Abi Bakr of Isfahan in 1235. Herman Contractus, the abbot of Reichman Abbey, examined the use of the astrolabe in Mensura Astrolai during the 11th century. Peter of Maricourt wrote a treatise on the construction and use of a universal astrolabe in the last half of the 13th century entitled Nova compositio astrolabii particularis. Universal astrolabes can be found at the History of Science Museum in Oxford. English author Geoffrey Chaucer compiled A Treatise on the Astrolabe for his son based on a work by Messahalla or Ibn al-Saffar; the same source was translated by others. The first printed book on the astrolabe was Composition and Use of Astrolabe by Christian of Prachatice using Messahalla, but original. In 1370, the first Indian treatise on the astrolabe was written by the Jain astronomer Mahendra Suri. A simplified astrolabe, known as a balesilha, was used by sailors to get an accurate reading of latitude while out to sea; the use of
Abū Sahl al-Qūhī
Abū Sahl Wayjan ibn Rustam al-Qūhī was a Persian mathematician and astronomer. He was from Kuh, an area in Tabaristan and flourished in Baghdad in the 10th century, he is considered one of the greatest Muslim geometers, with many mathematical and astronomical writings ascribed to him. Al-Qūhī was the leader of the astronomers working in 988 AD at the observatory built by the Buwayhid amir Sharaf al-Dawla in Badhdad, he wrote a treatise on the astrolabe. In mathematics he devoted his attention to those Archimedean and Apollonian problems leading to equations higher than the second degree, he discussed the conditions of solvability. For example, he was able to solve the problem of inscribing an equilateral pentagon into a square, resulting in a fourth degree equation, he wrote a treatise on the "perfect compass", a compass with one leg of variable length that allows users to draw any conic section: straight lines, ellipses and hyperbolas. It is that al-Qūhī invented the device. Like Aristotle, al-Qūhī proposed that the weight of bodies varies with their distance from the center of the Earth.
The correspondence between al-Qūhī and Abu Ishaq al-Sabi, a high civil servant interested in mathematics, has been preserved
The terms anno Domini and before Christ are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord", but is presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ"; this calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years from the start of this epoch, BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 follows the year 1 BC; this dating system was devised in 525 by Dionysius Exiguus of Scythia Minor, but was not used until after 800. The Gregorian calendar is the most used calendar in the world today. For decades, it has been the unofficial global standard, adopted in the pragmatic interests of international communication and commercial integration, recognized by international institutions such as the United Nations.
Traditionally, English followed Latin usage by placing the "AD" abbreviation before the year number. However, BC is placed after the year number, which preserves syntactic order; the abbreviation is widely used after the number of a century or millennium, as in "fourth century AD" or "second millennium AD". Because BC is the English abbreviation for Before Christ, it is sometimes incorrectly concluded that AD means After Death, i.e. after the death of Jesus. However, this would mean that the approximate 33 years associated with the life of Jesus would neither be included in the BC nor the AD time scales. Terminology, viewed by some as being more neutral and inclusive of non-Christian people is to call this the Current or Common Era, with the preceding years referred to as Before the Common or Current Era. Astronomical year numbering and ISO 8601 avoid words or abbreviations related to Christianity, but use the same numbers for AD years; the Anno Domini dating system was devised in 525 by Dionysius Exiguus to enumerate the years in his Easter table.
His system was to replace the Diocletian era, used in an old Easter table because he did not wish to continue the memory of a tyrant who persecuted Christians. The last year of the old table, Diocletian 247, was followed by the first year of his table, AD 532; when he devised his table, Julian calendar years were identified by naming the consuls who held office that year—he himself stated that the "present year" was "the consulship of Probus Junior", 525 years "since the incarnation of our Lord Jesus Christ". Thus Dionysius implied that Jesus' incarnation occurred 525 years earlier, without stating the specific year during which his birth or conception occurred. "However, nowhere in his exposition of his table does Dionysius relate his epoch to any other dating system, whether consulate, year of the world, or regnal year of Augustus. Among the sources of confusion are: In modern times, incarnation is synonymous with the conception, but some ancient writers, such as Bede, considered incarnation to be synonymous with the Nativity.
The civil or consular year began on 1 January but the Diocletian year began on 29 August. There were inaccuracies in the lists of consuls. There were confused summations of emperors' regnal years, it is not known. Two major theories are that Dionysius based his calculation on the Gospel of Luke, which states that Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar", hence subtracted thirty years from that date, or that Dionysius counted back 532 years from the first year of his new table, it has been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years with a calendar based on the incarnation of Christ was intended to prevent people from believing the imminent end of the world. At the time, it was believed by some that the resurrection of the dead and end of the world would occur 500 years after the birth of Jesus; the old Anno Mundi calendar theoretically commenced with the creation of the world based on information in the Old Testament.
It was believed that, based on the Anno Mundi calendar, Jesus was born in the year 5500 with the year 6000 of the Anno Mundi calendar marking the end of the world. Anno Mundi 6000 was thus equated with the resurrection and the end of the world but this date had passed in the time of Dionysius; the Anglo-Saxon historian the Venerable Bede, familiar with the work of Dionysius Exiguus, used Anno Domini dating in his Ecclesiastical History of the English People, completed in 731. In this same history, he used another Latin term, ante vero incarnationis dominicae tempus anno sexagesimo, equivalent to the English "before Christ", to identify years before the first year of this era. Both Dionysius and Bede regarded Anno Domini as beginning at the incarnation of Jesus, but "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i.e. the Annunciation on March 25". On the continent of Europe, Anno
Ahmad ibn Muhammad ibn Kathir al-Farghani
Abū al-ʿAbbās Aḥmad ibn Muḥammad ibn Kathīr al-Farghānī. Also known as Alfraganus in the West, was an astronomer in the Abbasid court in Baghdad, one of the most famous astronomers in the 9th century; the lunar crater Alfraganus is named after him. He was involved in the calculation of the diameter of the Earth by the measurement of the meridian arc length, together with a team of scientists under the patronage of the ʿAbbāsid caliph al-Ma'mūn in Baghdad, he moved to Cairo, where he composed a treatise on the astrolabe around 856. There he supervised the construction of the large Nilometer on the island of al-Rawda in the year 861; some modern sources describe him as Arab or Persian His textbook Kitāb fī Jawāmiʿ ʿIlm al-Nujūm or Elements of astronomy on the celestial motions, written about 833, was a descriptive summary of Ptolemy's Almagest, while using the findings and revised values of earlier Islamic astronomers. It was translated into Latin in the 12th century and remained popular in Europe until the time of Regiomontanus.
Dante Alighieri's knowledge of Ptolemaic astronomy, evident in his Divina Commedia as well as other works such as the Convivio, seems to have been drawn from his reading of Alfraganus. In the 17th century the Dutch orientalist Jacob Golius published the Arabic text on the basis of a manuscript he had acquired in the Near East, with a new Latin translation and extensive notes. In the 15th century, Christopher Columbus used al-Farghani's estimate for the Earth's circumference as the basis for his voyages to America. However, Columbus mistook al-Farghani's 7091-foot Arabic mile to be a 4856-foot Roman mile, causing him to underestimate the Earth's circumference, believing he could take a shortcut to Asia. List of Iranian scientists and scholars Sabra, Abdelhamid I.. "Farghānī, Abu'l-ʿAbbās Aḥmad Ibn Muḥammad Ibn Kathīr al-". Dictionary of Scientific Biography. 4. New York: Charles Scribner's Sons. Pp. 541–545. ISBN 0-684-10114-9. Jacobus Golius, كتاب محمد بن كثير الفرغاني في الحركات السماوية وجوامع علم النجوم، بتفسير الشيخ الفاضل يعقوب غوليوس / Muhammedis Fil.
Ketiri Ferganensis, qui vulgo Alfraganus dicitur, Elementa astronomica, Arabicè & Latinè. Cum notis ad res exoticas sive Orientales, quae in iis occurrunt, Amsterdam 1669. El-Fergânî, The Elements of Astronomy, textual analysis, translation into Turkish, critical edition & facsimile by Yavuz Unat, edited by Şinasi Tekin & Gönül Alpay Tekin, Harvard University 1998. Elements of Chronology and Astronomy - Muhamedis Alfragani Arabis Chronologica et astronomica elementa. Richard Lorch, Al-Farghānī on the Astrolabe. Arabic text edited with translation and commentary, Stuttgart, 2005, ISBN 3-515-08713-3. Yavuz Unat, El-Fergânî, Cevami İlm en-Nucûm ve Usûl el-Harekât es-Semâviyye, Astronominin Özeti ve Göğün Hareketlerinin Esası, T. C. Kültür ve Turizm Bakanlığı, Bilimin ve Felsefenin Doğulu Öncüleri Dizisi 14, Ankara 2012. Yavuz Unat, “Fergânî’nin ‘Astronominin Özeti ve Göğün Hareketlerinin Esasları’ Adlı Astronomi Eseri”, DTCF Dergisi, Cilt 38, Sayı 1-2, Ankara 1998, s. 405–423. DeYoung, Gregg. "Farghānī: Abū al‐ʿAbbās Aḥmad ibn Muḥammad ibn Kathīr al‐Farghānī".
In Thomas Hockey. The Biographical Encyclopedia of Astronomers. New York: Springer. P. 357. ISBN 978-0-387-31022-0. Paul Lunde, Al-Farghani and the “Short Degree”, 1992, Saudi Aramco World
Abd al-Rahman al-Sufi
'Abd al-Rahman al-Sufi (Persian: عبدالرحمن صوفی was a Persian astronomer known as'Abd ar-Rahman as-Sufi,'Abd al-Rahman Abu al-Husayn,'Abdul Rahman Sufi, or'Abdurrahman Sufi and in the West as Azophi and Azophi Arabus. The lunar crater Azophi and the minor planet 12621 Alsufi are named after him. Al-Sufi published his famous Book of Fixed Stars in 964, describing much of his work, both in textual descriptions and pictures. Al-Biruni reports, he lived at the Buyid court in Isfahan.'Abd al-Rahman al-Sufi was one of the famous nine Muslim astronomers. His name implies, he lived at the court of Emir Adud ad-Daula in Isfahan and worked on translating and expanding Greek astronomical works the Almagest of Ptolemy. He contributed several corrections to Ptolemy's star list and did his own brightness and magnitude estimates which deviated from those in Ptolemy's work, he was a major translator into Arabic of the Hellenistic astronomy, centered in Alexandria, the first to attempt to relate the Greek with the traditional Arabic star names and constellations, which were unrelated and overlapped in complicated ways.
He identified the Large Magellanic Cloud, visible from Yemen, though not from Isfahan. He made the earliest recorded observation of the Andromeda Galaxy in 964 AD; these were the first galaxies other than the Milky Way to be observed from Earth. He observed that the ecliptic plane is inclined with respect to the celestial equator and more calculated the length of the tropical year, he observed and described the stars, their positions, their magnitudes and their colour, setting out his results constellation by constellation. For each constellation, he provided two drawings, one from the outside of a celestial globe, the other from the inside. Al-Sufi wrote about the astrolabe, finding numerous additional uses for it: he described over 1000 different uses, in areas as diverse as astronomy, horoscopes, surveying, Qibla, Salat prayer, etc. Since 2006, Astronomy Society of Iran – Amateur Committee hold an international Sufi Observing Competition in the memory of Al-Sufi; the first competition was held in 2006 in the north of Semnan Province and the second was held in the summer of 2008 in Ladiz near the Zahedan.
More than 100 attendees from Iran and Iraq participated in the event. List of Iranian scientists List of Muslim scientists Astronomy in Islam Liber locis stellarum fixarum, 964 da www.atlascoelestis.com Liber locis stellarum fixarum, 964, manoscritto del 1417 riprodotto il 1730 da www.atlascoelestis.com Ulug Beg in www.atlascoelestis.com Al-Sufi's constellations Al-Sūfī’s Book of the Constellations of the Fixed Stars and its Influence on Islamic and Western Celestial Cartography
Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times. Earth's axis of rotation is tilted with respect to its orbital plane; the gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, slows its rotation. Earth is the largest of the four terrestrial planets. Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water by oceans; the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, a convecting mantle that drives plate tectonics. Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms; some geological evidence indicates. Since the combination of Earth's distance from the Sun, physical properties, geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.
Humans have developed diverse cultures. The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most spelled eorðe, it has cognates in every Germanic language, their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was being used to translate the many senses of Latin terra and Greek γῆ: the ground, its soil, dry land, the human world, the surface of the world, the globe itself; as with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, Norse mythology included Jörð, a giantess given as the mother of Thor. Earth was written in lowercase, from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, the earth became the Earth when referenced along with other heavenly bodies. More the name is sometimes given as Earth, by analogy with the names of the other planets.
House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name but writes it in lowercase when preceded by the, it always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago. By 4.54±0.04 Bya the primordial Earth had formed. The bodies in the Solar System evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, dust. According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 million years to form. A subject of research is the formation of some 4.53 Bya. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.
In this view, the mass of Theia was 10 percent of Earth, it hit Earth with a glancing blow and some of its mass merged with Earth. Between 4.1 and 3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. A crust formed; the two models that explain land mass propose either a steady growth to the present-day forms or, more a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics