Thehistory of astronomy focuses on the contributions civilizations have made to further their understanding of theuniverse beyond earth's atmosphere.[1]Astronomy is one of the oldestnatural sciences, achieving a high level of success in the second half of the first millennium. Astronomy has origins in thereligious,mythological,cosmological, calendrical, andastrological beliefs and practices of prehistory. Early astronomical records date back to theBabylonians around 1000 BC. There is also astronomical evidence of interest from early Chinese, Central American and North European cultures.[2]
The success of astronomy, compared to other sciences, was achieved because of several reasons. Astronomy was the first science to have a mathematical foundation and have sophisticated procedures such as usingarmillary spheres and quadrants. This provided a solid base for collecting and verifying data.[4][5]Throughout the years, astronomy has broadened into multiple subfields such asastrophysics,observational astronomy,theoretical astronomy, andastrobiology.[6]
Calendars of the world have often been set by observations of the Sun and Moon (marking theday,month, andyear) and were important toagricultural societies, in which the harvest depended on planting at the correct time of year. The nearly full moon was also the only lighting for night-time travel into city markets.
Ancient astronomical artifacts have been found throughoutEurope. The artifacts demonstrate that Neolithic and Bronze Age Europeans had a sophisticated knowledge ofmathematics and astronomy.
Among the discoveries are:
Paleolithic archaeologistAlexander Marshack put forward a theory in 1972 that bone sticks from locations like Africa and Europe from possibly as long ago as 35,000 BC could be marked in ways that tracked the Moon's phases,[11][page needed] an interpretation that has met with criticism.[12]
TheWarren Field calendar in the Dee River valley ofScotland'sAberdeenshire was firstexcavated in 2004 but was revealed in 2013 as a find of huge significance. It is to date the oldest known calendar, created around 8,000 BC and predating all other calendars by some 5,000 years. The calendar takes the form of an earlyMesolithic monument containing a series of 12 pits which appear to help the observer track lunar months by mimicking the phases of the Moon. It also aligns to sunrise at the winter solstice, thus coordinating the solar year with the lunar cycles. The monument had been maintained and periodically reshaped, perhaps up to hundreds of times, in response to shifting solar/lunar cycles, over the course of 6,000 years, until the calendar fell out of use around 4,000 years ago.[13][14][15][16]
Goseck circle is located inGermany and belongs to thelinear pottery culture. First discovered in 1991, its significance was only clear after results from archaeological digs became available in 2004. The site is one of hundreds of similarcircular enclosures built in a region encompassingAustria,Germany, and theCzech Republic during a 200-year period starting shortly after 5000 BC.[17]
TheNebra sky disc is aBronze Age bronze disc that was buried in Germany, not far from the Goseck circle, around 1600 BC. It measures about 30 cm (12 in) diameter with a mass of 2.2 kg (4.9 lb) and displays a blue-green patina (from oxidization) inlaid with gold symbols. Found by archeological thieves in 1999 and recovered in Switzerland in 2002, it was soon recognized as a spectacular discovery, among the most important of the 20th century.[18][19] Investigations revealed that the object had been in use around 400 years before burial (2000 BC), but that its use had been forgotten by the time of burial. The inlaid gold depicted the full moon, a crescent moon about 4 or 5 days old, and thePleiades star cluster in a specific arrangement, forming the earliest known depiction of celestial phenomena. Twelve lunar months pass in 354 days, requiring a calendar to insert a leap month every two or three years in order to keep synchronized with the solar year's seasons (making itlunisolar). The earliest known descriptions of this coordination were recorded by the Babylonians in the sixth or seventh centuries BC, over one thousand years later. Those descriptions verified ancient knowledge of the Nebra sky disc's celestial depiction as the precise arrangement needed to judge when to insert theintercalary month into a lunisolar calendar, making it an astronomical clock for regulating such a calendar a thousand or more years before any other known method.[20]
TheKokino site, discovered in 2001, sits atop an extinctvolcanic cone at an elevation of 1,013 metres (3,323 ft), occupying about 0.5 hectares overlooking the surrounding countryside inNorth Macedonia. ABronze Ageastronomical observatory was constructed there around 1900 BC and continuously served the nearby community that lived there until about 700 BC. The central space was used to observe the rising of the Sun and full moon. Three markings locate sunrise at the summer and winter solstices and at the two equinoxes. Four more give the minimum and maximum declinations of the full moon: in summer, and in winter. Two measure the lengths of lunar months. Together, they reconcile solar and lunar cycles in marking the 235lunations that occur during 19 solar years, regulating a lunar calendar. On a platform separate from the central space, at lower elevation, four stone seats (thrones) were made in north–south alignment, together with a trench marker cut in the eastern wall. This marker allows the rising Sun's light to fall on only the second throne, at midsummer (about July 31). It was used for ritual ceremony linking the ruler to the local sun god, and also marked the end of the growing season and time for harvest.[21]
Golden hats of Germany,France andSwitzerland dating from 1400 to 800 BC are associated with the Bronze AgeUrnfield culture. The Golden hats are decorated with a spiralmotif of theSun and theMoon. They were probably a kind ofcalendar used tocalibrate between thelunar andsolar calendars.[22][23] Modernscholarship has demonstrated that the ornamentation of the gold leaf cones of theSchifferstadt type, to which theBerlin Gold Hat example belongs, represent systematic sequences in terms of number and types of ornaments per band. A detailed study of the Berlin example, which is the only fully preserved one, showed that the symbols probably represent alunisolar calendar. The object would have permitted the determination of dates or periods in bothlunar andsolar calendars.[24]
The origins of astronomy can be found inMesopotamia, the "land between the rivers"Tigris andEuphrates, where the ancient kingdoms ofSumer,Assyria, andBabylonia were located. A form of writing known ascuneiform emerged among the Sumerians around 3500–3000 BC. Our knowledge of Sumerian astronomy is indirect, via the earliest Babylonian star catalogues dating from about 1200 BC. The fact that many star names appear in Sumerian suggests a continuity reaching into the Early Bronze Age.Astral theology, which gave planetary gods an important role inMesopotamian mythology andreligion, began with theSumerians. They also used asexagesimal (base 60) place-value number system, which simplified the task of recording very large and very small numbers. The modern practice of dividing a circle into 360degrees, or an hour into 60 minutes, began with the Sumerians. For more information, see the articles onBabylonian numerals andmathematics.
Classical sources frequently use theterm Chaldeans for the astronomers of Mesopotamia, who were originallya people, before being identified with priest-scribes specializing inastrology and other forms ofdivination.
The first evidence of recognition that astronomical phenomena are periodic and of the application of mathematics to their prediction is Babylonian. Tablets dating back to theOld Babylonian period document the application of mathematics to the variation in the length of daylight over a solar year. Centuries of Babylonian observations of celestial phenomena are recorded in the series ofcuneiform tablets known as theEnūma Anu Enlil. The oldest significant astronomical text that we possess is Tablet 63 of theEnūma Anu Enlil, theVenus tablet ofAmmi-saduqa, which lists the first and last visible risings of Venus over a period of about 21 years and is the earliest evidence that the phenomena of a planet were recognized as periodic. TheMUL.APIN contains catalogues of stars and constellations as well as schemes for predictingheliacal risings and the settings of the planets, lengths of daylight measured by awater clock,gnomon, shadows, andintercalations. The Babylonian GU text arranges stars in 'strings' that lie along declination circles and thus measure right-ascensions or time-intervals, and also employs the stars of the zenith, which are also separated by given right-ascensional differences.[26]
A significant increase in the quality and frequency of Babylonian observations appeared during the reign ofNabonassar (747–733 BC). The systematic records of ominous phenomena inBabylonian astronomical diaries that began at this time allowed for the discovery of a repeating 18-year cycle oflunar eclipses, for example. The Greek astronomerPtolemy later used Nabonassar's reign to fix the beginning of an era, since he felt that the earliest usable observations began at this time.
The last stages in the development of Babylonian astronomy took place during the time of theSeleucid Empire (323–60 BC). In the 3rd century BC, astronomers began to use "goal-year texts" to predict the motions of the planets. These texts compiled records of past observations to find repeating occurrences of ominous phenomena for each planet. About the same time, or shortly afterwards, astronomers created mathematical models that allowed them to predict these phenomena directly, without consulting records. A notable Babylonian astronomer from this time wasSeleucus of Seleucia, who was a supporter of theheliocentric model.
Astronomy in the Indian subcontinent dates back to the period ofIndus Valley Civilisation during 3rd millennium BC, when it was used to create calendars.[28] As the Indus Valley Civilization did not leave behind written documents, the oldest extant Indian astronomical text is theVedanga Jyotisha, dating from theVedic period.[29] The Vedanga Jyotisha is attributed to Lagadha and has an internal date of approximately 1350 BC, and describes rules for tracking the motions of the Sun and the Moon for the purposes of ritual. It is available in two recensions, one belonging to the Rig Veda, and the other to the Yajur Veda. According to the Vedanga Jyotisha, in ayuga or "era", there are 5 solar years, 67 lunar sidereal cycles, 1,830 days, 1,835 sidereal days, and 62 synodic months. During the sixth century, astronomy was influenced by the Greek and Byzantine astronomical traditions.[28][30][31]
Aryabhata (476–550), in his magnum opusAryabhatiya (499), propounded a computational system based on a planetary model in which the Earth was taken to bespinning on its axis and the periods of the planets were given with respect to the Sun. He accurately calculated many astronomical constants, such as the periods of the planets, times of thesolar andlunareclipses, and the instantaneous motion of the Moon.[32][33][page needed] Early followers of Aryabhata's model includedVarāhamihira,Brahmagupta, andBhāskara II.
Astronomy was advanced during theShunga Empire, and manystar catalogues were produced during this time. The Shunga period is known[according to whom?] as the "Golden age of astronomy in India".It saw the development of calculations for the motions and places of various planets, their rising and setting,conjunctions, and the calculation of eclipses.
By the sixth century, Indian astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the sixth century by the astronomersVarahamihira and Bhadrabahu. The tenth-century astronomerBhattotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.[34]
TheAncient Greeks developed astronomy, which they treated as a branch of mathematics, to a highly sophisticated level. The first geometrical, three-dimensional models to explain the apparent motion of the planets were developed in the 4th century BC byEudoxus of Cnidus andCallippus of Cyzicus. Their models were based on nested homocentric spheres centered upon the Earth. Their younger contemporaryHeraclides Ponticus proposed that the Earth rotates around its axis.
A different approach to celestial phenomena was taken by natural philosophers such asPlato andAristotle. They were less concerned with developing mathematical predictive models than with developing an explanation of the reasons for the motions of the Cosmos. In hisTimaeus, Plato described the universe as a spherical body divided into circles carrying the planets and governed according to harmonic intervals by aworld soul.[35] Aristotle, drawing on the mathematical model of Eudoxus, proposed that the universe was made of a complex system of concentricspheres, whose circular motions combined to carry the planets around the Earth.[36] This basic cosmological model prevailed, in various forms, until the 16th century.
Greek geometrical astronomy developed away from the model of concentric spheres to employ more complex models in which aneccentric circle would carry around a smaller circle, called anepicycle which in turn carried around a planet. The first such model is attributed toApollonius of Perga and further developments in it were carried out in the 2nd century BC byHipparchus of Nicea. Hipparchus made a number of other contributions, including the first measurement ofprecession and the compilation of the first star catalog in which he proposed our modern system ofapparent magnitudes.
TheAntikythera mechanism, anancient Greek astronomical observational device for calculating the movements of the Sun and the Moon, possibly the planets, dates from about 150–100 BC, and was the first ancestor of an astronomicalcomputer. It was discovered in an ancient shipwreck off the Greek island ofAntikythera, betweenKythera andCrete. The device became famous for its use of adifferential gear, previously believed to have been invented in the 16th century, and the miniaturization and complexity of its parts, comparable to a clock made in the 18th century. The original mechanism is displayed in the Bronze collection of theNational Archaeological Museum of Athens, accompanied by a replica.
Depending on the historian's viewpoint, the acme or corruption[citation needed][dubious –discuss] of Classical physical astronomy is seen withPtolemy, a Greco-Roman astronomer from Alexandria of Egypt, who wrote the classic comprehensive presentation of geocentric astronomy, theMegale Syntaxis (Great Synthesis), better known by its Arabic titleAlmagest, which had a lasting effect on astronomy up to theRenaissance. In hisPlanetary Hypotheses, Ptolemy ventured into the realm of cosmology, developing a physical model of his geometric system, in a universe many times smaller than the more realistic conception ofAristarchus of Samos four centuries earlier.
Segment of theastronomical ceiling of Senenmut's Tomb (circa 1479–1458 BC), depicting constellations, protective deities, and twenty-four segmented wheels for the hours of the day and the months of the year
The precise orientation of theEgyptian pyramids affords a lasting demonstration of the high degree of technical skill in watching the heavens attained in the 3rd millennium BC. It has been shown the Pyramids were aligned towards thepole star, which, because of theprecession of the equinoxes, was at that timeThuban, a faint star in the constellation ofDraco.[39] Evaluation of the site of the temple ofAmun-Re atKarnak, taking into account the change over time of theobliquity of the ecliptic, has shown that the Great Temple was aligned on the rising of themidwinter Sun.[40] The length of the corridor down which sunlight would travel would have limited illumination at other times of the year. The Egyptians also found the position of Sirius (the dog star), who they believed was Anubis, their jackal-headed god, moving through the heavens. Its position was critical to their civilisation as when it rose heliacal in the east before sunrise it foretold the flooding of the Nile. It is also the origin of the phrase "dog days of summer".[41]
Astronomy played a considerable part inreligious matters for fixing the dates of festivals and determining the hours of thenight. The titles of several temple books are preserved recording the movements and phases of theSun,Moon, andstars. The rising ofSirius (Egyptian: Sopdet,Greek: Sothis) at the beginning of the inundation was a particularly important point to fix in the yearly calendar.
Writing in theRoman era,Clement of Alexandria gives some idea of the importance of astronomical observations to the sacred rites:
And after the Singer advances the Astrologer (ὡροσκόπος), with ahorologium (ὡρολόγιον) in his hand, and apalm (φοίνιξ), the symbols ofastrology. He must know by heart theHermetic astrological books, which are four in number. Of these, one is about the arrangement of the fixed stars that are visible; one on the positions of the Sun and Moon and five planets; one on the conjunctions and phases of the Sun and Moon; and one concerns their risings.[42]
The Astrologer's instruments (horologium andpalm) are aplumb line and sighting instrument[clarification needed]. They have been identified with two inscribed objects in theBerlin Museum; a short handle from which a plumb line was hung, and a palm branch with a sight-slit in the broader end. The latter was held close to the eye, the former in the other hand, perhaps at arm's length. The "Hermetic" books which Clement refers to are the Egyptian theological texts, which probably have nothing to do withHellenisticHermetism.[43]
From the tables of stars on the ceiling of the tombs ofRameses VI andRameses IX it seems that for fixing the hours of the night a man seated on the ground faced the Astrologer in such a position that the line of observation of thepole star passed over the middle of his head. On the different days of the year each hour was determined by a fixed starculminating or nearly culminating in it, and the position of these stars at the time is given in the tables as in the centre, on the left eye, on the right shoulder, etc. According to the texts, in founding or rebuilding temples thenorth axis was determined by the same apparatus, and we may conclude that it was the usual one for astronomical observations. In careful hands it might give results of a high degree of accuracy.
Astronomy in China has a long history. Detailed records of astronomical observations were kept from about the 6th century BC, until the introduction of Western astronomy and the telescope in the 17th century. Chinese astronomers were able to precisely predict eclipses.
Much of early Chinese astronomy was for the purpose of timekeeping. The Chinese used a lunisolar calendar, but because the cycles of the Sun and the Moon are different, astronomers often prepared new calendars and made observations for that purpose.
Astrological divination was also an important part of astronomy. Astronomers took careful note of"guest stars" (Chinese:客星;pinyin:kèxīng;lit. 'guest star') which suddenly appeared among thefixed stars. They were the first to record a supernova, in the Astrological Annals of the Houhanshu in 185 AD. Also, the supernova that created theCrab Nebula in 1054 is an example of a "guest star" observed by Chinese astronomers, although it was not recorded by their European contemporaries. Ancient astronomical records of phenomena like supernovae and comets are sometimes used in modern astronomical studies.
Maya astronomicalcodices include detailed tables for calculatingphases of the Moon, the recurrence of eclipses, and the appearance and disappearance ofVenus as morning andevening star. The Maya based theircalendrics in the carefully calculated cycles of thePleiades, theSun, theMoon,Venus,Jupiter,Saturn,Mars, and also they had a precise description of the eclipses as depicted in theDresden Codex, as well as the ecliptic or zodiac, and theMilky Way was crucial in their Cosmology.[44] A number of important Maya structures are believed to have been oriented toward the extreme risings and settings of Venus. To the ancient Maya, Venus was the patron of war and many recorded battles are believed to have been timed to the motions of this planet. Mars is also mentioned in preserved astronomical codices and earlymythology.[45]
Although theMaya calendar was not tied to the Sun,John Teeple has proposed that the Maya calculated thesolar year to somewhat greater accuracy than theGregorian calendar.[46] Both astronomy and an intricate numerological scheme for the measurement of time were vitally important components ofMaya religion.
The Maya believed that the Earth was the center of all things, and that the stars, moons, and planets were gods. They believed that their movements were the gods traveling between the Earth and other celestial destinations. Many key events in Maya culture were timed around celestial events, in the belief that certain gods would be present.[47]
The Arabic and the Persian world underIslam had become highly cultured, and many important works of knowledge fromGreek astronomy,Indian astronomy, and Persian astronomy were translated into Arabic, which were then used and stored in libraries throughout the area. An important contribution by Islamic astronomers was their emphasis onobservational astronomy.[48] This led to the emergence of the first astronomicalobservatories in theMuslim world by the early 9th century.[49][50]Zij star catalogues were produced at these observatories.
In the ninth century, Persian astrologerAlbumasar was thought to be one of the greatest astrologer at that time. His practical manuals for training astrologers profoundly influenced Muslim intellectual history and, through translations, that of western Europe and Byzantium In the 10th century,[51] Albumasar's "Introduction" was one of the most important sources for the recovery of Aristotle for medieval European scholars.[52]Abd al-Rahman al-Sufi (Azophi) carried out observations on thestars and described their positions,magnitudes, brightness, andcolour and drawings for each constellation in hisBook of Fixed Stars. He also gave the first descriptions and pictures of "A Little Cloud" now known as theAndromeda Galaxy. He mentions it as lying before the mouth of a Big Fish, an Arabicconstellation. This "cloud" was apparently commonly known to theIsfahan astronomers, very probably before 905 AD.[53] The first recorded mention of theLarge Magellanic Cloud was also given by al-Sufi.[54][55] In 1006,Ali ibn Ridwan observedSN 1006, the brightestsupernova in recorded history, and left a detailed description of the temporary star.
In the late tenth century, a huge observatory was built nearTehran,Iran, by the astronomerAbu-Mahmud al-Khujandi who observed a series ofmeridiantransits of the Sun, which allowed him to calculate the tilt of the Earth's axis relative to the Sun. He noted that measurements by earlier (Indian, then Greek) astronomers had found higher values for this angle, possible evidence that the axial tilt is not constant but was in fact decreasing.[56][57] In 11th-century Persia,Omar Khayyám compiled many tables and performed a reformation of thecalendar that was more accurate than theJulian and came close to theGregorian.
Bhāskara II (1114–1185) was the head of the astronomical observatory at Ujjain, continuing the mathematical tradition of Brahmagupta. He wrote theSiddhantasiromani which consists of two parts:Goladhyaya (sphere) andGrahaganita (mathematics of the planets). He also calculated the time taken for the Sun to orbit the Earth to nine decimal places. The Buddhist University ofNalanda at the time offered formal courses in astronomical studies.
Other important astronomers from India includeMadhava of Sangamagrama,Nilakantha Somayaji andJyeshtadeva, who were members of theKerala school of astronomy and mathematics from the 14th century to the 16th century. Nilakantha Somayaji, in hisAryabhatiyabhasya, a commentary on Aryabhata'sAryabhatiya, developed his own computational system for a partiallyheliocentric planetary model, in which Mercury, Venus,Mars,Jupiter andSaturn orbit theSun, which in turn orbits theEarth, similar to theTychonic system later proposed byTycho Brahe in the late 16th century. Nilakantha's system, however, was mathematically more efficient than the Tychonic system, due to correctly taking into account the equation of the centre andlatitudinal motion of Mercury and Venus. Most astronomers of theKerala school of astronomy and mathematics who followed him accepted his planetary model.[63][64]
After the significant contributions of Greek scholars to the development of astronomy, it entered a relatively static era in Western Europe from the Roman era through the 12th century. This lack of progress has led some astronomers to assert that nothing happened in Western European astronomy during the Middle Ages.[65] Recent investigations, however, have revealed a more complex picture of the study and teaching of astronomy in the period from the 4th to the 16th centuries.[66]
Western Europe entered the Middle Ages with great difficulties that affected the continent's intellectual production. The advanced astronomical treatises ofclassical antiquity were written inGreek, and with the decline of knowledge of that language, only simplified summaries and practical texts were available for study. The most influential writers to pass on this ancient tradition inLatin wereMacrobius,Pliny,Martianus Capella, andCalcidius.[67] In the 6th century BishopGregory of Tours noted that he had learned his astronomy from reading Martianus Capella, and went on to employ this rudimentary astronomy to describe a method by which monks could determine the time of prayer at night by watching the stars.[68]
In the 7th century the English monkBede of Jarrow published an influential text,On the Reckoning of Time, providing churchmen with the practical astronomical knowledge needed to compute the proper date ofEaster using a procedure called thecomputus. This text remained an important element of the education of clergy from the 7th century until well after the rise of theUniversities in the12th century.[69]
The range of surviving ancient Roman writings on astronomy and the teachings of Bede and his followers began to be studied in earnest during therevival of learning sponsored by the emperorCharlemagne.[70] By the 9th century rudimentary techniques for calculating the position of the planets were circulating in Western Europe; medieval scholars recognized their flaws, but texts describing these techniques continued to be copied, reflecting an interest in the motions of the planets and in their astrological significance.[71]
Building on this astronomical background, in the 10th century European scholars such asGerbert of Aurillac began to travel to Spain and Sicily to seek out learning which they had heard existed in the Arabic-speaking world. There they first encountered various practical astronomical techniques concerning the calendar and timekeeping, most notably those dealing with theastrolabe. Soon scholars such asHermann of Reichenau were writing texts in Latin on the uses and construction of the astrolabe and others, such asWalcher of Malvern, were using the astrolabe to observe the time of eclipses in order to test the validity of computistical tables.[72]
By the 12th century, scholars were traveling to Spain and Sicily to seek out more advanced astronomical and astrological texts, which theytranslated into Latin from Arabic and Greek to further enrich the astronomical knowledge of Western Europe. The arrival of these new texts coincided with the rise of the universities in medieval Europe, in which they soon found a home.[73] Reflecting the introduction of astronomy into the universities,John of Sacrobosco wrote a series of influential introductory astronomy textbooks: theSphere, a Computus, a text on theQuadrant, and another on Calculation.[74]
In the 14th century,Nicole Oresme, later bishop of Liseux, showed that neither the scriptural texts nor the physical arguments advanced against the movement of the Earth were demonstrative and adduced the argument of simplicity for the theory that the Earth moves, andnot the heavens. However, he concluded "everyone maintains, and I think myself, that the heavens do move and not the earth: For God hath established the world which shall not be moved."[75] In the 15th century, CardinalNicholas of Cusa suggested in some of his scientific writings that the Earth revolved around the Sun, and that each star is itself a distant sun.
During the renaissance period, astronomy began to undergo a revolution in thought known as theCopernican Revolution, which gets the name from the astronomerNicolaus Copernicus, who proposed a heliocentric system, in which the planets revolved around the Sun and not the Earth. HisDe revolutionibus orbium coelestium was published in 1543.[76] While in the long term this was a very controversial claim, in the very beginning it only brought minor controversy.[76] The theory became the dominant view because many figures, most notablyGalileo Galilei,Johannes Kepler andIsaac Newton championed and improved upon the work. Other figures also aided this new model despite not believing the overall theory, likeTycho Brahe, with his well-known observations.[77]
Brahe, a Danish noble, was an essential astronomer in this period.[77] He came on the astronomical scene with the publication ofDe nova stella, in which he disproved conventional wisdom on the supernovaSN 1572[77] (As bright as Venus at its peak, SN 1572 later became invisible to the naked eye, disproving theAristotelian doctrine of the immutability of the heavens.)[78][79] He also created theTychonic system, where the Sun and Moon and the stars revolve around the Earth, but the other five planets revolve around the Sun. This system blended the mathematical benefits of the Copernican system with the "physical benefits" of the Ptolemaic system.[80] This was one of the systems people believed in when they did not accept heliocentrism, but could no longer accept the Ptolemaic system.[80] He is most known for his highly accurate observations of the stars and the planets. Later he moved to Prague and continued his work. In Prague he was at work on theRudolphine Tables, that were not finished until after his death.[81] The Rudolphine Tables was a star map designed to be more accurate than either theAlfonsine tables, made in the 1300s, and thePrutenic Tables, which were inaccurate.[81] He was assisted at this time by his assistant Johannes Kepler, who would later use his observations to finish Brahe's works and for his theories as well.[81]
After the death of Brahe, Kepler was deemed his successor and was given the job of completing Brahe's uncompleted works, like the Rudolphine Tables.[81] He completed the Rudolphine Tables in 1624, although it was not published for several years.[81] Like many other figures of this era, he was subject to religious and political troubles, like theThirty Years' War, which led to chaos that almost destroyed some of his works. Kepler was, however, the first to attempt to derive mathematical predictions of celestial motions from assumed physical causes. He discovered the threeKepler's laws of planetary motion that now carry his name, those laws being as follows:
The orbit of a planet is an ellipse with the Sun at one of the two foci.
A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.[82]
With these laws, he managed to improve upon the existing heliocentric model. The first two were published in 1609. Kepler's contributions improved upon the overall system, giving it more credibility because it adequately explained events and could cause more reliable predictions. Before this, the Copernican model was just as unreliable as the Ptolemaic model. This improvement came because Kepler realized the orbits were not perfect circles, but ellipses.
Galileo Galilei (1564–1642) crafted his own telescope and discovered that the Moon had craters, that Jupiter had moons, that the Sun had spots, and that Venus had phases like the Moon. Portrait byJustus Sustermans.
The invention of thetelescope in 1608 revolutionized the study of astronomy.Galileo Galilei was among the first to use a telescope[83] to observe the sky, after constructing a 20x refractor telescope.[84] He discovered the four largest moons of Jupiter in 1610, which are now collectively known as theGalilean moons, in his honor.[85] This discovery was the first known observation of satellites orbiting another planet.[85] He also found that the Moon had craters and observed, and correctly explained sunspots, and that Venus exhibited a full set of phases resembling lunar phases.[86] Galileo argued that these facts demonstrated incompatibility with the Ptolemaic model, which could not explain the phenomenon and would even contradict it.[86] With Jupiter's moons, he demonstrated that the Earth does not have to have everything orbiting it and that other bodies could orbit another planet, such as the Earth orbiting the Sun.[85] In the Ptolemaic system the celestial bodies were supposed to be perfect so such objects should not have craters or sunspots.[87] The phases of Venus could only happen in the event that Venus orbits around the Sun, which did not happen in the Ptolemaic system. He, as the most famous example, had to face challenges from church officials, more specifically theRoman Inquisition.[88] They accused him of heresy because these beliefs went against the teachings of the Roman Catholic Church and were challenging the Catholic church's authority when it was at its weakest.[88] While he was able to avoid punishment for a little while he was eventually tried and pled guilty to heresy in 1633.[88] Although this came at some expense, his book was banned, and he was put under house arrest until he died in 1642.[89]
Plate with figures illustrating articles on astronomy, from the 1728Cyclopædia
Sir Isaac Newton developed further ties between physics and astronomy through hislaw of universal gravitation. Realizing that the same force that attracts objects to the surface of the Earth held the Moon in orbit around the Earth, Newton was able to explain – in one theoretical framework – all known gravitational phenomena. In hisPhilosophiæ Naturalis Principia Mathematica, he derived Kepler's laws from first principles. Those first principles are as follows:
In an inertial reference frame, thevector sum of the forces F on an object is equal to themass m of that object multiplied by theacceleration a of the object: F = ma. (It is assumed here that the mass m is constant)
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.[90]
Thus while Kepler explained how the planets moved, Newton accurately managed to explain why the planets moved the way they do. Newton's theoretical developments laid many of the foundations of modern physics.
Cosmic pluralism is the name given to the idea that the stars are distant suns, perhaps with their own planetary systems.Ideas in this direction were expressed in antiquity, byAnaxagoras and byAristarchus of Samos, but did not find mainstream acceptance. The first astronomer of the European Renaissance to suggest that the stars were distant suns wasGiordano Bruno in hisDe l'infinito universo et mondi (1584). This idea, together with a belief in intelligent extraterrestrial life, was among the charges brought against him by the Inquisition.The idea became mainstream in the later 17th century, especially following the publication ofConversations on the Plurality of Worlds byBernard Le Bovier de Fontenelle (1686), and by the early 18th century it was the default working assumptions in stellar astronomy.
The Italian astronomerGeminiano Montanari recorded observing variations in luminosity of the starAlgol in 1667. Edmond Halley published the first measurements of theproper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancientGreek astronomers Ptolemy and Hipparchus.William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Waycore. His sonJohn Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[94] In addition to his other accomplishments, William Herschel is noted for his discovery that some stars do not merely lie along the same line of sight, but are physical companions that form binary star systems.[95]
Pre-photography, data recording of astronomical data was limited by the human eye. In 1840,John W. Draper, a chemist, created the earliest known astronomical photograph of the Moon. And by the late 19th century thousands of photographic plates of images of planets, stars, and galaxies were created. Most photography had lower quantum efficiency (i.e. captured less of the incident photons) than human eyes but had the advantage of long integration times (100 ms for the human eye compared to hours for photos). This vastly increased the data available to astronomers, which led to the rise ofhuman computers, famously theHarvard Computers, to track and analyze the data.
The science ofstellar spectroscopy was pioneered byJoseph von Fraunhofer andAngelo Secchi. By comparing the spectra of stars such asSirius to the Sun, they found differences in the strength and number of theirabsorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars intospectral types.[96] The first evidence of helium was observed on August 18, 1868, as a bright yellow spectral line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India.
The first direct measurement of the distance to a star (61 Cygni at 11.4light-years) was made in 1838 byFriedrich Bessel using theparallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[citation needed] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion.Edward Pickering discovered the firstspectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the starMizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such asFriedrich Georg Wilhelm von Struve andS. W. Burnham, allowing the masses of stars to be determined from the computation oforbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[97]In 1847,Maria Mitchell discovered a comet using a telescope.
With the accumulation of large sets of astronomical data, teams like theHarvard Computers rose in prominence which led to many female astronomers, previously relegated as assistants to male astronomers, gaining recognition in the field. TheUnited States Naval Observatory (USNO) and other astronomy research institutions hiredhuman "computers", who performed the tedious calculations while scientists performed research requiring more background knowledge.[98] A number of discoveries in this period were originally noted by the women "computers" and reported to their supervisors.Henrietta Swan Leavitt discovered thecepheid variable starperiod-luminosity relation which she further developed into a method of measuring distance outside of the Solar System.
A veteran of the Harvard Computers,Annie J. Cannon developed the modern version of the stellar classification scheme in during the early 1900s (O B A F G K M, based on color and temperature), manually classifying more stars in a lifetime than anyone else (around 350,000).[99][100]The twentieth century saw increasingly rapid advances in the scientific study of stars.Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing thevisual magnitude against thephotographic magnitude. The development of thephotoelectricphotometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921Albert A. Michelson made the first measurements of a stellar diameter using aninterferometer on theHooker telescope atMount Wilson Observatory.[101]
Comparison ofCMB (Cosmic microwave background) results from satellitesCOBE,WMAP andPlanck documenting a progress in 1989–2013
Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, theHertzsprung–Russell diagram was developed, propelling the astrophysical study of stars.InPotsdam in 1906, the Danish astronomerEjnar Hertzsprung published the first plots of color versusluminosity for these stars. These plots showed a prominent and continuous sequence of stars, which he named the Main Sequence.AtPrinceton University,Henry Norris Russell plotted the spectral types of these stars against their absolute magnitude, and found that dwarf stars followed a distinct relationship. This allowed the real brightness of a dwarf star to be predicted with reasonable accuracy.Successfulmodels were developed to explain the interiors of stars and stellar evolution.Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 doctoral thesis.[102] The spectra of stars were further understood through advances inquantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[103]As evolutionary models of stars were developed during the 1930s,Bengt Strömgren introduced the term Hertzsprung–Russell diagram to denote a luminosity-spectral class diagram.A refined scheme forstellar classification was published in 1943 byWilliam Wilson Morgan andPhilip Childs Keenan.
The existence of theMilky Way, Earth's home galaxy, as a separate group of stars was only proven in the 20th century, as was the knowledge that other galaxies exist and that most of them are moving away from each other. The "Great Debate" betweenHarlow Shapley andHeber Curtis, in the 1920s, concerned the nature of the Milky Way, spiral nebulae, and the dimensions of the universe.[104]
The Sun was found to be part of agalaxy made up of more than 1010 stars (10 billion stars). The existence of other galaxies, one of the matters ofthe great debate, was settled byEdwin Hubble, who identified theAndromeda nebula as a different galaxy, and many others at large distances and receding, moving away from our galaxy.
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^The Nebra Sky Disc, Landesamt für Denkmalpflege und Archäologie Sachsen-Anhalt / Landesmuseum für Vorgeschichte, archived fromthe original on 12 April 2014, retrieved15 October 2014
^Nebra Sky Disc, UNESCO Memory of the World Programme, retrieved2025-04-22
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^Richard Lemay,Abu Ma'shar and Latin Aristotelianism in the Twelfth Century, The Recovery of Aristotle's Natural Philosophy through Iranian Astrology, 1962.
^Kepple, George Robert; Sanner, Glen W. (1998),The Night Sky Observer's Guide, Volume 1, Willmann-Bell, Inc., p. 18,ISBN0-943396-58-1
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^Henry Smith Williams,The Great Astronomers (New York: Simon and Schuster, 1930), pp. 99–102 describes "the record of astronomical progress" from the Council of Nicea (325 AD) to the time of Copernicus (1543 AD) on four blank pages.
^David Juste, "Neither Observation nor Astronomical Tables: An Alternative Way of Computing the Planetary Longitudes in the Early Western Middle Ages," pp. 181–222 in Charles Burnett, Jan P. Hogendijk,Kim Plofker, and Michio Yano,Studies in the Exact Sciences in Honour of David Pingree, (Leiden: Brill, 2004)
^Nicole Oresme,Le Livre du ciel et du monde, xxv, ed. A. D. Menut and A. J. Denomy, trans. A. D. Menut, (Madison: Univ. of Wisconsin Pr., 1968), quotation at pp. 536–7.
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^abcJohn Louis Emil Dreyer,Tycho Brahe: a Picture of Scientific Life and Work in the Sixteenth Century, A. & C. Black (1890), pp. 162–3
^abcdeAthreya, A.; Gingerich, O. (December 1996). "An Analysis of Kepler's Rudolphine Tables and Implications for the Reception of His Physical Astronomy".Bulletin of the American Astronomical Society.28 (4): 1305.
^GINGERICH, O. (2011). Galileo, the Impact of the Telescope, and the Birth of Modern Astronomy.Proceedings of the American Philosophical Society,155 (2), 134–141.
^Brasch, Frederick (October 1931), "The Royal Society of London and its Influence upon Scientific Thought in the American Colonies",The Scientific Monthly,33 (4): 338.
^Morison, Samuel Eliot (March 1934), "The Harvard School of Astronomy in the Seventeenth Century",The New England Quarterly,7 (1):3–24,doi:10.2307/359264,JSTOR359264.
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