History of astronomy
Author: Romuald Rzeszutko
Astronomy is probably the oldest of the natural sciences, dating back to antiquity, with its origins in the religious practices of pre-history: vestiges of these are still found in astrology, a discipline long interwoven with astronomy, and not completely different from it until about 1750-1800 in the Western World. Early astronomy involved observing the regular patterns of the motions of visible celestial objects, especially the Sun, Moon, stars and naked eye planets. An example of this early astronomy might involve a study of the changing position of the Sun along the horizon or the changing appearances of stars in the course of the year, which could be used to establish an agricultural or ritual calendar. In some cultures astronomical data was used for astrological prognostication.
Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.
Ancient history
Early cultures identified celestial objects with gods and spirits. They related these objects (and their movements) to phenomena such as rain, drought, seasons, and tides. It is generally believed that the first "professional" astronomers were priests, and that their understanding of the "heavens" was seen as "divine", hence astronomy's ancient connection to what is now called astrology. Ancient structures with astronomical alignments (such as Stonehenge) probably fulfilled both astronomical and religious functions.
Calendars of the world have usually been set by the Sun and Moon (measuring the day, month and year), and were of importance to agricultural societies, in which the harvest depended on planting at the correct time of year. The most common modern calendar is based on the Roman calendar, which divided the year into twelve months of alternating thirty and thirty-one days apiece. In 46 BC Julius Caesar instigated calendar reform and created the leap year.
Mesopotamia
The origins of Western astronomy can be found in Mesopotamia, the "land between the rivers" Tigris and Euphrates, where the ancient kingdoms of Sumer, Assyria, and Babylonia were located. A form of writing known as cuneiform emerged among the Sumerians around 3500-3000 BC. The Sumerians only practiced a basic form of astronomy, but they had an important influence on the sophisticated astronomy of the Babylonians. Astral theology, which gave planetary gods an important role in Mesopotamian mythology and religion, began with the Sumerians. They also used a sexagesimal (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 360 degrees, of 60 minutes each, began with the Sumerians. For more information, see the articles on Babylonian numerals and mathematics.
Classical sources frequently use the term Chaldeans for the astronomers of Mesopotamia, who were, in reality, priest-scribes specializing in astrology and other forms of divination. The earliest activities of Babylonian astronomers were limited to the recording of significant astronomical phenomena that they regarded as omens. The best known example is the Venus tablet of Ammisaduqa, a record of the first and last visibilities of the planet Venus observed around the 16th century BC. The text of the Venus tablet was later included in a large compendium of omens called Enuma Anu Enlil.
A significant increase in the quality and frequency of Babylonian observations appeared during the reign of Nabonassar (747-733 BC). The systematic records of ominous phenomena in astronomical diaries that began at this time allowed for the discovery of a repeating 18-year cycle of lunar eclipses, for example. The Greek astronomer Ptolemy 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 the Seleucid Empire (323-60 BC). In the third century, 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 past records.
The Mesopotamian underpinnings of western astronomy are extensive. It was from the Mesopotamians that the Greeks gained their knowledge of the five visible planets and the constellations of the zodiac, centuries of recorded astronomical observations, and even the idea that the motions of the planets could be predicted with accuracy.
China
Astronomy in China has a long history. Houses at Banpo ca. 4000 BCE were oriented to a position coinciding with the culmination of the constellation Yingshi (part of what we call Pegasus), shortly after the winter solstice. This had the effect of orienting the houses for solar gain. Mosaics of two of the four mega-constellations (Dragon, Phoenix, Tiger, Turtle) flanked a Longshan burial in Puyang at roughly the same time. The astronomical observatory at Taosi (ca. 2300-1900 BCE) used the hills to the east as markers.
Oracle bones from the Yin Dynasty (2nd millennium BCE) record eclipses and novae. Detailed records of astronomical observations were kept from about the 6th century BCE, until the introduction of Western astronomy and the telescope in the 17th century. Chinese astronomers were able to precisely predict comets and 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" which suddenly appeared among the fixed stars. They were the first to record a supernova, in the Astrological Annals of the Houhanshu in 185 A.D. Also, the supernova that created the Crab 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.
The world's first star catalogue was made by Gan De, a Chinese astronomer, in 4th century BC.
It's worth mentioning, among other achievements of Chinese astronomy, making the record of the earliest known solar eclipse on 22nd of October 2137 BCE, record of meteor shower (687 AD) and also making earliest known dated record of sunspot in 28 BCE. The Chinese astronomer, mathematician, and inventor Zhang Heng catalogued some 2500 stars in his lifetime, along with recognizing over 1000 constellations.
Greece
The Ancient 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 by Eudoxus of Cnidus and Callippus of Cyzicus in the 4th century BC. Their models were based on nested homocentric spheres centered upon the Earth. Their younger contemporary Heraclides Ponticus proposed that the Earth rotates around its axis.
A different approach to celestial phenomena was taken by natural philosophers such as Plato and Aristotle. They were less concerned with developing mathematical predictive models than with developing an explanation of the reasons for the motions of the Cosmos. In his Timaeus Plato described the universe as a spherical body divided into circles carrying the planets and governed according to harmonic intervals by a world soul. Aristotle, drawing on the mathematical model of Eudoxus, proposed that the universe was made of a complex system of concentric spheres, whose circular motions combined to carry the planets around the earth. This basic cosmological model prevailed, in various forms, until the Sixteenth century.
Greek geometrical astronomy developed away from the model of concentric spheres to employ more complex models in which an eccentric circle would carry around a smaller circle, called an epicycle which in turn carried around a planet. The first such model is attributed to Apollonius of Perga and further developments in it were carried out in the 2nd century BC by Hipparchus of Nicea. Hipparchus made a number of other contributions, including the first measurement of precession and the compilation of the first star catalog in which he proposed our modern system of apparent magnitudes.
The study of astronomy by the ancient Greeks was not limited to Greece itself but was further developed in the 3rd and 2nd centuries BC, in the Hellenistic states and in particular in Alexandria. However, the work was still done by ethnic Greeks. In the 3rd century BC Aristarchus of Samos was the first to propose a fully heliocentric system, while Eratosthenes, using the angles of shadows created at widely-separated regions, estimated the circumference of the Earth with great accuracy.
The Antikythera mechanism, an ancient Greek device for calculating the movements of planets, dates from about 80 B.C., and was the first ancestor of an astronomical computer. It was discovered in an ancient shipwreck off the Greek island of Antikythera, between Kythera and Crete. The device became famous for its use of a differential 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 the National Archaeological Museum of Athens, accompanied by a replica.
The culmination of Greek astronomy is seen with Ptolemy of Alexandria, who wrote the classic systematic presentation of geocentric astronomy, the Megale Syntaxis (Great Synthesis), better known by its Arabic title Almagest, which had a lasting effect on astronomy up to the Renaissance.
India
India has one of the oldest and most well-documented tradition of scientific astronomy. The earliest references to astronomy are found in the Rig Veda, which are dated 2000 BC. During next 2500 years, by 500 AD, ancient Indian astronomy has emerged as an important part of Indian studies and its affect is also seen in several treatises of that period. In some instances, astronomical principles were borrowed to explain matters, pertaining to astrology, like casting of a horoscope. Apart from this linkage of astronomy with astrology in ancient India, science of astronomy continued to develop independently, and culminated into original findings, like the calculation of occurrences of eclipses, determination of Earth's circumference, theorizing about the theory of gravitation or determining that sun was a star and determination of number of planets under our solar system.
As an offshoot, geometry, trigonometry, and algebra too developed. Indian astronomers had accurately calculated the value of pi and other constants. The oldest known exclusive text on astronomy is the Vedanga Jyotisha, though astronomical references are found in much older works. The Rigveda refers to the 27 constellations associated with the motions of the Sun and also the 12 zodiacal divisions of the sky.
There are astronomical references of chronological significance in the Vedas. Some Vedic notices mark the beginning of the year and that of the vernal equinox in Orion; this was the case around 4500 BC. Fire altars, with astronomical basis, have been found in the third millennium cities of India. The texts that describe their designs are conservatively dated to the first millennium BC, but their contents appear to be much older.
Valmiki has described following astronomical events in Ramayana:
Sri Ram was born on Chaitra shukla 9 when the Sun was in Aris, Mars in Capricornus, Jupitor in cancer, Venus in Pisces, and saturn in Libra, Moon in castor. (6th Dec 7560BC Greg) SriRam was to be crowned on 25th birthday Chaitra shukla 9, when Mercury, Mars,Rahu, Jupitor,Crux and alpha Libra were surrounding Moon ( 30th Nov 7535BC Greg), Mercury, Mars & Rahu were in Airis. Ravan was killed on Falgun No moon day (after 14 years) when Mars was near alpha Libra (16th Nov 7521BC) Next day was Chiatra Shukla 1, Gudhipadva (17th Nov 7521BC). Gudipadva is still celebrated in India. 17th Nov 7521BC to 19th March 2007AD =117837 Lunar months (3479786 days).
Maharshi Vyas has described a number of astronomical events in Mahabharata, referring to motions of stars, but also joining these phenomena with terrestial issue, such as death of Bhishma on Magh Shukla 8 when moon was in Rohini and the sun crossed winter solastice (22 Dec 3009BCE Gregarian or 16 Jan 3008BCE Julian).
Yajnavalkya (perhaps 1800 BC) advanced a 95-year cycle to synchronize the motions of the sun and the moon. A text on Vedic astronomy that has been dated to 1350 BC, was written by Lagadha. It described rules for tracking the motions of the Sun and the Moon, and also develops the use of geometry and trigonometry for astronomical uses.
After the conquest of Alexander and the formation of Hellenistic states, Indian astronomy was influenced by Greek astronomy.
Around 500 CE, Aryabhata presented a mathematical system that took the Earth to spin on its axis and considered the motions of the planets with respect to the Sun. He also made an accurate approximation of the Earth's circumference and diameter, and also discovered how the lunar eclipse and solar eclipse happen. He gives the radius of the planetary orbits in terms of the radius of the Earth/Sun orbit as essentially their periods of rotation around the Sun. He was also the earliest to discover that the orbits of the planets around the Sun are ellipses.
Brahmagupta (598-668) was the head of the astronomical observatory at Ujjain and during his tenure there wrote a text on astronomy, the Brahmasphutasiddhanta in 628. He was the earliest to use algebra to solve astronomical problems. He also developed methods for calculations of the motions and places of various planets, their rising and setting, conjunctions, and the calculation of eclipses.
Bhaskara (1114-1185) was the head of the astronomical observatory at Ujjain, continuing the mathematical tradition of Brahmagupta. He wrote the Siddhantasiromani which consists of two parts: Goladhyaya (sphere) and Grahaganita (mathematics of the planets). He also calculated the time taken for the Earth to orbit the sun to 9 decimal places.
Other important astronomers from India include Madhava, Nilakantha Somayaji and Jyeshtadeva, who were members of the Kerala school of astronomy and mathematics from the 14th century to the 16th century. The University of Nalanda, considered by some to be one of the foremost historical universities, offered formal courses in astronomical studies.
Middle ages and Islamic astronomy
Greeks made some important contributions to astronomy, but the progress was mostly stagnant in medieval Europe. Western Europe entered the Middle Ages with great difficulties that affected the continent's intellectual production. Most astronomic treatises of classical antiquity (in Greek) were unavailable, leaving only simplified summaries and compilations. It flourished in the Arab world and priests in distant parishes needed elementary astronomical knowledge for calculating the exact date of Easter, a procedure called computus. The Arabic world under Islam had become highly cultured, and many important works of knowledge from ancient Greece were translated into Arabic, used and stored in libraries throughout the area. The late 9th century Persian astronomer al-Farghani wrote extensively on the motion of celestial bodies. His work was translated into Latin in the 12th century.
In the late 10th century, a huge observatory was built near Tehran, Iran, by the astronomer al-Khujandi who observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic, also known as the tilt of the Earth's axis relative to the Sun. In Persia, Omar Khayyám compiled many tables and performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian. An amazing feat was his calculation of the year to be 365.24219858156 days long, which is accurate to the 6th decimal place.
Starting around year 1100, Europe experienced increased appetite for the study of nature as part of the Renaissance of the 12th century. Astronomy was then one of the seven liberal arts, making it a core subject of any studium generale (now known as "Universities"). The model from the Greeks most remembered through the Middle Ages was the geocentric model, in which the spherical Earth was in the center of the cosmos or universe, with the Sun, Moon and planets each occupying its own concentric sphere. The fixed stars shared the outermost sphere.
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, and not 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." In the 15th century, cardinal Nicholas 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. He was not, however, describing a scientifically verifiable theory of the universe.
Mesoamerican civilizations
Maya astronomical codices include detailed tables for calculating phases of the Moon, the recurrence of eclipses, and the appearance and disappearance of Venus as morning and evening star. 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 early mythology.
Although the Maya calendar was not tied to the Sun, John Teeple has proposed that the Maya calculated the solar year to somewhat greater accuracy than the Gregorian calendar. Both astronomy and an intricate numerological scheme for the measurement of time were vitally important components of Maya religion.
The Copernican revolution
The Renaissance came to astronomy with the work of Nicolaus Copernicus, who proposed a heliocentric system, in which the planets revolved around the Sun and not the Earth. His De revolutionibus provided a full mathematical discussion of his system, using the geometrical techniques that had been traditional in astronomy since before the time of Ptolemy. His work was later defended, expanded upon and modified by Galileo Galilei and Johannes Kepler.
Galileo was among the first to use a telescope to observe the sky, and after constructing a 20x refractor telescope he discovered the four largest moons of Jupiter in 1610. This was the first observation of satellites orbiting another planet. He also found that our Moon had craters and observed (and correctly explained) sunspots. This, along with Galileo noting that Venus exhibited a full set of phases resembling lunar phases. Galileo argued that these observations supported the Copernican system and were, to some extent, incompatible with the favored model of the Earth at the center of the universe.
Uniting physics and astronomy
Although the motions of celestial bodies had been qualitatively explained in physical terms since Aristotle introduced celestial movers in his Metaphysics and a fifth element in his On the Heavens, Johannes Kepler was the first to attempt to derive mathematical predictions of celestial motions from assumed physical causes. Combining his physical insights with the unprecedentedly accurate naked-eye observations made by Tycho Brahe, Kepler discovered the three laws of planetary motion that now carry his name.
Isaac Newton developed further ties between physics and astronomy through his law of universal gravitation. Realising that the same force that attracted 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 his Philosophiae Naturalis Principia Mathematica, he derived Kepler's laws from first principles. Newton's theoretical developments lay many of the foundations of modern physics.
Modern astronomy
At the end of the 19th century it was discovered that, when decomposing the light from the Sun, a multitude of spectral lines were observed (regions where there was less or no light). Experiments with hot gases showed that the same lines could be observed in the spectra of gases. It was proved that the chemical elements found in the Sun (chiefly hydrogen and helium) were also found on Earth. During the 20th century spectrometry (the study of these lines) advanced, especially because of the advent of quantum physics, that was necessary to understand the observations.
Although in previous centuries noted astronomers were exclusively male, at the turn of the 20th century women began to play a role in the great discoveries. In this period prior to modern computers, women at the United States Naval Observatory (USNO), Harvard University, and other astronomy research institutions often served as human "computers," who performed the tedious calculations while scientists performed research requiring more background knowledge. A number of discoveries in this period were originally noted by the women "computers" and reported to their supervisors. For example, Henrietta Swan Leavitt discovered the cepheid variable star period-luminosity relation, Annie Jump Cannon organized the stellar spectral types according to stellar temperature, and Maria Mitchell was the first person to discover a comet using a telescope. Some of these women received little or no recognition during their lives due to their lower professional standing in the field of astronomy. And although their discoveries are taught in classrooms around the world, few students of astronomy can attribute the works to their authors.
Cosmology and the expansion of the universe
Most of our current knowledge was gained during the 20th century. With the help of the use of photography, fainter objects were observed. Our Sun was found to be part of a galaxy made up of more than 1010 stars (10 billion stars). The existence of other galaxies, one of the matters of the great debate, was settled by Edwin Hubble, who identified the Andromeda nebula as a different galaxy.
Physical cosmology, a discipline that has a large intersection with astronomy, made huge advances during the 20th century, with the model of the hot big bang heavily supported by the evidence provided by astronomy and physics, such as the redshifts of very distant galaxies and radio sources, the cosmic microwave background radiation, Hubble's law and cosmological abundances of elements.
New windows into the Cosmos open
Late in the 19th century, scientists began discovering forms of light which were invisible to the naked eye: X-Rays, gamma rays, radio waves, microwaves, ultraviolet radiation, and infrared radiation. This had a major impact on astronomy, spawning the fields of infrared astronomy, radio astronomy, x-ray astronomy and finally gamma-ray astronomy. With the advent of spectroscopy it was proved that other stars were similar to our own sun, but with a range of temperatures, masses and sizes. The existence of our galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe seen in the recession of most galaxies from us.
The article is strongly based on History of astronomy from English Wikipedia. The photo of Stonehedge taken by Frédéric Vincent available under Creative Commons Share-alike 2.0 license.
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