History of Astronomy

[Icarus]

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 (such as the Magi), 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 adopted a calendar based upon the 365 1/4 day year length originally proposed by 4th century BC Greek astronomer Callippus.

The Bible contains a number of unsophisticated statements on the position of the Earth in the universe and the nature of the stars and planets; see Biblical cosmology.


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 first evidence of recognition that astronomical phenomena are periodic and of the application of mathematics to their prediction is Babylonian. Tablets dating back to the Old 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 of cuneiform tablets known as the Enûma Anu Enlil. The oldest significant astronomical text that we possess is Tablet 63 of the Enûma Anu Enlil, the Venus tablet of Ammi-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. The MUL.APIN, contains catalogues of stars and constellations as well as schemes for predicting heliacal risings and the settings of the planets, lengths of daylight measured by a water-clock, gnomon, shadows, and intercalations. 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.

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 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 past records. A notable Babylonian astronomer from this time was Seleucus of Seleucia, who was a supporter of the heliocentric model.

Babylonian astronomy was the basis for much of what was done in Greek and Hellenistic astronomy, in classical Indian astronomy, in Sassanian Iran, in Byzantium, in Syria, in Islamic astronomy, in Central Asia, and in Western Europe.

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Indian Astronomy

Ancient Indian astronomy is based upon sidereal calculations. The sidereal astronomy is based upon the stars and the sidereal period is the time that it takes the object to make one full orbit around the Sun, relative to the stars. It can be traced to the final centuries BC with the Vedanga Jyotisha attributed to Lagadha, one of the circum-Vedic texts, which describes rules for tracking the motions of the Sun and the Moon for the purposes of ritual. After formation of Indo-Greek kingdoms, Indian astronomy was influenced by Hellenistic astronomy (adopting the zodiacal signs or râúis).

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 Buddhist University of Nalanda offered formal courses in astronomical studies.

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The astronomy of East Asia began in China. Solar term was completed in Warring States Period. The knowledge of Chinese astronomy was introduced into East Asia.

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 Shang 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.

This is an abridged timeline of Chinese records and investigations in astronomy.

4000 BCE - Astronomy used to orient houses at Banpo to the constellation Yingshi (part of Pegasus).

4000 BCE - Astronomical mosaics of the Dragon and Tiger flanked a male burial at Xishuipo, Puyang.

2300 BCE +/- 250 y - Yaodian (Book of Yao) records astronomical markers for calendrical purposes, using the leading-stars of the four mega-constellations.

2137 BCE - Classic of History; records the earliest known solar eclipse on October 22.

ca. 2000 BCE - Chinese determine that Jupiter needs 12 years to complete one revolution of its orbit.

ca. 1400 BCE - Chinese record the regularity of solar and lunar eclipses and the earliest known Solar prominence and two novas.

ca. 1200 BCE - Sky divided into twenty eight regions (Chinese constellation); for recognitions of the stars.

ca. 1100 BCE - First determination of the spring equinox.

776 BCE - The earliest reliable record of solar eclipse.

613 BCE, July - A Comet, possibly Comet Halley, record in Spring and Autumn Annals.

532 BCE - A nova was recorded in Records of the Grand Historian and Zuo Zhuan.

28 BCE - Chinese history book Book of Han makes earliest known dated record of sunspot.

78-139 CE - The astronomer, mathematician, and inventor Zhang Heng catalogued some 2500 stars in his lifetime, along with recognizing over 1000 constellations.

185 CE - The earliest recorded and verifiable supernova of RCW 86

687 - Chinese make earliest known record of meteor shower.

1054 - On July 4, Chinese astronomers noted the appearance of a guest star, the supernova now called the Crab Nebula, Messier's M1.

1088 - In his Dream Pool Essays, the polymath Chinese scientist Shen Kuo (1031-1095) wrote of his findings for the improved meridian measurement between the polestar and true north, which was an invaluable concept for aiding navigation by use of the magnetic compass. Shen Kuo also argued for spherical celestial bodies by using evidence of lunar eclipse and solar eclipse, which promoted spherical earth theory and went against flat earth theory. Along with his colleague Wei Pu, he accurately plotted the orbital paths of the sun, moon, and planets over a five year period, and supported retrogradation.

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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 . 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 suggest a heliocentric system, although only fragmentary descriptions of his idea survive. 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.

Depending on the historian's viewpoint, the acme or corruption of physical Greek astronomy is seen with Ptolemy of Alexandria, who wrote the classic comprehensive 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. In his Planetary 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 of Aristarchus of Samos four centuries earlier.

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MesoAmerican

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. The Maya based their calendrics in the carefully calculated cycles of the Pleiades, The Sun, The Moon, Venus, Jupiter, Saturn, Mars, and also they had a precise description of the eclipses as depicted in the Dresden Codex, as well as the ecliptic or zodiac, and the Milky Way was crucial in their Cosmology. (Source:Maya Astronomy). 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.

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 Khayy¨¢m 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.

Muslim advances in astronomy included the construction of the first observatory in Baghdad during the reign of Caliph al-Ma'mun, the collection and correction of previous astronomical data, resolving significant problems in the Ptolemaic model, the development of universal astrolabes, the invention of numerous other astronomical instruments, the beginning of astrophysics and celestial mechanics after Ja'far Muhammad ibn M¨±s¨¡ ibn Sh¨¡kir discovered that the heavenly bodies and celestial spheres were subject to the same physical laws as Earth, the first elaborate experiments related to astronomical phenomena and the first semantic distinction between astronomy and astrology by Ab¨± al-Rayh¨¡n al-B¨©r¨±n¨©, the use of exacting empirical observations and experimental techniques, the separation of natural philosophy from astronomy by Ibn al-Haytham, the first non-Ptolemaic models by Ibn al-Haytham and Mo'ayyeduddin Urdi, and the first empirical observational evidence of the Earth's rotation by Nas¨©r al-D¨©n al-T¨±s¨© and Ali al-Qushji.

Several Muslim astronomers also considered the possibility of the Earth's rotation on its axis and perhaps a heliocentric solar system. It is known that the Copernican heliocentric model in Nicolaus Copernicus' De revolutionibus was adapted from the geocentric model of Ibn al-Shatir and the Maragha school (including the Tusi-couple) in a heliocentric context, and that his arguments for the Earth's rotation were similar to those of Nas¨©r al-D¨©n al-T¨±s¨© and Ali al-Qushji. Some have referred to the achievements of the Maragha school as a "Maragha Revolution", "Maragha School Revolution", or "Scientific Revolution before the Renaissance".

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 13th century the most famous astronomers were Johannes de Sacrobosco and Guido Bonatti from Forl¨¬, in Italy. Bonatti was one of the consultants of Frederick II, Holy Roman Emperor.

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.

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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. Galileo noted 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.

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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, specific lines corresponding to unique elements. 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 discovered 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.

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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, and many others at large distances and receding, moving away from our 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.

(all from wikipedia)

[tomw2005]

We effectively did this today, in my first Intoduction to Astronomy lecture.

Tom w

[Icarus]

Tom,

You'll probably really enjoy Astronomy class. I know I did. I was lucky enough to have a "theoretical physics" guy as my professor, so he elaborated on all sorts of stuff....alien intelligence, blasters...ion rifles.....lightsabers.....the geometry of the pyramids and the alignment of those monuments to astronomical phenomena... It was awesome! Hope you enjoy it as much as I did.

[tomw2005]

It's been good so far, and a break from the harder maths, I'm a big fan of all this small angle approximating I get to do.

Tom W