Astronomy

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The astronomical clock in Prague, Photo Andrew Shiva

Astronomy is the science of celestial bodies or cosmic matter. As opposed to astrology it does not attempt to find meaning in the movements of these bodies but concentrates on investigating their origins, development, arrangement, composition and movement. This understanding is based on observation of the cosmos and measurement of radiation which is then assessed according to the laws of physics and chemistry. Astrophysics, which attempts to gain a more accurate picture of cosmic events, has developed as a specialised branch of astronomy.

There are records of astronomical observation which go back at least three thousand years, although they may have begun much earlier. Astronomy is therefore considered to be the oldest branch of science. The Egyptians, Sumerians, Babylonians, Chinese, Indians and the Indians of Central America all made systematic written records of planetary movements which enabled them to recognise the fundamental patterns on which they based their calendars. The earliest type was probably based on the cycles of the Moon. The knowledge that these cultures gathered helped in developing astrological thought.

The Greeks, with their highly developed mathematics, tried to systematise their observations in the hope of being able to draw further conclusions. They recognised the oblique nature of the ecliptic and the orbits of the various planets. The idea that the Earth may be a globe surfaced five centuries before the Christian Era, and one hundred years later Aristarch speculated the heliocentric model of our solar system with the Sun at its centre. Ptolemy recorded the knowledge of antiquity around 150 AD in his book "Algamest" which remained the most important astrological work in the Western World until the modern era.

After the downfall of the classical world astronomical science declined in Europe and its center shifted to Arabian Cultural circles, with Baghdad becoming the most important centre of learning. Astrology then experienced a renewed renaissance in modern times that made a decisive contribution to the newly emerging universal form of human consciousness. Nicolas Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei and Isaac Newton made discoveries which, among other things, helped to establish the heliocentric world model with the planets orbiting around the Sun and the laws of gravity. The invention and rapid improvements of telescopes enabled much more detailed observation of the cosmos.

Research into invisible radiation (including radio and gamma rays), manned and unmanned space exploration and the possibility to electronically process data enabled astronomy to make great advances in the twentieth century. This not only made it possible to use science to speculate on the age of the universe, but also to postulate the existence of twin planets and antimatter.

The astronomical insights into the origins and development of the universe led to growing numbers of scientists not necessarily rejecting the idea of some kind of divine plan. However, this has not led to a modern rehabilitation of astrology.

History of astronomy

Stonehenge[1]

Prehistoric Europe

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 astronomers were priests, and that they understood celestial objects and events to be manifestations of the divine, hence early astronomy's connection to astrology.

Ancient structures with possibly astronomical alignments such as Stonehenge probably fulfilled astronomical, religious, and social functions. It has also been suggested that drawings on the wall of the Lascaux caves in France dating from 33,000 to 10,000 years ago could be a graphical representation of the Pleiades, the Summer Triangle, and the Northern Crown.

The Nebra Sky Disk

Nebra sky disc

Berlin Gold Hat

The Nebra sky disc is a Bronze Age bronze disc that was buried in Germany around 1600 BC. It measures about 30 cm diameter with a mass of 2.2 kg 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. 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 the Pleiades 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 it lunisolar).[2] Those descriptions verified ancient knowledge of the Nebra sky disc's celestial depiction as the precise arrangement needed to judge when to insert the intercalary 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.

Gold hats of the Bronze Age

A Gold Hat probably was a priest's ritual instrument. It can be used to calculate a lunisolar calendrical system, i.e. for a direct reading in either lunar or solar dates, as well as the conversion between them. It allows the calculation of 12, 24, 36, 48, 54, and 57 synodic months in the lunar system and of 12, 18, 24, 36, 48, 54, and 57 solar months (twelfths of a tropical year). The excavated mysterious gold hats of the Bronze Age are 3,000 years old and are very similar to each other regarding their form and symbolism and decoration like disks, circles, and wheel symbols. The so-called 'Berlin Gold Hat' probably originates from Swabia or Switzerland, and is dated to ca. 1000–800 BC. The conical hats could have been used to calculate the movements of the sun and the moon in advance, and were probably used to predict the correct time to sow, plant, and harvest. They are composed of 10 to 20 zones filled with a different number of symbols. The number of circles of each symbol and the number of symbols from one or more zones would be multiplied in a first step, the total amount meaning a number of days. The number of days is compared to astronomical cycles like the synodic month or the tropical year. The 1,739 sun and half-moon symbols decorating the hat's surface make up a scientific, still undeciphered code which resembles 18.67-year cycle called the "Metonic Cycle"

Babylonian tablet recording Halley's comet in 164 BC

Mesopotamia

The origins of Western astronomy can be found in Mesopotamia, 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. 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 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, or an hour into 60 minutes, began with the Sumerians.

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 Enūma Anu Enlil. The oldest significant astronomical text that we possess is Tablet 63 of the Enūma 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.[3]

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 Babylonian 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 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 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.

India

Astronomy in the Indian subcontinent dates back to the period of Indus Valley Civilisation during 3rd millennium BCE, when it was used to create calendars. As the Indus Valley civilization did not leave behind written documents, the oldest extant Indian astronomical text is the Vedanga Jyotisha, dating from the Vedic period. 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 a yuga or "era", there are 5 solar years, 67 lunar sidereal cycles, 1,830 days, 1,835 sidereal days and 62 synodic months. During the 6th century, astronomy was influenced by the Greek and Byzantine astronomical traditions.

Aryabhata (476–550), in his magnum opus Aryabhatiya (499), propounded a computational system based on a planetary model in which the Earth was taken to be spinning 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 the solar and lunar eclipses, and the instantaneous motion of the Moon.[4]Early followers of Aryabhata's model included Varāhamihira, Brahmagupta, and Bhāskara II.

Astronomy was advanced during the Shunga Empire and many star catalogues were produced during this time. The Shunga period is known 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, Astronomical conjunctions, and the calculation of eclipses.

Indian astronomers by the 6th century believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varahamihira and Bhadrabahu, and the 10th-century astronomer Bhattotpala listed the names and estimated periods of certain comets, but it is unfortunately not known how these figures were calculated or how accurate they were.

Bhāskara II (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. The Buddhist University of Nalanda at the time offered formal courses in astronomical studies.

Other important astronomers from India include Madhava of Sangamagrama, Nilakantha Somayaji and Jyeshtadeva, who were members of the Kerala school of astronomy and mathematics from the 14th century to the 16th century. Nilakantha Somayaji, in his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, developed his own computational system for a partially heliocentric planetary model, in which Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system later proposed by Tycho 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 and latitudinal motion of Mercury and Venus. Most astronomers of the Kerala school who followed him accepted his planetary model.

Historical Jantar Mantar observatory in Jaipur

Greece and Hellenistic world

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.[5] 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.[6] This basic cosmological model prevailed, in various forms, until the 16th century.

In the 3rd century BC Aristarchus of Samos was the first to suggest a heliocentric system.[7] Eratosthenes estimated the circumference of the Earth with great accuracy.

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 Antikythera mechanism, an ancient 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 astronomical computer. It was discovered in an ancient shipwreck off the Greek island of Antikythera.[8] 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.

Calendars

Calendars of the world have often been set by observations of the Sun and Moon (marking the day, month and year), and were important to agricultural societies, in which the harvest depended on planting at the correct time of year, and for which the nearly full moon was the only lighting for night-time travel into city markets.

The common modern calendar is based on the Roman calendar. Although originally a lunar calendar, it broke the traditional link of the month to the phases of the Moon and divided the year into twelve almost-equal months, that mostly alternated between thirty and thirty-one days. Julius Caesar instigated a calendar reform in 46 BCE and introduced what is now called the Julian calendar, based upon the leap year (365& 1|4 day year length) originally proposed by the Greek astronomer Callippus.

See also

Antikythera Mechanism (shipwreck fragment)

Weblinks

A Complete Guide to the Solar System and the Night Sky (Visibiliy from Greenwich, United Kingdom)
Photographies of all the sky show the wonders of the universe, from our starry landscapes to atmospheric phenomena, from the visible constellations to the galaxies far away. Discover the fascination of astronomy and astrophotography

Notes and References

  1. ↑ Stonehenge is a prehistoric monument on Salisbury Plain in Wiltshire, England
  2. ↑ The earliest known descriptions of this coordination were recorded by the Babylonians in 6th or 7th centuries BC, over one thousand years later.
  3. ↑ Pingree, 1998, and Rochberg, 2004
  4. ↑ Thurston, H, Early Astronomy. Springer, 1994, p. 178–188
  5. ↑ Plato, Timaeus, ca. 36D BC
  6. ↑ Aristotle, Metaphysics, 1072a18-1074a32
  7. ↑ Pedersen, Early Physics and Astronomy, pp. 55–6
  8. ↑ Antikythera is a Greek island north of Crete.

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