Astronomy in the Ancient Times
The story of the skies
author Paul Boșcu, April 2017
Astronomy is a science that studies celestial objects and phenomena. Humanity has studied astronomy since ancient times. Astronomy, as an orderly pursuit of knowledge about the heavenly bodies and the universe, did not begin in one moment at some particular epoch in a single society. Every ancient society had its own concept of the universe (cosmology) and of humanity's relationship to the universe. In most cases, these concepts were certainly molded by three forces: theology (religion), nature (climate, floods, winds, natural disasters), and the assumed influence of the stars and planets on the fortunes and fate of people and their societies (astrology).

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Astronomy is a science that studies celestial objects and phenomena. Humanity has studied astronomy since ancient times.

Astronomers, scientists who study the universe, organize the sky according to fixed constellations, or groups of stars and other heavenly bodies that are arranged in a particular way. The International Astronomical Union (IAU) recognizes 88 “official” constellations, which form a map of the entire sky when combined together.

Of all the scientific disciplines Astronomy stands out as the all-encompassing one in that all the other sciences grew out of astronomy and are still influenced by it. A history of astronomy, to be complete, should trace these interscience relationships to some extent, but the story of astronomy is not subject to these constraints.

Astronomy, as an orderly pursuit of knowledge about the heavenly bodies and the universe, did not begin in one moment at some particular epoch in a single society. Every ancient society had its own concept of the universe (cosmology) and of humanity's relationship to the universe. In most cases, these concepts were certainly molded by three forces: theology (religion), nature (climate, floods, winds, natural disasters), and the assumed influence of the stars and planets on the fortunes and fate of people and their societies (astrology).

Because theology deals with the creation of the universe and everything in it, the various religions were quite naturally the precursors of the ancient astronomies. To the ancients the apparent division of their universe into water, land, and sky pointed to a creator or creators who could dwell primarily in the sky. The study of the sky became an important phase of religion around the world. The astronomy that stemmed from these studies was, of course, extremely primitive. Gods were believed by many ancient societies to inhabit not only the heavens but also the highest mountains and the deepest oceans.

We can see the profound influence of religion on the development of astronomy most clearly when we consider that from the time of the Babylonians to the Roman era, astronomical knowledge and the management of the calendar were confined to the priesthood in most cultures. The common elements in most of the ancient theological concepts of the universe, however, were the initial void, a creator, and the act of creation.

The influence of nature on the development of astronomy became more important as societies grew and the contributions of individuals to the well-being of society became increasingly specialized. Although most of the people in any particular society may not have been overly concerned about climate, the food-producing population was deeply concerned about it and ultimately turned to and depended upon the astronomer for information about the changing seasons. In the earliest years, much of this information was probably guesswork, but in time it became apparent that careful records needed to be kept; these records were the precursor of the modern precise calendar.

The importance of the understanding of natural forces to the development or understanding of astronomy, and the importance of astronomy to the understanding of natural forces, are dramatically illustrated by the annual flooding of the Nile, which is of utmost importance to Egyptian agriculture. The ancient Egyptians knew that the flooding of the Nile occurs when the star Sirius is first visible on the eastern horizon shortly after the sun sets. Arriving at this conclusion required careful study of the rising and setting of the stars and the Egyptians eventually learned to track and record the positional astronomy of the stars.

The flow of time and its measurement probably influenced the development of astronomy more than any other natural phenomenon. Before the first dock and watches were introduced, various crude devices such as the hourglass and water clocks were used to measure short periods of time; these periods were measured by the sun and stars moving across the sky. The length of the day measured by these apparent motions was divided into basic equal intervals which later became the hour.

As civilizations developed from primitive tribal organizations into complex interrelated societies, the need for methods and devices for keeping records of societal activities over long periods of time grew, ultimately becoming societal imperatives. These needs were met by the invention of the calendar, which appeared in various forms in different societies. Because all calendars divide the year into smaller time intervals, knowledge of the length of the year in days was absolutely essential for constructing a calendar. Astronomy thus became indispensable to calendar makers.

The very earliest calendars were based on "year lengths" ranging from 354 to 365 days. The accurate length of the sidereal year in solar time unite as measured today, with modern astronomical methods, is 365 solar days, 5 hours, 48 minutes, and 45.5 seconds.

Because the changes of phase of the moon made it easy to divide the year into lunar months, the earliest calendars were lunisolar. The Babylonians introduced a lunisolar calendar based on 12 lunar months, each of 30 days, which added up to 360 days. To keep the calendar in step with the season, they added months whenever necessary.

The ancient Egyptians were the first to construct and use a solar calendar with a 365-day year, which did not refer to the moon. This year was divided into 12 months, each 30 days long; five extra days were added to this calendar in Egyptian chronology. The Egyptians may have known that the length of the year is 365 days, 6 hours because King Ptolemy III, in 238 BC, ordered that one full day be added to the Egyptian calendar every fourth year. The Egyptians charged their priests with the task of keeping their calendar in step with the true length of the year. The priesthood, therefore, acquired considerable power.

As the celebration of special feast days and religious holidays was very important in all early cultures, it was natural that priests became the keepers of the calendar. Thus priests themselves were astronomers of a sort, principally concerned with the study of the apparent motion of the sun and the appearance and disappearance of the well-known constellations from the evening sky. In time, as astronomy began to evolve into a precise science, the professional astronomer became the calendar authority, laying down the guideline for calendar improvements.

Although astronomy did not originate with the construction of the early calendars, the need for accurate calendars certainly encouraged and contributed to the study of the apparent motion of the sun and thus to astronomy.

Early Chinese astronomy was driven primarily by the need to construct an accurate calendar. By the year 2000 BC, the Chinese astronomers had determined that the length of the year is slightly more than 365 days, but there is no evidence that they had refined their estimates of the length of the year beyond 365 days, 6 hours by the beginning of the common era. Being deeply concerned about the occurrence of lunar and solar eclipses, the Chinese populace held astronomers in great esteem, maintaining them as officials of the imperial court. The main activity of these astronomers was the prediction of such eclipses.

Two technologies, map making and navigation, that are extremely important to trade and commerce owe much to astronomy and in turn contributed to its study. Map making dates back to the Babylonians who constructed maps on clay tiles about 40 centuries ago. The concepts of latitude and longitude were carried over to navigation, which is the technique of finding the position of a ship on the sea. In the early days of navigation, one did this by finding the altitude of the sun which is the height in degrees of the sun above the horizon. Because no sun is available to the navigator at night time, he had to use the stars to navigate the oceans.

Because maps introduce directions relative to an observer on the surface of the earth, it was natural for the map makers to base their definitions of directions on the position of the sun. The east was defined as the direction of the rising sun and the west as the direction of the setting sun.

The practice of navigating the ship led to a sophisticated navigational technique called "celestial navigation," which required introducing a system of great circles on the sky equivalent to the circles of longitude on the earth and a system of parallel small circles on the sky similar to circles of latitude on the earth. Thus the celestial coordinate system that is still used in astronomy was first introduced by the early navigators.

These early navigators who plied the Mediterranean, Thracian, and Aegean seas as well as the shallow waters along the European and African shores, were well acquainted with the stars of the circumpolar constellations. These are constellations, such as Ursa Major which circle the north celestial pole without setting. The further north one moves, the more of these constellations one sees. As one goes south, however, the number of circumpolar constellations visible to the observer decreases. These very early navigators could have interpreted these observations in only one way: the waters of the earth lie on a spherical surface.

That the earth is round must therefore have occurred to the early navigators even though such speculations cannot be verified because there are no written records of their astronomical or cosmological concepts.

In the earliest days of humanity’s fascination with the heavens, observers noticed that although the patterns of the stars never changed, some points of light seemed to travel through them on a fixed path. These objects were called planets, from the Greek root planasthai, meaning “wanderers.” Such initial observations helped ancient astronomers track the seasons and, eventually, allowed for the revolutionary understanding of how the solar system is arranged. The Earth’s place in that solar system was, one of the keenest points of interest.

The basic European northern constellations originate in Greek mythology and were said to have been created by Zeus and his divine cohorts (according to myriad ancient Greek writers, that is). Homer, a poet who lived in the seventh century BCE, wrote of arrangements of stars resembling certain shapes, such as the Ram. Later, other ancient Greek authors wrote of star arrangements based on the gods and their antics.

Roman authors such as Ptolemy (of the second century CE) catalogued the stars into at least 48 different constellations, some of which involved Roman mythology. Although this list was limited to those constellations that could be seen in the Northern Hemisphere, it formed the basis for today’s categorization.

Although astronomy was actively pursued by Hindu astronomers, they observed the heavens primarily in conjunction with their deep interest in numerology. Thus they did not treat astronomy as requiring exact observations, contenting themselves with a year length of 366 days. The notion that the length of the year lies between 365 and 366 days was repugnant to their sense of propriety, which was governed by numerology. Because astrology was important to the early Hindus, they worked with year lengths that ranged from 324 to 378 days, according to their fancy.

That the ancient Britons had a flourishing ceremonial-based astronomy as an adjunct to their religious practices is indicated by the famous Stonehenge ritual monuments. Careful studies of these massive stone structures indicate that they date from the Late Stone and Early Bronze Ages. A computer analysis of these megaliths shows that they were used as recently as 1500 BC to predict the summer and winter solstices and the vernal and autumnal equinoxes, as well as solar and lunar eclipses. These predictions must certainly have led them to the construction of a calendar. But astronomy as a science, did not arise from these astronomical studies.

The astronomical pursuits of the early Mayan Indians in South America and their relationship to astronomy as a science were similar to those of the Stonehenge Britons. Although the Mayans developed a very elaborate calendar, the most accurate until the Gregorian calendar was introduced in Europe in the sixteenth century, it did not lead them to a science of astronomy.

Because megaliths are found in all parts of the world, from Carnac in France to Easter Island west of the coast of Chile, we may conclude that celestial observations were widely pursued by most of the ancient civilizations. But only the ancient Mediterranean civilizations produced the science of astronomy. The contributions of much of the rest of the ancient world were to amount to little more than unsystematic observations and the creation of elaborate myths to coincide with and to explain those observations.

When ancient astronomers first observed the planets, they also noticed that the wanderers seemed to travel through a set of 12 constellations. These 12 constellations made up the ancient Babylonian system of navigation in which each station was given an animal sign. This system was later called the zodiac, which meant “circle of animals” in Greek. Over time, these cultures created a calendar where each constellation corresponded approximately to one month of the year. These 12 signs of the zodiac makeup what’s called the Ecliptic, which is the plane of the solar system as seen on the sky.

Different countries around the world have different zodiacs. For example, the western one is unlike the traditional Chinese zodiac, which is also unlike the Hindu or Celtic versions. The 12 signs of the western zodiac are as follows: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces.

The solar system is laid out like concentric circles on a flat plane, with the Sun at the center and the planets moving in ever-increasing circles around it. When someone on Earth looks to the sky and observes the solar system’s other planets, those planets are much closer than the fixed stars that make up the constellations. The planets therefore appear to move with respect to the constellations and drift through the Ecliptic plane, whose background is the 12 signs of the zodiac.

Astronomy was also stimulated by the spiritual needs of people, their religious beliefs, and their belief that the stars and planets influence their affairs. This belief led to the birth of astrology which greatly stimulated the study of astronomy. Indeed, the original study of the positions of the heavenly bodies was called astrology. Though later recognized as a pseudoscience and dropped from the true science labeled "astronomy," it did much up until the time of Newton to contribute to knowledge of the stars and their apparent motions in the sky.

Astrology was probably practiced by the ancient Egyptians, Hindus, Chinese, Etruscans, and Chaldeans of Babylonia. Knowing that the sun greatly influenced their lives, they believed the moon, the planets, and the stars did so as well. A complicated system of predicting the "influences" on human destiny of the positions of the planets in the various constellations along the ecliptic (the signs of the zodiac) was thus developed. This method of "predicting" human events—astrology—is still popular but, has no basis in scientific facts.

It was easy enough for astrologers to acquire prestige and achieve a lofty status in society by using coincidences between terrestrial phenomena and celestial events. Thus the ancient Sumerians referred to the seven stars in Pleiades as "wicked demons" because they noted that torrential storms occurred when the Pleiades arose early in the evening in spring at the time of the new moon. They argued then that the Pleiades caused these storms, which naturally occur in spring owing to climatic changes.

The Chinese astrologers, observing the apparent retrograde motion of Jupiter, labeled Jupiter "an omen of famine" instead of "a harbinger of good fortune" as they had previously considered it. Here we see how early astronomy had played into the hands of astrologers. Thus astronomy and astrology affected each other in a very asymmetrical way. Astronomy became the handmaiden of astrology, presenting astrologers with celestial data that astrologers used in any way that suited them. Astrologers today, as in the past, freely use whatever astronomical data they believe can be twisted to serve their purposes.

Because the ancient Greeks drew no distinction between philosophers and astronomers, we cannot discuss one without the other. Of the early Greek philosopher-astronomers, Thales of Miletus and his younger disciple Anaximander were the first to propose celestial models that are based, at least to some degree, on the movements of heavenly bodies and not merely the manifestations of mythological beasts and superstitions. These ideas, so elementary to us, represented tremendous progress in our understanding of the universe as an orderly system. Anaximander is also credited with introducing the sundial and inventing cartography.

Anaximander did not progress much beyond Thales but carried on and propagated Thales' ideas and teachings. He discovered the obliquity of the ecliptic, which is expressed as the tilt of the plane of the earth's equator to the plane in which the earth revolves around the sun (the plane of the ecliptic). Anaximander did not know anything about the motion of the earth around the sun, but he observed that the height of the sun above the horizon at noon changes from season to season.

This model, however primitive, represented a great departure from the pre-Grecian anthropomorphic cosmic mythology. It was, indeed, the beginning of the epicycle concept developed hundreds of years later by Hipparchus and Ptolemy.

Like all early Greek philosophers Anaximander had his own theory of the origin of all things. He postulated that the universe originated from the separation of opposites, Hot "naturally" separated from the cold, followed by the separation of the dry from the wet He completed these ideas by postulating that all things eventually return to their original elements.

Anaxagoras, a younger contemporary, and probably a student of Ananimander's (they were both of the Ionian School of Greek philosophy), taught that all matter had existed originally as "atoms or molecules," infinitely numerous and infinitesimally small. Anaxagoras declared that these atoms had existed "from all eternity" and that all forms of matter are different aggregates of these basic atoms. In this statement, he laid the basis for the Greek philosopher Democritus, who is credited today with having created atomic theory. Leucippus probably greatly influenced Democritus, who was his student, in his formulation of the atomic theory.

What little we do know about Leucippus stems from Aristotle's commentaries—not Leucippus' surviving works. Aristotle himself thought very highly of Leucippus, crediting him with the invention of atomic theory.

Democritus and Leucippus had similar ideas about the relationship of the sun and moon to the earth and the stars. They both presented theories about solar and lunar eclipses which were probably not developed in isolation from one another.

Anaximenes of Miletus, Xenophanes of Kolophon, Parmenides of Elea, and Heracleitus of Ephesus, all contemporaries or disciples of Thales, speculated about the nature of the sun, moon, planets, and the stars and developed primitive cosmologies, which have one thing in common; they were all based on the atomic theory of Democritus of Abdera. This was the beginning of unity in astronomy, but still a far cry from the Greek astronomy that finally began to emerge from these ancient cosmologies in the fourth century BC.

Other Greek philosophers such as Metrodorus of Chios and Empedocles of Sicily, who lived in the fifth century BC, offered cosmic theories that contributed to the intellectual dominance of Athens. This proud city-state became the undisputed home of philosophers, who flocked there from all corners of the known earth. The importance of these philosophers for the growth of astronomy was not in the correctness of their primitive speculations, but in their insistence on careful observations of the celestial bodies and their motions.

Mythology was thus beginning to give way to rationalism as the proper way to understand the universe and to explain natural phenomena.

Of all the early Greek philosopher-astronomers, Pythagoras was probably the most influential in turning the attentions of astronomers to the importance and usefulness of mathematics in constructing cosmological models that could be compared more or less accurately with the observed motions of the celestial bodies. Pythagoras founded a school of philosophy whose main concern was to interpret all natural phenomena in terms of numbers.

Born in Samos and spending most of the 50 years of his life in Croton, he laid down the basic principle of his philosophy: number is everything. This school of thought became so popular and dominant that it survived for 200 years—not because of its mathematics and astronomy but because of the religious mysteries surrounding it.

With their emphasis on number as the basis of natural phenomena, the Pythagoreans quite naturally sought in nature phenomena that would provide a basis for their numerology, and they were quick to find this justification in the great regularity they observed in the motions of the celestial bodies. The sun rose without fail every day as did the stars. Everything seemed to repeat itself in a precise period to which a definite integer could be assigned. This regularity led the Pythagoreans to the concept of cosmic harmony; indeed, Pythagoreans introduced the word "cosmos" to designate the universe.

The Pythagoreans were encouraged in their search for harmony in the universe by Pythgoras' discovery that the most harmonious sounds, pleasing to the ears, are those whose vibrations are related to each other in simple numerical ways. The Pythagoreans extended this harmony to the heavens and called it the "harmony of the spheres," In this cosmology, the universe is described as a "cosmic union" with one celestial skin inside another, each revolving at a different rate.

lthough historians differ in their evaluation of the specific contribution of Pythagoras to astronomy they all agree that his contribution to the way one must think about nature and about astronomy, in particular, was a revolutionary departure from everything that had occurred before. Specifically, however, it is believed that Pythagoras promulgated the doctrine of a spherical earth in conformity with the spherical appearance of the sky which does not change as one moves from point to point on the surface of the earth. There is also some historical evidence that Pythagoras believed that the earth rotates, thus producing day and night.

The Pythagorean philosophers who followed Pythagoras stated that he was the first to discover that Phosphorus and Hesperus, the "morning and evening" stars, are the same celestial body, later called the early planet Venus by the Romans.

The Pythagoreans made remarkably accurate observations, discovering that the phases of the moon complete a cycle every 29 days, 12 hours. The moon itself was pictured as revolving around "the central fire" during that period. They contrasted this prediction with the sun's motion around the same "central fire," finding that the sun completed its journey once every 364 days, 12 hours—a period of time which they identified with the length of the year. At no time did they identify the sun with the "central fire"; instead they placed the earth at the center, without worrying about how the earth could exist at such a center.

Pythagoras is best known today for his famous theorem about the sides of a right triangle, but his greatest contribution was his invention of a consistent philosophy and procedure for explaining natural phenomena which did not call upon the gods, fairies, or spirits. In spite of the great attraction that Pythagorean philosophy and numerology held for people, the Pythagorean influence began to fade and practically vanished by the beginning of the Christian era.

An interesting but incorrect conclusion about the number of planets or non stellar objects associated with the earth stemmed from Philolaus' devotion to numbers as the rules of celestial harmonies. The nine visible bodies: earth, moon, sun, the five planets, and the sphere of the fixed stars, left a gap in what Philolaus considered the perfect numerical harmony that 10 bodies would constitute. He therefore proposed a tenth body which he assumed to be "another earth" on the opposite side of the sky which he called the "antichthon" or "counter earth."

This error persisted for some time, but we can clearly see the significance of the fact that it was predicted by the Pythagoreans as a consequence of their requirement that numerical harmony apply and was not based on any mythological or religious reasons. This perhaps was the first instance in which a theory, however faulty, led to a prediction. Indeed, this is the way science progresses today, with scientists using basic physical laws to guide them in their predictions rather than fanciful notions about the predictive powers of numbers.

Although Plato himself did not develop any original cosmological models, he carried on the Pythagorean principle that only the application of pure thought can fathom the "true harmony" of the universe, Plato also argued that symmetry is the basis of all natural phenomena, which led him to the belief that the sphere is the only admissible shape for celestial bodies and the circle is the only admissible orbit of a celestial body, Plato's concept of symmetry also led him to a numerical model of the distances between the sun, moon, and planets which is completely Pythagorean.

Using the sequences 1, 2, 4, 8 and 1, 3, 9, 27, which are the successive powers of 2 and 3, Plato assigned the distances 1, 2, 3, 4, 8, 9, 27 to the moon, sun, Venus, Mercury, Mars, Jupiter, and Saturn, each moving in its own circle with a radius given by the number in the sequence assigned to it, Plato did not believe that his deductions about the motions of the planets required any observational verification for he was firmly convinced that pure reason could lead to no other numerical model.

Although Plato's influence, as a philosopher, dominated the intellectual lives of the Greek philosophers of that period, his younger disciples and students included Eudoxus, Aristotle,Herakleides, and Aristarchus, all of whom advanced science (astronomy, in particular) far more than had Plato.

Eudoxus attended Plato's Academy in Athens for some months but then went to Egypt where he studied planetary motion with the priests of Heliopolis. Though this model is incorrect, Eudoxus' work set an important standard for all astronomers who followed him: to let the observations guide the astronomers in their search for models of the motions of celestial bodies. Eudoxus was an excellent mathematician. Indeed, some historians believe that he wrote Euclid's fifth book of geometry.

Plutarch states that Plato considered Eudoxus to be the foremost mathematician in Athens. He knew the length of the year quite accurately and suggested that a solar year of 365 days be accepted for 4 years and that every fifth year consist of 366 days. This suggestion later became the basis of the Julian calendar, which was the first of our modern calendars.

Eudoxus produced a very ingenious model of the apparent planetary motions which depicted them in accordance with the observational data known at the time. The model introduced by Eudoxus is known as the homocentric sphere model because Eudoxus assigned a sphere to each planet and assumed that these spheres nest in each other and are concentric with the earth which he assumed to be at the common center of these spheres.

The book On Velocities written by Eudoxus, which describes this theory in detail, is lost, but Aristotle knew about this book, and had probably read it, because he gave a detailed account of Eudoxus' homocentric spheres.

In spite of his practical philosophy, Aristotle applied metaphysical reasoning and guidance in developing a model of the universe. He discussed his astronomical speculations in his books On the Heavens, and Meteorologica. His main concern was to explain the apparent motions of the celestial bodies. Because he had no laws of motion nor any real understanding of the nature of motion he had to introduce some very primitive concepts about how the celestial bodies acquired their motion. But Aristotle had a true scientific attitude and approach to the problems and puzzles that confronted him. He argued that the Earth is spherical in shape.

Aristotle differentiated between perfect motion, which he pictured as the "quickest," from the so-called lesser motions. He assigned the "most perfect" motion to the celestial sphere of the stars and lesser motions to the moon, sun, and planets. He assumed that these were primordial motions that were assigned to the various celestial spheres by a "divine power." He further assumed that this divine power acted to keep the celestial bodies moving continuously. In particular, he did not understand the concept of inertia which remained for Galileo to discover some 20 centuries later.

Aristotle wondered why stars (as opposed to planets) twinkle, and explained this difference in terms of the eye, which he suggested tears and shakes when viewing distant objects such as stars but remains vigorous and steady when viewing closer objects such as the planets. He explained the "circular motions" of the stars and planets by arguing that the material of the stars and planets is "circular motion" material, as opposed to earth material which can travel in straight lines, like flowing water and rising fire.

Aristotle accepted the spherical shape for the celestial bodies because, he argued, the sphere is a perfect shape and therefore the only one fitting for celestial bodies. He arrived at the spherical shape for the earth by reasoning that the earth was formed from particles that all moved toward a common center. These particles then formed a series of concentric spheres, the result of which formed the surface of the earth. He estimated the diameter of the earth to be 400,000 stadia or 12361 miles, almost one and one-half times its true value, which was a remarkably accurate estimate for that time.

The importance of Aristotle's speculations about astronomy is that he established a new standard for pursuing and investigating natural phenomena. Nothing was to be accepted without presenting a reasonable explanation. Reasonable in this context meant either mathematically acceptable, consistent with experience, or both, but without recourse to gods or myths.

Of the disciples of Plato, Herakleides and Aristarchus were closer to modern astronomers in their thinking than Eudoxus and Aristotle. Both Herakleides and Aristarchus discarded the concept of a rotating sun, replacing that concept with that of a rotating Earth.

Although Herakleides is said to have suggested that the Earth rotates on its own axis and that it probably revolves around a central fire, historians do not believe that he proposed a heliocentric model of the solar system. However, he is credited, with having introduced the concept of the epicycle to explain the observed motions of the planets.

Aristarchus of Samos was the closest to modern astronomers in spirit and approach to the solution of astronomical problems. Indeed, he was the first to propose a self-consistent heliocentric model of the solar system, with the planets, starting with Mercury, arranged in the order from the sun which we accept today. He was the first to attempt to determine the dimensions and distances from the Earth of the sun and moon. His approach in these exercises was mathematically and physically impeccable, which he described in a treatise. The Dimensions and Distances of the Sun, an Moon, This is the only one of his original treatises that has survived.

The early Greek astronomers were greatly puzzled by the apparent periodic retrograde motions of the planets (from east to west). Herakleides argued that this motion can be explained by assigning to each planet a loop in its observed orbit. He discovered that the observed motion of Jupiter around the earth can be explained by assigning 12 loops to its complete circular orbit; Saturn's observed motion, by contrast, requires 29 loops.

Because all but a few fragments of Herakleides' own writings are lost, we find it difficult to separate what Herakleides actually discovered and proposed from what commentators wrote about following his death. But the many references made to Herakleides by Aristotle and other contemporary philosophers suggest that Herakleides was one of the most influential of the early Greek philosophers.

Aristarchus presented a brilliant analysis of the relationship between the phases of the moon and the geometrical arrangements of the Earth, moon, and sun for the various phases. From this analysis he concluded correctly that the sun is at a much greater distance from the earth than is the moon, and that the sun must therefore be many times larger than the moon and the earth.

Archimedes, a younger contemporary of Aristarchus, commented on this solar system model of Aristarchus, noting that this model implies that "the world [cosmos] is many times larger than had previously been thought." The Greek biographer and historian Plutarch in his book On the Face in the Disk of the Moon remarked that Aristarchus proposed the hypothesis that the "heavens stand still and the earth moves in an elliptic circle at the same time as it turns round its axis." This is essentially the Copernican heliocentric model of the solar system, so that Copernicus may be called the "Aristarchus of the modern era."

Interestingly enough, Copernicus, in seeking ancient authoritative support for his heliocentric solar system, referred to Aristarchus.

Archimedes, a younger contemporary of Aristarchus, did not accept Aristarchus' model of the earth revolving around the sun because it meant to Archimedes that the earth would have to be revolving around the stellar sphere as well. This was unacceptable to Archimedes because, he argued, the earth could not revolve around a sphere of which it was the center. Archimedes placed the earth at me center of the celestial sphere because of the apparent motions of the stars around the earth.

According to Plutarch, Aristarchus was charged with impiety for daring to propose that the earth is not at the center of the universe.

The Alexandrian Library was famous for the great Greek literary men of letters, who were its librarians. It is most famous among astronomers and geographers for having had the Greek mathematician, astronomer, geographer, and poet Eratosthenes for its librarian and director. Eratosthenes is most famous for having measured the circumference of the Earth, with great accuracy.

As Eratosthenes' procedure for measuring the circumference of the Earth is similar to what geographers and surveyors do today to determine the shape, as well as the size of the Earth.

We can see the limits that the absence of theory imposes on pure observational astronomy, however precise such observations may have been, when we consider the observations of Hipparchus who was, by far, the greatest of the Greek astronomers of the pre- Christian era. Hipparchus' emphasis on observational precision led him to perhaps his greatest discovery, the precession of the equinoxes. Hipparchus’ most important contribution to the development of astronomy was his introduction of precision and systematic recording in observational astronomy.

Born in Nicaea, Bithynia, Hipparchus spent most of his life in Rhodes, one of the most prominent states of Greece. Like Alexandria, Rhodes was a center of intellectual life with great activity in literature, astronomy, and mathematics.

Hipparchus is renowned also for his investigations into the orbits of the planets, his attempt to determine the sizes of the sun and moon, and his contributions to mathematics. In his study of planetary motions he was most concerned about the irregularity in their motions and their retrograde motions. He tried to explain both of these by introducing different centers for their circular orbits. Because this did not account for both their irregular motions and their retrograde (from east to west) motions, he proposed the concept that two different circles of motion must be assigned to each planet.

The only writing of Hipparchus still extant is his book in which he expounded upon the importance of a continuous pursuit of accuracy in tracing the apparent motions of the sun and the planets, particularly the apparent motion of the sun. Most of what we know about Hipparchus comes from the writings of Ptolemy in his famous work, The Almagest.

The equinoxes are two imaginary points on opposite sides of the visible sky (the celestial sphere) where the two imaginary great celestial circles—the celestial equator and the ecliptic—intersect. These circles are defined by the diurnal (daily) rotation of the earth and its annual revolution around the sun.

Up to the time of Hipparchus, all observational astronomy was more or less of a random occupation. The need for systematic observation of the celestial bodies was not recognized because the early Greeks believed that everything was precisely ordered and, hence, unaltered from eon to eon. Thus nothing would be gained by a night to night observation of the sky.

Hipparchus pursued astronomy in the manner of modern observers, which meant keeping careful records of the stars and the planets. The only book by Hipparchus which still exists was written before he discovered the precession of the equinoxes. This book deals primarily with the rising and setting of the stars and lists the times of culmination (passing across his meridian) of stars from hour to hour. The accuracy of these observations tells us that the clock that Hipparchus used was probably an accurate water clock.

He introduced the classification of stellar brightness by arranging the stars on an importance scale which he associated with brightness. Thus to Hipparchus, the brightest stars were the most important "stars and therefore, stars of the first magnitude." "Magnitude" in this context has nothing to do with the size of a star, which could not be measured until 2000 years later. In his stellar catalogue Hipparchus divided the visible stars into six magnitude classes, with those in the first class the brightest to the naked eye, and those in the sixth class barely visible to the naked eye.

Claudius Ptolemaeus (Ptolemy) was probably born in Greece but his Latin named indicated that he possessed Roman citizenship. His earliest celestial observations are dated in the eleventh year of the Roman emperor Hadrian. In his The Almagest, Ptolemy examined and commented on every problem in astronomy that had challenged his predecessors. But the principal problem that concerned him was the explanation of the motions of the planets. In this area of his work he accepted the concept of epicycles as proposed by Hipparchus, but improved on Hipparchus' work by developing a very elegant epicycle model of the motions of Venus and Mercury.

Because he flourished in Alexandria, he is often called Ptolemy the Egyptian, He deserves his title of Ptolemy the Great for his summary of the work of the outstanding Greek astronomers such as Aristarchus and Hipparchus.

Ptolemy had to explain why the apparent motions of Mercury and Venus differ drastically from the apparent motions of Mars, Jupiter and Saturn. Unlike the planets Mars, Jupiter, and Saturn which rise and set at all times of day without regard to the sun, the rising and setting of Venus and Mercury closely coincide with the risings and settings of the sun. To explain this "strange" behavior of the planets, Ptolemy accepted the loops proposed by Herakleides for the motions of Mars, Jupiter, and Saturn, but such loops would not work for Venus and Mercury.

Ptolemy solved the problem of Venus and Mercury very neatly by picturing Venus as moving around a small circle (the epicycle) whose center is on a large circle at the center of the earth. Ptolemy then had the line from the center of the earth to the center of the epicycle pass through the center of the sun. By choosing the radius of Venus' epicycle just right, Ptolemy kept Venus in its observed relationship with respect to the sun as the center of the epicycle revolved around the earth.

Ptolemy’s model was so clever (which also applied to Mercury but with a smaller epicycle) that the epicycle concept for planetary motions dominated astronomers' thinking for some 1500 years and was swept away only with the theories later proposed by Copernicus in the sixteenth century.

The last Roman emperor was deposed in AD 476, but, in time, the domination of the barbarian gave way to the Holy Roman Empire as Christianity was gradually adopted throughout Europe. This development had its good and bad points. Christian monks preserved the work of the Greek astronomers. But their rigorous interpretation of Christianity did not allow them to expand on the work of the ancients. It is no wonder, then, that hardly any significant astronomy was produced during the medieval period in Europe from AD 500 to 1500.

The good points were that Christianity, to some extent, encouraged a devotion to contemplation and study and that the convents and monasteries became the repositories of manuscripts of all kinds including the writings of the Greek astronomers which might otherwise have been lost.

The bad points were that the Christian hierarchy insisted on a narrow-minded literal interpretation of every word in the Scriptures. Every departure from these strictures and any attempt to question Christian authority were rejected, scorned, and severely punished.

A further deterrent to the growth of independent creative thinking and, therefore, of astronomy, was that such thinking in general was limited to those who had time to devote themselves to thinking, as the Greek and Alexandrian philosophers did. Instead of such free thinkers who flourished in the early pre-Christian era and exchanged their ideas in such free institutions as Plato's Academy and the great library in Alexandria, the thinkers in the European medieval era were associated with monasteries and convents and therefore constrained by the religious rules of the institutions to which they were bound.

When the mathematician Gerbert ascended the papal throne in AD 999 as Sylvester III, the restrictions on accepting the earth as a sphere were removed. Some freedom of thought about astronomy was allowed. Geographers thus became the leaders of astronomical thinking. This was a far cry from the intellectual freedom of the pre-Christian Greeks and Alexandrians. But the writers of this period were free to read the early Greek and Roman histories of the Greeks. Most important among these was the work of Pliny whose history of the early Greek astronomers stimulated many of the medievalists to stray into forbidden astronomical territories.

The combined efforts of all of these brave intellectual explorers did very little to ease the grip of Christian theology on astronomical thinking or to offer any alternative to Ptolemy's epicycles and his geocentric model of the solar system. One need only read the writings of the Church fathers to see what an uphill struggle it was to free astronomy from doctrinaire religion, let alone to advance it beyond Ptolemy.

Even those medievalist writers who dared to wander into astronomy did not go beyond Aristotle, who was accepted as the authority on all things scientific. This adherence to Aristotelian thinking evolved into what we now call the scholastic school led by Thomas Aquinas and Albertus Magnus. If anything, scholasticism was more of a drag on astronomy than was theology since it bore the imprint of a great Greek philosopher.

One independent thinker who went against the theology and scholasticism of the Middle Ages was the British scholar Roger Bacon In the 13th century, who knew the Greek philosophers very well and had mastered the Ptolemaic model of the solar system. Bacon was even acquainted with the Arabian astronomy of that period. In all his writings, Bacon argued passionately for the need to separate the Scriptures from the study of the stars and planets. His books were banned and he was imprisoned for some ten years.

Bacon was courageous enough to point out difficulties and contradictions in various passages of the Old Testament. He ridiculed the writings of the Church leaders on astronomy, arguing that they were without merit because they were based not on observations and measurements but on speculation and superstition. Bacon would certainly have projected astronomy into its modern mode if he had not been ordered by his supervisors in the Franciscan monastery to which he belonged to give up all scientific work, particularly astronomy.

The extent of astronomical knowledge in the Middle Ages is best exemplified by Dante's Divine Comedy in which Dante attempted to incorporate Ptolemaic astronomy into a complete cosmological treatise in agreement with the accepted Catholic theology of the day. However, he was guided more by Aristotle's philosophy than Ptolemy's astronomy.

Dante seems to have accepted the idea of a spherical earth because in his description of his descent into Hell, he remarks on passing through the "center of Hell" and looking back so that they (Dante and his guide Virgil) saw Lucifer "upside-down" which meant that they had "commenced their ascent to the other side of the earth."

That very little astronomy was pursued in medieval Europe does not mean that astronomy was dead everywhere. Indeed, it flourished in countries such as India, the Arab countries, and China, but this activity influenced the development of Western astronomy only very slightly, primarily because the contact between medieval Europe and India and the Arab countries was very superficial. The contrast between Arabian astronomy and medieval European astronomy is that no theological restrictions were placed on Arabian astronomers. In any case, no real progress was made by the Arabs or the Europeans since neither advanced beyond Ptolemy.

The Indian astronomers taught that the earth is a sphere, floating freely in space, with all the other planets revolving around the earth. Some Hindu astronomers promulgated the concept of a rotating earth to explain the diurnal rising and setting of the stars, but the idea of a spinning earth was rejected by most Indian astronomers as contrary to the observations that objects on the earth are not flying about helter-skelter as they "would be if the earth were spinning."

The conquest of Persia by the Arabs in the 7th century A.D. brought the Arabs in contact with the Hindus whose philosophies and science had penetrated into Persia. The Caliphs of that period had become interested in the motions of the stars and planets and had ordered various of the Hindu books on the stars and planets to be translated into Arabic. These books greatly encouraged not only the study of astronomy but also of mathematics.

The Caliph Mamun was a patron of science and, in fact, had enlarged the observatory near Damascus that had been constructed by the Umayyad Caliphs.

The Chinese were brilliant astronomers and superb observers. The best example of their astronomical achievements was their observations of the Crab Nebula in Taurus, which the Chinese described as a "guest star" (now known as the remnants of a supernova) which exploded on July 4,1054. The Chinese followed this "guest star" for many days until it faded from sight. It was discovered in the 20th century with the aid of modern telescopes. That the medieval European astronomers have left no records of this amazing event shows how far ahead the Chinese astronomers were of their European counterparts at that time.

Early views of the solar system’s design assumed that the Earth was at the center of the cosmos. After all, the Sun, Moon, planets, and stars all appeared to rise and set each day and seemed to circle the Earth. Why shouldn’t the Earth be front and center on the universe’s stage? At the time, such a layout made theological sense because it reinforced humanity’s view of the Earth as “special.”

This geocentric, or Earth-centered, view of the universe was supported by both mainstream science and religion. Several scientists in the 1500s and 1600s disputed this view with scientific discoveries.