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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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."
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.
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.
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.
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.
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.