Astronomy is probably the oldest of the Natural Sciences, and its history closely follows and often leads the history of the intellectual developments of humankind.
The regular apparent motions of the Sun, Moon and stars would be obvious to anyone spending significant amounts of time outdoors, and the phases of the moon are also easily identifiable. The explanation of these phenomena was intimately related with the cosmovision of humankind. In an age where natural world was seen as populated by gods and spirits, it was only natural to identify these sources of light with gods. But these "gods" were predictable. Moreover, the periodicity of their movements whose closely related with other phenomena that affected directly the life of agricultural communities: rain, and droght, cold and heat, where related with the positions of the celestial objects. It was not strange that the first astronomers where also priests, and their predictions of natural events through observations was seen by the uninitiated as a demonstration of power, or at least a close communication with the divine. Remains of ancient constructions with probable ceremonial purposes usually have some astronomical alineations, Stonehenge beeing the most famous example of standing stones with probable astronomical alignments.
While the ancients were able to notice the difference between the stars and the planets, as the planets move an appreciable amount during only a few years, whereas the constellations remain relatively fixed over centuries. The word planet come from the greek for 'wanderer'.
As the calendars of the world have usually been set by the sun and moon (measuring the day, month and year), civilisation has found it important to keep the calendar up-to-date with the seasons and sunrise. This information was vitally important to agricultural societies, in which the harvest depended on whether the seeds were planted at the correct time of year. Our most familiar calendar today is based on the Roman calendar, which divided the year into twelve months of alternating thirty and thirty-one days apiece. Various Roman emperors altered the calendar subsequently. Julius Caesar instigated calendar reform, creating the leap year, and the most accurate atomic clocks are moderated by precise astronomical measurements and the insertion and removal of 'leap seconds' at the very end of June and December.
Greek Philosophers thought of several models to explain the movements of stars, planets, the Sun and the Moon. Eratosthenes, using the angles of shadows created at widely-separated regions, calculated the curvature of the Earth, and thus its size. Hipparchus was a Greek astronomer who made a number of important contributions, including the first measurement of precession and the compilation of the first star catalog. Ptolemy later referred to this work in his important Almagest, which had a lasting effect on astronomy up to the Rennaisance.
During the Middle Ages, astronomy, as most of the sciences, didn't advance much in Europe, and many important works could have been forgotten but for the work of scholars of the Arabic world. The Arabic world under the Islam had become a higly cultured one, and many important works of knowledge from Ancient Greece were translated into Arabic, used and stored in libraries throughout the area. New works were also written, and even the name of algebra tells us about his origin. In astronomy, Arab scholars also left a legacy, that is easily seen in the names still used for most of the brilliant stars in the sky (see, e.g. Ursa Major)
Meanwhile in Europe, the model from the Greeks most remembered through the Middle Ages was the geocentric model, in which the Earth was in the center of the Universe, with the Sun, Moon and planets each occupying its own concentric sphere. Stars used the outermost one.
The Copernican Revolution
Copernicus proposed a heliocentric system, in which the Sun was in the center. The model had some flaws, and did not predict the positions of the planets better that the old Ptolemaic system (the version of the geocentric model that was most accepted), but had its supporters. Two of the most famous supporters were Johannes Kepler and Galileo Galilei.
Kepler, using precise naked eye observations made by Tycho Brahe, discovered the three laws of planetary movement that carry his name (though he published them mixed with some other not-so-correct ideas, and didn't give them the importance that we do).
Galileo was not the first one to use the telescope to observe the sky, although he after constructing a 20x refractor he discovered the moons of Jupiter and introduced Sunspots to Europe, He is perhaps most famous for his problems with the Catholic Church (though the real history is more complex than usually believed). Galileo's greatest contribution to knowledge was not in astronomy, but in dynamics, where he studied the motion of objects, but his effort in popularizing the Copernican model was very significant.
Physics marries Astronomy (a longlasting and happy union)
Isaac Newton was the first scientist to marry physics with astronomy, discovering that the same force that causes objects to fall on earth, causes the motion of planets and the moon. Using his Law Of Gravity, the Laws of Kepler are explained, and the heliocentric system gained a sound physical basis. Newton also found out that the white light from the sun can be decomposed into its component colors, this fact is crucial for most of the 20th century research.
At the end of the 19th century it was discovered that, when decomposing the light from the sun, 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.
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 by more than 1010 stars, and 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 out galaxy.
New windows into the Cosmos open
The 20th century was an exciting time for astronomy, with each advance in instrumentation leading to a new breakthrough in the understanding of the universe.