History of astronomy

Astronomy is the branch of physics that studies celestial bodies and the universe as a whole.

From this perspective, the study of celestial bodies can be reasonably said to have begun when at some point humanity looked up and began to observe the moon and the stars and the planets regardless of how they may have thought of them.

These ancient beginnings are often indicated by structures studied by archeologist. Stonehenge, constructed sometime between 3100 to 2000 BC may have constituted an astronomical site, possibly an observatory or the structure may have been designed upon observations previously made. Either way, it seems clear that Stonehenge was meant to take advantage of astronomical phenomena since the "heelstone" in the circle of stones is aligned with the rising Sun on Midsummer's Day (June 21, the Summer Solstice). This represents a true astronomical alignment. Many other Megalithic sites also demonstrate such alignments.

The Megalithic Passage Tomb at Newgrange, built about 3200 BC.also demonstrates knowledge of astronomical phenomena. The passage and single chamber of the tomb are illuminated by a shaft of sunlight that shines through the roof box over the entrance and penetrates the passage, lighting up the chamber at winter solstice sunrise. This happens at dawn from the 19th to the 23rd of December for 17 minutes.

Earlier evidence of astronomical observations can be found in Vedic India in the Rg Veda which contains a verse observing the winter solstice in the constellation Aries. This would have placed it at around 6500 BC. The Myth of Janus, a four headed god of of the Vedic people of India, presents the possibility of astronomical observations around 4,000 BC. Each head of Janus represented a phase of the moon which in turn represented one of the four seasons: one full moon represented the spring equinox, one full moon represented the autumn equinox, the waning moon the winter solstice and a waxing moon representing the summer solstice.This dating is disputed but it does indicate a very early study of both the constellations and the moon.

Halley's Comet (considered a guest star) was noted by Chinese astronomers as early as 240 BC and perhaps as early as 1059 BC.

Branches and subdisciplines

Celestial mechanics

Celestial mechanics, a subfield of astronomy, began with the application of Newton's theory of mechanics and gravitation (as elucidated in the Principia) to the movement of planets. Eventually Einstein's theory of general relativity and modern computing technology overtook the field of classic physics.

Cosmology

Cosmology is defined as the science of the universe, the branch of astronomy which studies the origin, evolution, and structure of the universe, the study of "the contents, structure, and evolution of the universe from the beginning of time to the future", a branch of astronomy that studies the "origin, large-scale properties, and the evolution of the observable universe.".

Astronomy underwent significant changes in the period following 1970 when a union of particle physics ("the study of the unbelievably small" ) and astronomy ("the study of the incomprehensibly large") had begun to take place. This has had a significant impact on cosmology. The scope of cosmology arguably begins approximately 10^-42 seconds following the origin of the universe when the universe was smaller than a proton.

Planetology

Also referred to as planetary science, this branch of astronomy is involved with the study of other planets, including meteorology, geology, location, orbits, origins. Given the fact that the earth is our primary source of information about other planets, there is a great deal of comparative study of earth and other planets. The primary focus has been on the planets of this solar system but as new planets are discovered, there is a growing amount of data on planets in orbit around other stars.

Astronomy in ancient Greece

Any assertion as to where astronomy began faces the problem of providing dated evidence that shows a reasonable conclusion. With astronomy, there are a great many pieces of evidence in the form of ancient documents and archaeological finds that make such a claim for any place or time difficult to sustain. In other words, it is not really possible to state exactly where astronomy in its earliest forms began. However, it is possible to trace the roots of the study of the skies and the objects visible to the unaided eye with some degree of certainly even if only to establish a theory of its beginnings and where the influences of these early impressions and thoughts eventually spread. For the western world, that is Europe and the European influenced Americas, and the ancient civilisations of North Africa and the Middle East, some of those roots can be traced to the earliest Greek philosophers.

Aristotle was not alone in the development of the Hellenic foundation of humanity's perspective on the celestial but his name is the most prominent of the early Greeks. Aristotle's perspective was derived from what he thought things should be, it was an aesthetic view of the cosmos rather than a scientifically derived view. For Aristotle, the Prime Mover set the universe in motion both perfect (in Aristotle’s point of view) and eternal. There was no such thing as vacuum, no emptiness. All the nearby objects, the Sun, the Moon and the planets as well as the far distant were set in eight crystalline spheres that revolved around the Earth. For Aristotle there were the four basic elements we have on Earth, fire, and water, earth and air. In the heavens there was a fifth from which the crystalline spheres were composed, aether--a perfect substance that could neither be changed nor destroyed.

Things moved about Earth, they moved in perfect circles, they were embedded in a perfect substance, they would never stop in their perfect movement--and all of this was based on Aristotle’s vision of perfection.

Aristotle’s view was later incorporated by Ptolemy in Alexandria, North Africa who made some changes in the Aristotelian perspective to account for anomalies he had observed--the planets occasionally moved in reverse. (Unlike Aristotle, Ptolemy actually observed the phenomena he studied. While he was not the first, this approach to the study of physical phenomena was not required nor evidently even expected of those who made claims about the world,) Ptolemy's work and his writings carried Aristotle’s views forward into the 16th century when Copernicus's work on the calendar led him to make his own changes--in this case a paradigm change. Copernicus, like Aristotle and unlike Ptolemy, did not make his own observations. However, he did incorporate the work of others and he added his contribution by placing the Sun at the centre of the universe. This was to have a major impact on the study of the celestial, marking the beginning of the end or Aristotelian influence, and the politics of the day.