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.

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.