The earliest attempts at explaining the origins of the Universe were generally centered in religion. The gods, frequently a male with female counterpart, were commonly assumed to have created the known world and the heavens above it. It was also common to believe they were responsible for the movement of the Sun, Moon and other heavenly bodies. Ancient civilizations, Greek, Roman, Norse, etc, and even more recent peoples, such as the Inuit and various Native American tribes, had their creation story, or myth. This was sufficient and filled their need to understand their world.. But soon enough the concept of physical law replaced the whimsy of the Gods as a controlling factor.
Early scientific theory was concerned more with the arrangement of the heavens than with their beginnings. Aristotle, circa 330 BC, published a thesis detailing the general nature of the cosmos. This included the belief that the earth was a stable object around which the Sun, Moon and other planets revolved.
In his work, The Almagest, Ptolemy supported much of Aristotle's work. His major contribution was in proposing a geometric or mathematical explanation of the motions and positions of the sun, moon and planets against a backdrop of unmoving stars. Ptolemy assumed that each planet moved in a circle called an epicycle, whose center was in turn carried around the earth in a second circular orbit called a deferent. Ptolemy's theories dominated until the 16th century.
Through much of the Middle Ages the Church was a great hindrance to the furtherance of this field. Even speaking of theories that contradicted their religious teachings was enough to warrant ex- communication.
As early as 280 BC, Aristarchus of Samos, a Greek astronomer, theorized that the Earth revolved around the Sun. But Aristotle, citing a number of physically based arguments pretty much dismissed this theory out of existence.
The theory of an Earth-centered Universe held unchallenged until sometime between 1507 and 1515 when Nicolaus Copernicus proposed that the planets revolve around the sun and that the Sun, not the Earth, was the center of our Universe. It should be noted that the Copernican system was not a wholesale dismissal of Aristotlian and Ptolemic understanding: the planets are assumed to move in circles around the sun. Copernicus, whose theory of the order of the Universe would reign for centuries, did not publish his thesis until just before his death in 1543. During the remainder of that century and into the next Copernian theory had few followers; a few notable exceptions are covered below .
In 1599, a third alternative was offered by Tycho Brahe who suggested that the various planets, with the exception of the Earth, orbited the Sun and that the Sun in turn orbited a stationary Earth which was at the center of the then-known Universe.
Johannes Kepler, once an assistant to Tycho Brahe, was one of a handful of scientist who accepted Copernicus' theory of planetary movement. Using Brahe's observations of planetary position, he went on to conclude that the planets revolved around the sun in elliptical, as opposed to circular, orbits and gave us the means for calculating their individual distance from the Sun.
Galileo Galilei, also a believer in Copernicus' theories, made many contributions to the field of astronomy. In 1609, he modified the principla of a simple spyglass and developed a series of telescopes. With these creations, Galileo became the first to view the mountains of the Moon, the moons of Jupiter, and proved the Milky Way was made up of a multitude of tiny stars. Other significant contributions to the field include his studies of the laws of motion, including much that he used in support of Copernian theory. This eventually lead to Galileo being censured by the Inquisition. Galileo was placed on house arrest and forbidden to publish. Luckily, Galileo continued to work throughout the rest of his life and smuggled his works out of Italy.
In the late 1680s, Sir Isaac Newton established the modern science of dynamics by formulating his three laws of motion. When Newton applied these to the laws of orbital motion formulated by Kepler, he formulated the law of universal gravitation. Universal gravitation states that all objects are effected by a force, gravity, and that the strength of this force varies in accordance to the mass and distance between the objects. In 1668, Newton built the first reflecting telescope.
In the early 1920's, Edwin Hubble proved that there were galaxies beyond the Milky Way. Using the Doppler effect and light's tendency to shift toward red as it moves away, Edwin Hubble also determined that these other galaxies were moving away from us and each other in all direction. His observations about the red-shift of light gave us a means of determining our distance from these galaxies and objects within our own.
Hubble's work in combination with Einstein's 1915 theory of gravitation led to the inescapable conclusion that all the galaxies, and the whole Universe, had originated in a great explosion. This theory, commonly referred to as The Big Bang theory, espouses that the universe was created approximately 15 billion years ago. This theory marks the beginning of modern cosmology.
