Early Theories on the Solar System
  2006-08-24 11:03:18      CRIENGLISH.com

During the first millennium B.C., astronomy became more scientific. Middle Eastern and Chinese cultures observed the Sun, stars and the planets more precisely, attempting to learn more about our position in the universe. They studied intently the rise and set times of the stars and planets, and developed calendars useful for agriculture. Star positions also became important tools in understanding directions, thereby aiding navigation. Although not always correct in its beliefs, the most mathematically influential society during this time period was ancient Greece; not only did they think that the Earth was the centre of the universe, but one philosopher stated in 434 B.C. that the Sun was a ball of fire 60 kilometres in diameter, hovering 6500 kilometres above Earth’s surface. The Greeks did, however, use mathematics to estimate the circumference of the Earth and developed extensive star catalogues. Around 130 B.C., Ptolemy wrote Almagest, a huge collection of astronomical data including mathematical models, information about eclipses, and planetary and stellar positions and movements. It remained the main astronomical almanac for hundreds of years, and was not seriously challenged until Copernicus disputed the geocentric model of the solar system in the 1500’s.

A quiet period occurred in astronomical research after the fall of the Greek Empire, with few theories being introduced for over a thousand years. During this stagnant period, most astronomical research was conducted either by the Roman Catholic Church or by astrologers. The Church employed astronomers to study the heavens. They had a firm philosophy about the objects in the night sky, believing that the Earth was the centre of the universe and that the sky was static and unchanging. In contrast to the Church’s pursuit of astronomy was astrology, the study of the planets and the stars and their effect on human life. In spite of the fact that astrology depended on supernatural beliefs rather than on scientific findings, the scientific field of astronomy benefited greatly from the research of ancient astrologers. Astrologers observed and recorded the motions of the stars and the planets with great detail, giving astronomers valuable information about their movement in the night sky. Although the Church held fast to its erroneous beliefs and astrologers’ information was used for supernatural philosophy, the research of both contributed significantly to astronomical advances.

By the 16th century, when the tools used to measure stellar positions gave relatively accurate results, astronomers began to note irregularities in the accepted model of the solar system and the night sky. In the early 1500s, Nicolaus Copernicus noted that the planets had slight discrepancies between their observed and presumed positions. When the theory that the planets orbited the Earth in perfectly circular orbits could not account for the observed motions, Copernicus speculated that the Sun was the centre of the solar system. This Heliocentric mode had been postulated in the third Century B.C., but had not been taken seriously and was subsequently ignored. In 1572 another astronomer, Tycho Brahe, observed a supernova explosion in the constellation of Cassiopeia. This “new star” proved that the sky was not permanent and unchanging. Both of these observations were seen as sacrilege by the Church because they went against accepted dogma. Their later published works were not officially recognized by the Church, and they were forced to renounce their heliocentric theories.

A breakthrough for astronomy came with the invention of the telescope. The spyglass was invented in 1608, but an Italian named Galileo Galilei was the first to construct a telescope in 1610 and use it to look at the night sky. His small handheld refractor telescope did not provide sharp images and had a magnification of only 20 times, but what Galileo saw was unlike anything anyone had ever seen before. Over the first few months of observations Galileo had discovered more about the solar system and the universe than anyone had previously achieved. He first studied the Sun and Moon and discovered their surfaces were not perfect; the Moon had numerous craters and mountains and there were visible “blemishes” which rotated around the surface of the Sun. He observed the planets, noting that they were circular disks and not pinpoints of light like the stars. The phases of Venus were discovered and signified that planets shone by reflected sunlight. He also noticed the rings of Saturn (although he did not realize what they were) and the four large moons of Jupiter which are now named after him. The motion of the four satellites from one side of the planet to the other convinced Galileo that they were in orbit around Jupiter, proving that not every object in the sky was in orbit around the Earth. Galileo noted many more stars were visible through the telescope than with the naked eye, and the cloudy haze of the Milky Way was actually made up of thousands of faint individual stars not visible with the naked eye. Because Galileo suggested that the objects in the night sky were not perfect in form, were in constant change, and were more numerous than previously believed, he was excommunicated from the Roman Catholic Church.

Galileo’s findings revolutionized astronomy as a science and began the fall of the Church’s astronomical beliefs, but it was not until Kepler and Newton backed the observations with mathematical calculations that the heliocentric model of the solar system was accepted as truth. While Galileo was making his breakthrough observations, Johannes Kepler used the accurate recorded observations of Brahe to develop a new planetary model, and formulated the three laws of planetary motion. Essentially, the first law stated that the planets orbited the Sun in an ellipse with the Sun at one focus, the second that the orbital speed of a planet slows down the further it is from the Sun, and the third gave a mathematical relationship between a planet’s orbital period and its distance from the Sun. (The square of the orbital period of the planets is proportional to the cube of their average distance from the Sun) These simple but innovative laws were in agreement with the observed planetary movements, and allowed astronomers to calculate the distances from the planets to the Sun.

In the late 1600s a mathematician named Sir Isaac Newton developed his own three laws of motion involving forces, along with the universal law of gravity. Newton’s three laws were the law of inertia, the relation between the force applied to an object, the object’s mass, and its acceleration, and the third law is the famous “for every action there is an equal and opposite reaction”. His proposal of the law of gravity, which described mathematically that the force of attraction between two bodies was proportional to the product of their masses divided by the square of the distance between them, was a monumental concept and explained how the planets remained in orbit around the Sun. The theory of gravity finally convinced astronomers that the Sun was the centre of the solar system and governed the motions of the planets.

(Source: ioncmaste.ca)



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