The Universe consists of billions of systems of stars called galaxies. The galaxy to which we belong is called The Milky Way. It is a family of stars which form a great disc, which is about 100,000 light years in diameter, which bears a thickness of about 250 light year near our Sun. Our Sun is about 30,000 light years from the center of the Galaxy. The stars that we see with the naked eye in the nights are at a large range of distances from the Solar System, from a few light years to hundreds of light years
The bodies in the Universe are constantly moving through space, which implies that the galaxies are changing positions. Our Sun, which is traveling at about 500,000 miles per hour, completes an orbital path around the center of the Galaxy in about 230 million years. This movement through The Milky Way, may result in the Sun with its satellites, passing through areas of dense clouds of cold dust. Some scientists believe that this occurance could be responsible for significant climatic changes on Earth
. The stars or the suns in the galaxies form systems called solar systems. At the center of each system is the star. Our Solar System has the Sun at the center, and revolving around it, nine planets, five of which are visible with the naked eye. These planets which are of varying sizes, occupy varying elliptical orbits at different distances around the Sun. Some of these planets have satellites like the Moon and other large objects in orbit around them. There are about 50,000 smaller bodies called Planetoids or Asteroids, and countless smaller objects which are found in our Solar System.
The question is often asked “Why do the planets orbit the Sun?” The same is applicable to all orbiting bodies. This question which puzzled scientists for thousands of years was eventually solved by Isaac Newton’s Laws of motion. Newton stated that if a body was in a state of rest or uniform motion, it would continue in that state unless it was disturbed by another force . That is considered the reason why the planets instead of following straight lines of path, they move in elliptical orbits. He further stated that every particle of matter in the Universe attracts every particle with a force that varies directly as the product of their masses and inversely as the square of the distance between them. Therefore, the greater the masses of the bodies, the greater is the mutual attraction. Since the distance between the attracting bodies affects the force between them, it means that the farther the bodies are apart, the lesser the attraction. The converse is true, the lesser the distance apart, the greater the attraction. As the planets orbit the Sun, the Sun which is much larger in mass, exerts a tremendous pull on it. This could result in the planet falling into the Sun. However, because of the tendency for the planet to keep in a straight path, a compromise between the two extremes is reached. This results in the planet taking an elliptical path.
Historical View of Planetary Motion
Astronomy is the science which involves the study of the universe. It is considered to be the oldest science. In early times, the movement of the Sun and the Moon were of particular interest for reasons that they caused changes in light and darkness. In addition to the natural events that the movement and positioning of these bodies effected, some societies attached social, political and religious significants.
Men in particular, priests and magicians, could be considered the forerunners of the practitioners of the study of Astronomy. They devoted inordinate amounts of time in the study of planetary movements. Their persistence and commitment provided the world with astonishing accuracy of recorded observations. That attitude continued even with the rise and fall of great civilizations in the Middle East, Greece and Rome.
Among the earlier astronomers were the Egyptians who discovered important patterns of solar eclipses. Any one place on Earth with a similar size of Egypt would witness a total eclipse once every eight years. This was an indication that the Egyptians must have devoted a great number of years of observations in developing those particular astronomical predictions.
With the decline of the Egyptians civilization, the art was lost for thousands of years. Later on, the existing knowledge of the Egyptians was utilized by the Greeks, who later became the torch bearers of astronomical knowledge. The Arabs continued the practice after the Greek civilization fell and were credited for providing the modern terminologies of Astronomy. After the Arabs, the interest in the science declined until the Middle Ages when the Renaissance produced the so-called fathers of modern Astronomy.
The persisting central concept for over two thousand years was the belief that the Earth was the center of the Universe. The prevailing thought was that the Earth was stationary and the other celestial bodies moved around it in uniform circular patterns.
Claudius Ptomley(90 AD—168 AD) a Greek astronomer, was credited with compiling and developing a great body of work on Greek Astronomy. He published a book called “The Almagest” which compiled ideas of the old philosophers and from the then new astronomical developments. His work provided impressive models for the planetary motions which were useful for the next thousand years in predicting planetary positions. One of his core ideas was the existing concept that the Earth was the center of the Universe. One of the serious difficulties that he encountered was the observation that some planets showed a retrograde motion—an apparent reversal in their orbital path. This was a marked contrast to the to the existing idea of uniform orbital patterns.
A Polish mathematician, canon and astronomer, Nicholas Copernicus, born in 1473, initiated what was regarded as the start of modern Astronomy. As an astronomer, he compiled a table of planetary motions which was useful up until the end of the sixteenth century. Quite noticeably, he produced a central shift in the core of the existing astronomical concepts that the Earth was the center of the Universe. He then suggested that it was the Sun which was the center of the Universe around which the planetary bodies orbit. He claimed that it was that principle which was responsible for the seemingly irregular planetary motions.
Tycho Brahe was born in 1546, three years after the death of Copernicus whose idea he viewed with suspicion. He was considered the father of observational astronomy; he fundamentally improved the designs of observational instruments. His greatest contribution in understanding the universe was the precision of his recorded observations. Prior to his death, he passed on to his assistant, Johnnes Kepler, the observational measurements he had collected.
Johnnes Kepler was born on December 27, 1571 in Weil Der Stadt, Germany, to a father who was a tavern keeper, and a mother who was a herbalist. In his youthful years he was plagued with bad digestion, boils, a stint of smallpox, crippled hands and various skin disorders. Kepler entered the University of Tubingen as a teenager, and graduated at the age of twenty. He had the ambition of becoming a Lutheran minister, instead, he was encouraged to pursue the study of Astronomy. He devoted nearly ten years of his life working with Tycho Brahe’s data, and the Copernicus theory that the planet Mars, orbits in a perfect circular pattern. As a mathematician, he employed his full mathematical skills in finding a calculation that would fit. Reluctantly, he accepted his own finding that the only mathematical fit was an elliptical orbit. From this, Kepler established the three laws of planetary motions.
Kepler’s First Law
Firstly, he dispensed with the concept of circular orbit. In his work he discovered that the orbit of Mars matched best with a curve called an ellipse. He stated that
the orbit of a planet is an ellipse with the Sun at one focus.
Kepler’s Second Law
“The line joining the sun to the planet(called the radius vector) sweeps through an equal area in any given interval of time”. This law implies that when an orbiting planet is closest to the Sun, its orbiting velocity must increase in order to sweep through an equal area within the same interval of time as when it was farther away.
Kepler’s Third Law
The square of the period of rotation about the sun (the sidereal period) is proportion to the cube of the mean distance, P
2
= D
3
. Where P represents the sidereal period, and D is the average distance of the planet from the sun in Astronomical Units”.
Later, Isaac Newton’s laws of motion allowed for the derivation of Kepler’s laws of elliptical planetary motion as a general consequence of gravitational force acting on the bodies in the Solar System.
