Introduction and brief history of astronomy
Introduction
Astronomy is a field of physical science that studies the motion and nature of celestial bodies. It was originally founded on astrology and on naked-eye observation of a spherical and limited universe. With time it has developed into an exact science which tries to answer many questions which occur one after the other. In other words there will always be questions which will be asked about the universe, and after answering those questions new ones will occur but to be able to answer those questions we must be aware of some things...let us just start here: There are mainly two main divisions or points of view, of the origin of the universe, the creation concept and that of evolution (starting with the big bang theory). Through this website we are going to show you the scientific facts and which seem to be more relevant and exact in the creation model but you are free to believe in the one that is more reasonable to you, but know there is only one truth. so read the information carefully and with an open mined and I am sure you will benefit from this site. Especially from the essays and all the other linked opportunities. Here is a short historic introduction to the theme with a preparing conclusion for the final conclusion which you must give to yourself.
The origin of astronomy
Since ancient time, man has noticed that stars rise and set just the sun and Moon do. Man has also noticed that five stars are shiner than the others and wonder about noticeably. These are the planets (from Greek: wondering): Venus, Mars, Jupiter, Mercury, and Saturn. On the other hand, the other stars always have the same position with respect to each other. They seem to be fixed on a same moving surface. Therefore they are called fixed stars. The neighboring stars are grouped and form unchanging figures. These groups of stars or constellations were given names of familiar animals: scorpion, cancer, lion, etc. Knowing constellations was essential for night travel either on land or by sea.
The ancients also noticed the regularity of the celestial phenomena such as the return of the Sun, the alternation of the seasons and the phases of the Moon , and the presence of a specific star at a specific position at a specific time of the year. The emergence of agriculture and farming as well as the periodic floods of the rivers have led the ancients (Mesopotamians, Egyptians, Chinese etc.) to establish calendars based on the observation of the sky in order to specify the periods of harvesting and sowing. One of the first known calendars was elaborated in Egypt in the third millennium before our era.
Moreover, the ancients have attributed to the celestial bodies a divine character and supernatural effect on people’s lives. This has promoted the development of astrology. Astrology which, is an augural art founded on the study of stars in order to determine their influence on terrestrial events, on lives and characters of people, and on their destiny.
For example, it was necessary to locate the star that rises at the child’s birth in order to predict his/her destiny. Instruments such as the alidade and the gnomon were invented for this purpose in order to determine the directions of the stars and the time respectively.
Thus, astronomy, which is the science that studies the position, motion, and structure of the celestial bodies – planets, stars, galaxies, etc.- was born from the development of astrology in order to meet the requirements of everyday life.
In spite of its irrational and unscientific foundations, astrology has contributed to the development of astronomy. Until the 17th century, all the great astronomers were astrologers. But nowadays, astronomy has rid itself of those sources. It has become an exact science and field of physics.
The Greeks pursued the astronomic observations of their predecessors. However, they focused more on the thought of the nature, origin, and transformation of matter on one hand, and on the architecture of the universe and laws that govern it, on the other.
This has given birth to cosmology, which is a field of astronomy that studies the structure and origin of the universe (cosmos) as a whole.
The Geocentric theory of Plato and Aristotle.
The two Greek philosophers Plato (428-348 BC) and Aristotle (384- 322 BC) have established in dogma the geocentric theory that describes the structure of the universe and laws that govern that motion of the celestial bodies.
The postulates and the principles of this theory are the following:
- The universe is spherical.
- All celestial bodies, including earth, are spherical.
- Earth is immobile and is at center of the universe.
- Any celestial motion must be circular and uniform around earth, and any appearance of a non-circular motion should be explained by a combination of uniform circular motions.
- The celestial bodies complete their revolution around the earth at different distances. The fixed stars are the farthest from Earth and are carried by a sphere whose period of revolution is on day.
Greek astronomers have elaborated systems that describe the architecture of the Universe according to this theory, and that explain the data obtained from observing the sky.
The Geocentric Ptolemaic system
In the second century of our era, Ptolemy (70-147) of Alexandria produced a system of the world. The system
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was based on astronomical observations performed for hundreds of years in Alexandria, and on work of other Greek astronomers trying to apply the dogma of uniform circular motion to the data obtained from their observations. The Ptolemaic system in his book titled Syntax which was translated into Arabic under the title Almagest (the great). According to this system:
- - Earth is immobile and is at the center of the universe..
- - The Moon and the Sun move uniformly along two circular orbits having the earth as center,
- - The other planets move uniformly along small circles called epicycles. The center of each epicycle moves along a circular orbit around earth . this is called the deferent
- The planets are located around the Earth in the following order from closest to the farthest: Moon, Mercury, Venus, The Sun, Mars, Jupiter, and Saturn.
Ptolemy carefully selected the various parameters of motion (epicycle and deferent radii, planet velocity, etc.) and was able in to interpret some observed abnormalities such as retro gradation, which is a planet’s motion in
an opposite direction to its original direction, and why a planet gets closer to or moves farther away from the earth.
The development of Astronomy in the 16th and 17th centuries.
The Ptolemaic geocentric system remained almost unchanged for fourteen centuries. During this long period, astronomy remained stationary in Europe but was developed by the Arabs. They built observatories, perfected the observational instruments, and made developed several observations. But they did not look for any simpler system than that of Ptolemy.
The Heliocentric Theory Of Copernicus
The renaissance of astronomy took place in the beginning of the 16th century when the Polish astronomer Nicolas Copernicus (1473-1543) revealed his heliocentric theory (helio, from Greek :Sun) a few weeks before his death in his book De revolutuionbus.
Copernicus realized that the only way to resolve the contradictions that existed between the astronomical observations and the Ptolemaic system was by abandoning the geocentric theory. Moreover, he introduced the concept of relative motion in order to demonstrate the apparent stillness of Earth. According to Copernicus’s theory
- The Sun is immobile and is at the center of the spherical universe.
- Earth is a plant like any other.
- The planets are carried by solid and transparent spheres, and each planet is driven by uniform rotational motion of its sphere around the Sun. The fixed stars are carried by the sphere that has the longest radius.
- The period of revolution of a planet around the Sun depends on the distance that separates it from the Sun.
- The Earth revolves around the Sun in one year and at the same time rotates on itself in twenty-four hours.
- The Moon is a satellite of the earth and has a uniform circular motion around it.
- The planets are located around the Sun in the following order from closest to farthest: Mercury, Venus, Earth, Mars, Jupiter, and Saturn.
Copernicus theory has led to a more accurate prediction of the motions of the planets and to an understanding
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of the variation in brightness of planets far from the Sun (Jupiter and Saturn). It has also led to the accurate determination of the periods of the revolution of the planets around the Sun and to the other Phenomena. This theory proved to be a decisive one: It is based on this theory that scientific revolution was triggered in the 17th
century against Aristotle’s physics.
In addition, Copernicus ideas marked the beginning of modern astronomy. Observations became more frequent and more accurate to verify these ideas.
The observations of Tycho Brahe and the laws of Johannes Kepler
Danish astronomer, Tycho Brahe (1546-1601), never accepted Copernicus’s theory yet he was a remarkable observer. He received financial help from the king for a long time and was therefore able to build a big observatory. During more than twenty-five years he accumulated precious data on the motions of the planets. He also invented several measuring instruments with which he performed invaluable observations with the naked eye. Brahe died in 161 in Prague leaving all his paperwork to his young assistant, the German astronomer Johannes Kepler(1572-1630). Convinced by Copernicus’s theory, Kepler tried to match Brahe’s observations with the model of a motion around the Sun. After long calculations, he published the three empirical laws which are:
- The first law: The planets move along ellipses around the Sun that is at one of the foci.
- The second law: the second law links the planet velocity to its distance from the Sun: the velocity decreases as the distance increases and vice versa.
- The third law: links the period of revolution of the planet to its average distance.
With this three laws that give a complete description of the motion of the planets, Kepler put an end to the dogma of the geocentric uniform circular motion.
Galileo Galilei
Convinced by Copernicus’ theory, the Italian Physicist Galileo (1564-1642) looked for the experimental proof.
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In 1609, he made the first telescope designed for astronomical observations. He observed the mountains and
craters of the Moon, the Milky Way as a tight formation of stars, the phases of Venus predicted by Copernicus’ theory. In 1616, after the publication of these results, Pope Paul V condemned the idea of the Earth revolving around the Sun as contrary to the scriptures.
Isaac Newton
In order to answer the question: “ What make the planets revolve around the Sun?”, the British scientist Isaac Newton (1643-1727)established the fundamental law of celestial mechanism in his book Principia. He deduced the law of universal gravitation from Kepler’s laws and from Galileo’s mechanism. This law states:
Any two bodies attract each other with a force that varies with the inverse of the square of distance between them and with the product of their masses.
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This attraction is the cause of the fall of objects on earth and of the motion of the stars. Thus, the same laws apply in the sky as well as on the earth. Hence it becomes possible to compute accurately the position of a star when its velocity and its position at a certain time are known. It is thanks to Newton’s work that a lot of phenomena such as the tides and the decrease of the acceleration of gravity going from the pole to the equator were explained. Moreover, the accurate prediction of the return of Halley’s comet in 1758 was a spectacular confirmation of the universal gravitational law.
Newton’s accomplishment in optics contributed as well to the sky observation techniques. His discovery of separation of white light by prism incited him to construct the first refracting telescope that avoids the dispersion of colors by refraction.
The development of astronomy in the 18th and 19th centauries
Most astronomers in the 18th and 19th centauries were developed to verify the gravitational law. Astronomers perfected observational instruments in order to resolve the problems they were facing. They constructed large –mirror telescopes and produced the achromatic lens that refracts light without scattering it. They also produced clocks and accurate stopwatches, and invented the micrometer to measure accurately the dimensions of the images of the stars.
The mechanical processing of the images of results of the observations led to the improvement of the methods of mechanical analysis and then to the development of a new science: Celestial Mechanics.
The development of the instruments enabled the English astronomer Edmund Halley (1656-1742) to detect the motion of the stars and hence to put an end to the concept of sphere of fixed stars.
In 1781, the British astronomer William Herschel (1738-1822)
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discovered the seventh planet in the solar system, Uranus, by using a 7-foot(2.10 meters) long telescope. He also discovered the double stars.
French astronomer Urbain Leverrier (1811-1877)came up with the hypothesis of an unknown planet while trying to interpret the disturbances observed in the motion of Uranus. In 1846 he discovered the eight planet, Neptune, at the exact position predicted by the computations done according to the gravitational law.
The exact measurements of the celestial distances have led the establishment of star catalogs that indicate the positions of thousands of stars.
In the middle of the 19th century, the study and interpretation of the solar light spectrum led to the development of a new field of astronomy, astrophysics: it is a science that studies the constitution the physical properties, and the origin of the stars. In fact, the study of the light spectrum emitted by a star leads to the understanding of the composition of its atmosphere and estimate the temperature, pressure, and even the speed at which the star moves away from us.
The use of big telescopes and the development of photography have led to the observation of low- brilliance stars and to extend the field of studies to the galaxies.
How one might have noticed that the development of astronomy had many stages and corrections which are still being continued through the modern ages or the technological era. How I already said in the introduction that there are many questions which in turn after being answered create new ones, so let us study in an scientific way all the things which we can know, to answer the questions of this era. But we must be aware that only the scientific research and work will not be sufficient to answer these new questions, but there is something else which one should experience or be aware of to be able to come closer to the truth and so the answers.
To finish I want to tell a short story.
On a warm and clear summer evening two persons go along the same sea shore. They hear the sound of the waves and observe the beautiful star sky. Suddenly both see a light going on and off on the sea. How do they react and what do they do?
The first person, a retired professor of physics, runs directly to his car where he always has some physical instruments to measure different things at different occasions. With a watch he measures the time of the on going and off going of the lights, he also measures the brightness of the light, he notes the position of the light and with a spectrometer measures the spectrum. On his way back home he once again takes notes of the different position of the light.
As he reaches home his wife says “ why are you so exited darling, did you see something exiting today?”
He answers Yes, I was watching and observing something, that I could identify as an electric light in an silicate frame, which was regularly going on and off whose light beams had an intensity of 2500 lumen, and at a distance of about 850m from the shore.”
The second person: on the same sea shore at the same evening is a young scout who is tired from the day and is going home he sees the same light. As he reaches home his mother asks him: “ why are you so exited son? Did you see something exiting?” and he answers, “Yes, I saw a boat on the sea which was sending SOS signals because it was sinking. So I ran directly to the coast guard and informed them, who directly helped and saved the people on board.”
In your opinion who of the two could guess and get the idea of the situation better? I would say the second. The first was to busy with only the measuring and did not get the main idea. But the other got the idea without even measuring anything.
I am not trying to say that measuring and observing is unimportant, in turn it is even very important to realize things but if we only observe we will not come and understand, and so act on what is really the aim of that what is the truth. So let us study, observe, and sea the facts to come to an answer which will give sense to the question. Hope you can benefit from this website and you come closer to some facts and so the Truth.
Written by Jonathan Spangenberg