MIRA :: Field Trips to the Stars ::

MIRA's Field Trips to the Stars Internet Education Program
Back to the MIRA Website \| Field Trips to the Stars Home >
A Brief History of Astronomy "The gods have not revealed all things from the beggining, but men seek and so find out better in time." -Xenophanes Astronomy is the first science. Diagram showing the alignment of major astronomical events with Stonehenge. All civilizations studied the sky. Indirect evidence of astronomic obsevations can be seen in the highly precise placement of ancient monuments, such as Stonehenge, where important events such as the solstice and equinox would align with certain aspects of the layout. Early civilizations used the Moon as a timekeeping devices, helping to keep track of when to plant crops, when to harvest, and most importantly, when to perform religious ceremonies. The periodic cycles of the Moon last roughly 29 1/2 days, with twelve cylces adding to be almost a full year, which determined the first calendars. It is not hard to imagine that when observing the Moon, early astronomers could not help but notice the motions of the planets, especially Venus. The first direct recording of observations date back almost 4,000 years and are written on Babylonian cunieform tablets. After the Babylonians, the rise of the Assyrian empire brought with it an obsession for astrology. Assyrian priests attempted to find meaning and portence in the movent of the planets, sun, and moon. They produced detailed recordings stretching out over many centuries, but there is is no recorded evidence that they attempted to understand the physical design of the universe. The reasons behind the motions of the heavens were the in realm of supersition and myth, and were not to be understood by humanity. It wasn't until the ancient Greeks that Astronomy became a science, almost 2,500 years ago. The Greeks Pythagoras and the Mathematics of the Natural World Pythagoras of Samos (580 - 500 B.C.E.) was a cult-leader, a mystic, and a mathematician. His followers, the Pythagoreans, worshipped numbers. Pythagorean's Theorem, which relates the lengths of sides in a right triangle, is still taught to this day: where a and b are the two sides, and c is the hypoteneuse. Numbers were more than abstract ideas to the Pythagoreans. They held a deeper meaning, rooted in the fundamental construction of the world. Meaning arose from physical interpretations, for example, a square number is an amount of objects that can be arranged into a square: Square numbers: 1, 4, 9, 16. The Pythagoreans also discovered a numerical relationship in musical harmony. The frequency of vibration (the pitch) of a plucked string is related to the tension of the string. They found that if two strings are held at the same tension, but one is twice the length of the other (length ratio of 2:1), the shorter string vibrates at twice the frequency as the longer. This is known as an octave. When the length ratio of strings is 3:2, the shorter string will create a tone that is a fifth above the longer string. For a ratio of 4:3, a fourth. In fact, all harmonious tones can be described using whole number length ratios. It appeared that whole numbers were deeply related to music, an area that, until that point, was a completely seperate area. Pythagoras of Samos The greater meaning of this discovery was not lost to the Pythagoreans: phenomena in the natural world are mathematical. Since numbers seemed to be so deeply embedded in nature, the Pythagoreans looked for other meaningful relationships. The number 10 was of special importance. It is the sum the four most sacred Pythagorean numbers: 1, 2, 3, and 4, also known as the tetractys. The Pythagoreans believed that the tetractys represented, among other things, the four elements (earth, fire, water, air) and the four geometric elements (point, line, surface, solid). Because 10 was the sum of these sacred numbers, it was perfect. And because the heavens were obviously perfect, there should be 10 bodies in the universe if nature was inheretly mathematical. Unfortunately, there were only eight known bodies at the time: the Earth, Sun, Moon, and the five visible planets (Mercury, Venus, Mars, Jupiter, and Saturn). In order to reach the number ten, the Pythagoreans invented two new astronomical bodies, the central fire, around which everything revolved, and the counter-Earth which exists on the opposite side of the central fire from Earth. The two new bodies were not seen because the surface of the Earth always pointed away from them. Regardless of the validity of these new concepts, one idea is clear: no longer did humanity have to rely on the whims of gods and other supernatural beings to understand the world around them. There was a certain order inherent in nature and mathematics was the key to its understanding. For the first time in human history a secular explanation for natural phenomena began to form. In the words of Anaxagoras: "Reason rules the world." Plato and Aristotle The Geocentric model of the Universe Greek society flourished. Among its greatest achievements was the embracement of reason as a foundation of human thought. The famous greek philosopher Plato (427 - 347 B.C.E.) glorified reasoning and applied its power to many subjects, among them politics, society, and mathematics. He reasoned that the heavens were perfect and unchanging. Being perfect, their movement could be explained by the epitomes of perfection in geometry: the circle and the sphere. The motions in the sky should be reflection of perfect circular motion around the Earth, which was central and stationary. The stars were the most perfect, for they did not change. Planets became less and less perfect the more movement they displayed. The Moon was the least perfect celestial object because it changed nightly. Obviously, the Earth was not perfect at all because everything changes. Thus physical world of the Earth and the universe were different; the laws used to describe the world had nothing to do with the divine heavens above. It was a split that would not be reconsiled for thousands of years. Plato's student, Aristotle, embraced and expanded his ideas. Since it appears that the sky moves around the Earth, the Earth must be at the center of the universe. After all, if the Earth did move, the apparent locations of the stars would change (parallax), unless they were very far away, and this was not observed. Aristotle's model of the universe is called geocentric. Every astronomical object existed on it's own rotating sphere, which also had other spheres acting on them to correct their behavior. The more heavenly (the less mutable) the object, the further away from Earth it must be. Thus the order of bodies in the Aristotelian universe was the Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and finally the fixed stars. The major difference between Aristotle and Plato was that Aristotle actually believed that the universal existed physicall this way, with the spheres made up of some unknowable material. Plato did not believe the physical existence of his theory, merely that it mathematically represented the motions of the sky in a pleasing way. Aristotle's idea of the geocentric universe dominated human thought for the next 1,500 years, though it had some minor refinements. Ptolemy The Aristotelian geocentric universe, while intellectually pleasing, was not particularly good at predicting positions of planets in the sky. Many problems existed, the most egregious being retrograde planetary motion. The planets move in a constant direction with relation to the stars. At certain times of the year, however, certain planets (like Mars) will appear to stop their motion, move backwards, and then resume their normal direction. The Aristotelian system has no way to explain this, for it claims that Mars is on a rotating sphere and should trace a circle in the sky. Claudius Ptolemy (100 - 170 C.E.) was an Greek astronomer and mathematician who lived in Hellenized Egypt. He created a solution to this problem by this problem embracing an idea by a previous astronomer, Hipparchus, in which the planets would move in smaller circles placed on their spheres, called epicylces. He also placed the Earth slightly off-center of the nested sphere system. His altered system proved much more precise in predicting planetary motion, and was used until the Copernican revolution in the 15th century. The Copernican Revolution Ptolemy's model is known as the Geocentric model of the solar system, with Earth in the middle surrounded by the heavens, the planets moving in perfect circles and at uniform speeds. There are several problems with this model. It predicts incorrect positions for

MIRA's Field Trips to the Stars Internet Education Program

Back to the MIRA Website | Field Trips to the Stars Home >

A Brief History of Astronomy

"The gods have not revealed all things from the beggining, but men seek and so find out better in time."
-Xenophanes

Astronomy is the first science.

Diagram showing the alignment of major astronomical events with Stonehenge.

All civilizations studied the sky. Indirect evidence of astronomic obsevations can be seen in the highly precise placement of ancient monuments, such as Stonehenge, where important events such as the solstice and equinox would align with certain aspects of the layout. Early civilizations used the Moon as a timekeeping devices, helping to keep track of when to plant crops, when to harvest, and most importantly, when to perform religious ceremonies. The periodic cycles of the Moon last roughly 29 1/2 days, with twelve cylces adding to be almost a full year, which determined the first calendars. It is not hard to imagine that when observing the Moon, early astronomers could not help but notice the motions of the planets, especially Venus. The first direct recording of observations date back almost 4,000 years and are written on Babylonian cunieform tablets. After the Babylonians, the rise of the Assyrian empire brought with it an obsession for astrology. Assyrian priests attempted to find meaning and portence in the movent of the planets, sun, and moon. They produced detailed recordings stretching out over many centuries, but there is is no recorded evidence that they attempted to understand the physical design of the universe.

The reasons behind the motions of the heavens were the in realm of supersition and myth, and were not to be understood by humanity. It wasn't until the ancient Greeks that Astronomy became a science, almost 2,500 years ago.

The Greeks

Pythagoras and the Mathematics of the Natural World
Pythagoras of Samos (580 - 500 B.C.E.) was a cult-leader, a mystic, and a mathematician. His followers, the Pythagoreans, worshipped numbers. Pythagorean's Theorem, which relates the lengths of sides in a right triangle, is still taught to this day:

where a and b are the two sides, and c is the hypoteneuse.

Numbers were more than abstract ideas to the Pythagoreans. They held a deeper meaning, rooted in the fundamental construction of the world. Meaning arose from physical interpretations, for example, a square number is an amount of objects that can be arranged into a square:

Square numbers: 1, 4, 9, 16.

The Pythagoreans also discovered a numerical relationship in musical harmony. The frequency of vibration (the pitch) of a plucked string is related to the tension of the string. They found that if two strings are held at the same tension, but one is twice the length of the other (length ratio of 2:1), the shorter string vibrates at twice the frequency as the longer. This is known as an octave. When the length ratio of strings is 3:2, the shorter string will create a tone that is a fifth above the longer string. For a ratio of 4:3, a fourth. In fact, all harmonious tones can be described using whole number length ratios. It appeared that whole numbers were deeply related to music, an area that, until that point, was a completely seperate area.

Pythagoras of Samos

The greater meaning of this discovery was not lost to the Pythagoreans: phenomena in the natural world are mathematical.

Since numbers seemed to be so deeply embedded in nature, the Pythagoreans looked for other meaningful relationships. The number 10 was of special importance. It is the sum the four most sacred Pythagorean numbers: 1, 2, 3, and 4, also known as the tetractys. The Pythagoreans believed that the tetractys represented, among other things, the four elements (earth, fire, water, air) and the four geometric elements (point, line, surface, solid). Because 10 was the sum of these sacred numbers, it was perfect. And because the heavens were obviously perfect, there should be 10 bodies in the universe if nature was inheretly mathematical. Unfortunately, there were only eight known bodies at the time: the Earth, Sun, Moon, and the five visible planets (Mercury, Venus, Mars, Jupiter, and Saturn). In order to reach the number ten, the Pythagoreans invented two new astronomical bodies, the central fire, around which everything revolved, and the counter-Earth which exists on the opposite side of the central fire from Earth. The two new bodies were not seen because the surface of the Earth always pointed away from them.

Regardless of the validity of these new concepts, one idea is clear: no longer did humanity have to rely on the whims of gods and other supernatural beings to understand the world around them. There was a certain order inherent in nature and mathematics was the key to its understanding. For the first time in human history a secular explanation for natural phenomena began to form.

In the words of Anaxagoras: "Reason rules the world."

Plato and Aristotle

The Geocentric model of the Universe

Greek society flourished. Among its greatest achievements was the embracement of reason as a foundation of human thought. The famous greek philosopher Plato (427 - 347 B.C.E.) glorified reasoning and applied its power to many subjects, among them politics, society, and mathematics. He reasoned that the heavens were perfect and unchanging. Being perfect, their movement could be explained by the epitomes of perfection in geometry: the circle and the sphere. The motions in the sky should be reflection of perfect circular motion around the Earth, which was central and stationary. The stars were the most perfect, for they did not change. Planets became less and less perfect the more movement they displayed. The Moon was the least perfect celestial object because it changed nightly. Obviously, the Earth was not perfect at all because everything changes. Thus physical world of the Earth and the universe were different; the laws used to describe the world had nothing to do with the divine heavens above. It was a split that would not be reconsiled for thousands of years.

Plato's student, Aristotle, embraced and expanded his ideas. Since it appears that the sky moves around the Earth, the Earth must be at the center of the universe. After all, if the Earth did move, the apparent locations of the stars would change (parallax), unless they were very far away, and this was not observed. Aristotle's model of the universe is called geocentric. Every astronomical object existed on it's own rotating sphere, which also had other spheres acting on them to correct their behavior. The more heavenly (the less mutable) the object, the further away from Earth it must be. Thus the order of bodies in the Aristotelian universe was the Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and finally the fixed stars. The major difference between Aristotle and Plato was that Aristotle actually believed that the universal existed physicall this way, with the spheres made up of some unknowable material. Plato did not believe the physical existence of his theory, merely that it mathematically represented the motions of the sky in a pleasing way.

Aristotle's idea of the geocentric universe dominated human thought for the next 1,500 years, though it had some minor refinements.

Ptolemy
The Aristotelian geocentric universe, while intellectually pleasing, was not particularly good at predicting positions of planets in the sky. Many problems existed, the most egregious being retrograde planetary motion. The planets move in a constant direction with relation to the stars. At certain times of the year, however, certain planets (like Mars) will appear to stop their motion, move backwards, and then resume their normal direction. The Aristotelian system has no way to explain this, for it claims that Mars is on a rotating sphere and should trace a circle in the sky.

Claudius Ptolemy (100 - 170 C.E.) was an Greek astronomer and mathematician who lived in Hellenized Egypt. He created a solution to this problem by this problem embracing an idea by a previous astronomer, Hipparchus, in which the planets would move in smaller circles placed on their spheres, called epicylces. He also placed the Earth slightly off-center of the nested sphere system. His altered system proved much more precise in predicting planetary motion, and was used until the Copernican revolution in the 15th century.

The Copernican Revolution

Ptolemy's model is known as the Geocentric model of the solar system, with Earth in the middle surrounded by the heavens, the planets moving in perfect circles and at uniform speeds. There are several problems with this model. It predicts incorrect positions for