Astronomy
Lecture Ancient Four
Ancient Astronomy
Salisbury University
"Eppur Si Muove" -"(And, yet it moves!")
Some Early Cosmologies: "Models of the Universe" - A History Brief
Plato & Aristotle: (around 350 B.C.)
Plato remains the best known of all the Greeks. The original name of this Athenian aristocrat was Aristocles, but in his school days he received the nickname Platon (meaning "broad") because of his broad shoulders. The heavenly bodies, he believed, exhibited perfect geometric form. This he expresses most clearly in a dialogue called "Timaeus" in which he presents his scheme of the universe. He describes the five (and only five) possible regular solids -- that is, those with equivalent faces and with all lines and angles, formed by those faces, equal. These are the four-sided tetrahedron, the six-sided hexahedron (or cube), the eight-sided octahedron, the twelvesided dodecahedron, and the twenty-sided icosahedron. Four of the five regular solids, according to Plato, represented the four elements (Earth, Fire, Air, Water), while the dodecahedron represented the universe as a whole. (Kepler would later incorporate these perfect solids into his own view of the universe.) Plato also decided that since the heavens were perfect, the various heavenly bodies would have to move in exact circles (the perfect curve) along with the crystalline spheres (the perfect solid) that held them in place.
Figure 1: Plato A student of Socrates and mentor to Aristotle.
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Ancient Astronomy
Aristotle (Platos pupil) accepted the heavenly spheres and added further to them, reaching a total of 54. He seemed to think of the spheres as having an actual physical existence. Aristotle also accepted the four elements but restricted them to Earth. He suggested a fifth element "aether," of which all the heavens were composed. (We still use phrases such as "ethereal heights" and luminiferous ether today.)
Figure 2: Aristotle
This line of reasoning led him to agree with the Pythagoreans that Earth and heaven were subjected to two different sets of natural law. On Earth all things were changeable and corrupt, while in the heavens all was permanent and immutable (unchanging). On Earth the four elements each had its own place, and motion was an attempt to reach that place. Earth was in the center, water above it, air above that, and fire highest of all the earthly substances. Therefore an object composed largely of earth, such as a rock, would, if suspended in air, fall downward, while bubbles of air trapped under water would move upward. Again rain fell, but fire rose.
It also seemed to Aristotle that the heavier an object was, the more eagerly it would strive to achieve its proper place since the heaviness was the manifestation of its eagerness to return. Hence a heavier object would fall more rapidly than a lighter one. Nineteen centuries later, a reconsideration of this problem by Galileo was to lead to momentous consequences. The motion of heavenly objects, on the other hand, was no attempt to get anywhere. It was a steady, permanent motion, even and circular.
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Ancient Astronomy
Geocentric Model
Earth Centered, uniform circular motion (perfect) Besides everyone knew that the Earth could *not* possible move!! Why?? NO Observed Stellar Parallax!! (Via naked eye technology of the time) No parallax meant no Earthly motion, therefore Earth did not move! (THEY WERE WRONG, but for valid reasons.) Some "stars" did, however "wander" through the heavens. These "wandering stars" (planetsplanets) were observed to show "Retrograde Motion" - HmmmmThese motions against the fixed background of non-wandering stars had to be explained while still keeping to earlier dogma.
Figure 3: Possible retrograde patterns of a "wandering star" (planet).
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Ancient Astronomy
Ptolemy's Geocentric Model - A Fantastically Successful Model
Ptolemy (aka Claudius Ptolemaeus, Ptolomaeus, Klaudios Ptolemaios, Ptolemeus) lived in Alexandria (in Egypt) from approx. 87 -150 AD. Very little is known about his personal life or history other than his academic works. Ptolemy synthesized and extended Hipparchus's system of epicycles and eccentric circles to explain his geocentric theory of the solar system.
Ptolemy's system involved at least 80 epicycles to explain the motions of the Sun, the Moon, and the five planets known in his time. He believed the planets and sun to orbit the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic System. It predicts the positions of the planets accurately enough for naked-eye observations. The Ptolemaic explanation of the motions of the planets remained the accepted wisdom until the Polish scholar Copernicus proposed a heliocentric view in 1543.
Figure 4: Ptolemy.
This is described in the book Mathematical Syntaxis (widely called the Almagest), a thirteen book mathematical treatment of the phenomena of astronomy. It contains a myriad of information ranging from earth conceptions to sun, moon, and star movement as well as eclipses and a breakdown on the length of months. His work - "The Almagest" - accounted for Retrograde Motion of the "wandering stars" (planets). Corrections Slowly Added: Epicycles added to epicycles Precision timing to keep Mercury and Venus always near the Sun Earth was also placed "off-center" (Equant - see picture)
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Ancient Astronomy
The Ptolemaic System
Jupiter
Mars
Saturn Earth & Moon
Venus Mercury Sun
Figure 5: The Ptolemaic system is a geocentric model where the moon, all the planets known at the time, and fixed stars all orbit the Earth using perfect circles and epicycles.
The system provided the intellectual framework for all discussion of the universe for nearly 1000 years!! So in a very true sense, this idea was stupendously successful even though we now know that it was incorrect.
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Ancient Astronomy
Earth
Equant
Deferent Epicycle
Figure 6: The planet moves along its epicycle as the epicycle moves along the deferent around the Earth. To make the observations as accurate as possible, it was necessary to place the Earth slightly off center of the orbits, but to preserve symmetry that meant that there was an equal place (Equant) opposite the Earth from the center. The combined motion of the planet and the resulting retrograde motion are shown in blue.
Times do change! And New (Radical) Ideas soon follow.
The Successful (??) Beginnings of the Heliocentric Model:
(Sun-centered)yes, the Greeks, Muslims, and others had already proposed a lot of the coming ideas much earlier, but it wasn't until all the ideas where combined with physics, math, and astronomy, and careful repeatative observation that the tide of new ideas held firm.
Nicholas Copernicus:
Figure 7: Copernicus.
Born on Feb. 19, 1473, in Thorn (Torun), Poland, Nicolaus Copernicus was destined to become, through the publication of his heliocentric theory 70 years later, one of the seminal figures in the history of scientific thought. Copernicus entered the University of Krakow, which was famous for its mathematics, philosophy, and astronomy curriculum. This experience stimulated the Copernicus to study further liberal arts at Bologna (1496-1501), medicine at Padua, and law at the University of Ferrara, from which he emerged in 1503 with the doctorate in canon law.
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Ancient Astronomy Not only did Copernicus faithfully performed his ecclesiastical duties, but also practiced medicine, wrote a treatise on monetary reform, and turned his attention to a subject in which he had long been interested--astronomy.
Heliocentric Model (Sun Centered) De revolutionibus orbium coelestium
(On the Revolutions of the Heavenly Spheres, 1543 - published after his death). This classic work challenged the geocentric cosmology that had been dogmatically accepted since the time of Aristotle. His ideas were in direct opposition to Aristotle and to 2nd century astronomer Ptolemy. Copernicus hesitated a long time, whether to publish it or to imitate the Pythagoreans, who transmitted the mysteries of their philosophy only orally to their own disciples for fear of exposing them to the contempt of the multitude. Copernicus proposed that a rotating Earth revolving with the other planets, about a stationary central Sun, could account (in a simpler way) for the observed phenomena of the daily rotation of the heavens, the annual movement of the Sun through the ecliptic, and the periodic retrograde motion of the planets. 6
Planet's apparent path around sky.
6
5 4 3 2 1 5
Sun
4 3 2
Earth Outer Planet 1
Fixed Stars
Figure 8: Earth overtakes slow outer planet so the outer planet appears to slow down, move in reverse, and then move forward again with respect to the fixed stars.
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Ancient Astronomy Anticipated in various aspects by the Pythagoreans and Aristarchus, and by the Muslim astronomer Ibn al-Shatir and certain Christian writers (whose ideas there is no conclusive evidence he knew). However, Copernicus still adhered to the ancient Aristotelian doctrines of solid celestial spheres and perfect circular motion of heavenly bodies. Protestant theologians raised very heated opposition to the Copernican system. The main objections were based on Biblical reasons. On the Catholic side, opposition only commenced seventy-three years later. On March 5, 1616, Copernicus' work was banned from being taught and discussed by the Congregation of the Index "until corrected." It stayed on this list of prohibited books and teachings until 1822!!!! Earth is not the center of everything All the planets revolve around the Sun Stars are very much farther away than the Sun Any motion of the stars is a result of the Earth's rotation Any movement of the Sun is due to Earth's rotation and revolution Retrograde Motion was much easier to explain! One last compromise attempt!! (The Gold Nosed Rastafarian)
Tycho Brahe The Great Compromise
He allegedly challenged a fellow student to a duel with swords in a dispute over who was the better mathematician. Brahe's nose was partially cut off, and he was said to wear a gold and silver replacement upon which he would continually rub oil. [[Note: This is the definition of sacrificing for ones science.]] He was given the little island of Hven in the Sont near Copenhagen, and there he built his observatory, Uraniburg, which became the finest observatory in Europe.
Figure 9: Tycho Brahe
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Ancient Astronomy
Tycho Brahe's contributions to astronomy were enormous.
He not only designed and built instruments, he also calibrated them and checked their accuracy periodically. He thus revolutionized astronomical instrumentation. He also changed observational practice profoundly. Whereas earlier astronomers had been content to observe the positions of planets and the Moon at certain important points of their orbits (e.g., opposition*, quadrature *, station), Tycho and his cast of assistants observed these bodies throughout their orbits. As a result, a number of orbital anomalies never before noticed were made explicit by Tycho. Without these complete series of observations of unprecedented accuracy, Kepler could not have discovered that planets move in elliptical orbits. Tycho was also the first astronomer to make corrections for atmospheric refraction*. In general, whereas previous astronomers made observations accurate to perhaps 15 arc minutes, those of Tycho were accurate to perhaps 2 arc minutes, and it has been shown that his best observations were accurate to about half an arc minute. If Tycho destroyed the dichotomy between the corrupt and ever changing sublunary world and the perfect and immutable heavens, then the new universe was clearly more hospitable for the heliocentric planetary arrangement proposed by Nicholas Copernicus in 1543. Was Tycho therefore a follower of Copernicus? He was not. Tycho gave various reasons for not accepting the heliocentric theory, but it appears that he could not abandon Aristotelian physics. (Aristotle felt that all objects had a notion of place; heavy bodies fall to their natural place, the Earth, which is the center of the universe.) If the Earth were not the center of the universe, physics, as it was then known, was utterly undermined.
Tycho developed a system that combined the best of both worlds.
He kept the Earth in the center of the universe, so that he could retain Aristotelian physics (the only physics available). The Moon and Sun revolved about the Earth, and the shell of the fixed stars was centered on the Earth. But Mercury, Venus, Mars, Jupiter, and Saturn revolved about the Sun. He noted a circular path for the comet of 1577 between Venus and Mars.
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Ancient Astronomy This Tychonic world system became popular early in the seventeenth century among those who felt forced to reject the Ptolemaic arrangement of the planets (in which the Earth was the center of all motions) but who, for various reasons, could not accept the Copernican alternative. He made the most precise observations that had yet been made by devising the best instruments available before the invention of the telescope. His observations of planetary motion, particularly that of Mars, provided the crucial data for later astronomers like Kepler to construct our present model of the solar system. In addition, he made observations of a supernova (literally: nova= "new star") in 1572 (we now know that a supernova is an exploding star, not a new star). This was a "star" that appeared where suddenly none had been seen before, and was visible for about 18 months before fading from view. Since this clearly represented a change in the sky, prevailing opinion held that the supernova was not really a star but some local phenomenon in the atmosphere (remember: the heavens were supposed to be unchanging in the Aristotelian view). Brahe's meticulous observations showed that the supernova did not change positions with respect to the other stars (no parallax). Therefore, it was a real star, not a local object. This was early evidence against the immutable nature of the heavens, although Brahe did not interpret the absence of parallax for stars correctly, as we discuss below. Brahe made careful observations of a comet in 1577. By measuring the parallax for the comet, he was able to show that the comet was further away than the Moon. This contradicted the teachings of Aristotle, who had held that comets were atmospheric phenomena ("gases burning in the atmosphere" was a common explanation among Aristotelians). As for the case of the supernova, comets represented an obvious change in a celestial sphere that was supposed to be unchanging; furthermore, it was very difficult to ascribe uniform circular motion to a comet. Tycho made the best measurements that had yet been made in the search for stellar parallax. Upon finding no parallax for the stars, he (correctly) concluded that either; (1) the earth was motionless at the center of the Universe, or (2) the stars were so far away that their parallax was too small to measure. (Unfortunately, he chose incorrectly.)
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Ancient Astronomy Not for the only time in human thought, a great thinker formulated a pivotal question correctly, but then made the wrong choice of possible answers: Brahe did not believe that the stars could possibly be so far away and so concluded that the Earth was the center of the Universe and that Copernicus was wrong. Brahe proposed a model of the Solar System that was intermediate between the Ptolemaic and Copernican models (it had the Earth at the center). It proved to be incorrect, but was the most widely accepted model of the Solar System for a time. Thus, Brahe's ideas about his data were not always correct, but the quality of the observations themselves was central to the development of modern astronomy. Therefore: Tychonic Model: Sun and Moon orbit around the Earth others orbit around Sun!! (Very Complex and messy.)
Galileo Galilei: (Pisa 1564-1642)
Italy, telescope into Astronomy around 1609 (Invented in Holland) Galileo built a telescope with which he made celestial observations, the most spectacular of which was his discovery of the satellites of Jupiter. In 1610 he was nominated the foremost Mathematician of the University of Pisa and given the title of mathematician to the Grand Duke of Tuscany. He studied Saturn and observed the phases of Venus. In 1611 he went to Rome. He became a member of the Accademia dei Lincei and observed sunspots. In 1612 he began to encounter serious opposition to his theory of the motion of the earth that he taught based on the Copernican. In 1614, Father Tommaso Caccini denounced the opinions of Galileo on the motion of the Earth from the pulpit of Santa Maria Novella, judging them to be erroneous. Galileo therefore went to Rome, where he defended himself against charges that had been made against him but, in 1616,
Figure 10: Galileo Galilei
he was admonished by Cardinal Bellarmino and told that he could not defend Copernican astronomy because it went against the doctrine of the Church. In 1630 he returned to Rome to obtain the right to publish his Dialogue on the Two Chief World Systems which was eventually
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Ancient Astronomy published in Florence in 1632. In October of 1632 Galileo was summoned to Rome by the Holy Office & Pope. The tribunal passed a sentence condemning him and they ultimately compelled Galileo to solemnly recant his theory. He was sent to exile in Siena and finally, in December of 1633, he was allowed to retire to his villa in Arcetri, the Gioiello. His health condition was steadily declining, - by 1638 he was completely blind, and also by now bereft of the support of his daughter, Sister Maria Celeste, who died in 1634. Galileo died in Arcetri on 8 January 1642.
1610 - "Sideras Numicus" (Starry Messenger)
Observations of note contained within Book. Sunspots, lunar craters, - imperfections? 4 Moons of Jupiter (Galilean Satellites) Earth not center of all things! Galileo observed that Venus showed phases entirely like those of the moon from full to crescent, which it must do if the Copernican theory was correct. According to the Ptolemaic theory Venus would have to be a perpetual crescent. The discovery of the phases of Venus definitely demonstrated, by the way, the fact that planets shine by reflected sunlight. Galileo discovered that the night side (that is, the dark portion) of the moon when the moon was less than full had a dim glow, which he explained as caused by light shining upon it from Earth ("earthshine"). It had been seen before but had been explained otherwise. Poseidonius thought it was sunlight shining through a partly transparent moon. Reinhold thought the moon's surface was phosphorescent. Earthshine showed that Earth like the planets, gleamed in the Sun, and removed one more point of difference between Earth and the heavenly bodies. Phases of Venus Earth moves and Venus not around us! Venus always stays within 47o of the Sun. In a Ptolemaic System, Venus would always have to appear as a crescent (only)! So Galileo's observations did not support Ptolemy's model. Galileo observed all the phases, gibbous, crescent, waxing, waning, etc 1. Venus, not centered on Earth 2. Large Crescent (near), small gibbous (far) 3. Copernicus' Model *did* predict this observation! Cosmos is Heliocentric!
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Ancient Astronomy Galileo got into serious trouble with the church (Roman Catholic) Was "commanded and forbidden" from teaching these "heretical" views! Galileo visited Rome in 1611, where he was greeted with honor and delight, though not everyone was happy. The thought of imperfect heavens, of invisible objects shining there, and, worst of all, of the Copernican system enthroned and Earth demoted from its position as center of the universe was most unsettling. Galileo also rather unwisely ventured to write a book giving his views on the bible and generally discussing theological subjects to the offense of theologians. Galileo's conservative opponents persuaded Pope Pius V to declare Copernicanism a heresy, and Galileo was forced into silence in 1616.
1632: "Dialogo Dei Due Massim Sistemi" (In Italian! Not Latin! For the common people!) He therefore published his masterpiece, Dialogue on the Two Chief World Systems, in which he had two people, one representing the view of Ptolemy and other the view of Copernicus, present their arguments before an intelligent layman. Galileo of course gave the Copernican the brilliant best of the battle. The Pope was persuaded that Simplico, the character who upheld the views of Ptolemy in the book, was a deliberate and insulting caricature of himself. The book was all the more damaging to those who felt themselves insulted, because it was written in vigorous Italian for the general public (and not merely for the Latin-learned scholars) and was quickly translated into other languages -- including Chinese! 1. Greatly offended the current Pope (former friend) 2. Very intimidated by the Inquisition recanted in 1633 (Bruno burned at stake!) 3. But the truth was already very apparent. Galileo's Dialogue was not removed from the Catholic "Index of Prohibited Books" until 1835. In 1965, Pope Paul VI, on a visit to Pisa, spoke highly of Galileo -- an even clearer admission that on this issue the Church had been in the wrong.
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Ancient Astronomy
Johannes Kepler: (1571-1630)
"...a sickly child, with thin limbs and a large, pasty face surrounded by dark curly hair. He was born with defective eyesight-myopia plus anocular polyopy (multiple vision). His stomach and gall bladder gave constant trouble; he suffered from bo...