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Historical Astronomy Historical Astronomy (Neolithic record of Moon Phases) Introduction Arguably the history of astronomy IS the history of science. Many cultures carried out astronomical observations. However, very few formed mathematical or physical models based on their observations. It is those that did that we will focus on here, primarily the Babylonians and Greeks. Other Examples At the same time, that focus should not cause us to forget the impressive accomplishments of other cultures. Other Examples ∼ 2300 year old Chankillo Big Horn Medicine Wheel, Observatory, near Lima, Wyoming Peru Other Examples Chinese Star Map - Chinese records go back over 4000 Stonehenge, England years Babylonian Astronomy The story we will follow in more detail begins with the Babylonians / Mesopotamians / Sumerians, the cultures that inhabited the “fertile crescent.” Babylonian Astronomy Their observations and mathematics was instrumental to the development of Greek astronomy and continues to influence science today. They were the first to provide a mathematical description of astronomical events, recognize that astronomical events were periodic, and to devise a theory of the planets. Babylonian Astronomy Some accomplishments: • The accurate prediction of solar and lunar eclipses. • They developed mathematical models to predict the motions of the planets. • Accurate star charts. • Recognized the changing apparent speed of the Sun’s motion. • Developed models to account for the changing speed of the Sun and Moon. • Gave us the idea of 360◦ in a circle, 600 in a degree, 6000 in a minute. Alas, only very fragmentary records of their work survives. Early Greek The conquests of Alexander the Great are oen credited with bringing knowl- edge of the Babylonians science and mathematics to the Greeks. Alexander ordered many translations of their work. The Greeks did benefit from these observations, particularly their records of eclipses, observations of the motion of the Sun, and their star charts. The Greeks also learned many techniques of time measurement from the Babylonians. Unfortunately, for most of them, only scraps of their work remains, oen as mentions by other authors. This is partly due to the decline of Alexandria. Greek Astronomers Solar System Models Starting with the Greeks, we will see a tension between models for the solar system, a geocentric or Earth-centered model, and a heliocentric or Sun-centered model. Although we know today that the heliocentric model is correct and the Earth really does move, remember that for most of recorded history the geocentric model was accepted as “obviously” correct. Solar System Models Parallax Some of the resistance to the heliocentric model was due to Aristotle’s influence, some because they did not observe stellar parallax. Astronomers realized it might mean the stars were very far away, but that much empty space was inconceivable. Anaximander of Miletus Anaximander was a student of Thales, who is oen called the first scientist. Anaximander drew the first known map of the world. In addition, he believed life began in weer environments and simpler forms gave rise to more advanced forms. He even traced human ancestors to fish! Anaximander of Miletus Anaximander’s astronomical ideas were radical for his time. He was the first to propose that Earth floated freely in space. He described the Sun, Moon, and stars as coming from rings of fire with openings in them that could change size or close. Note that this puts them at dierent distances from the Earth. Euxodus of Cnidus Euxodus developed many of the foundational ideas in Greek mathematics and astronomy. • He was the first to give a mathematical framework for a model of the “Universe” (solar system), based on spherical geometry. • It consisted of nested celestial spheres to explain celestial motions, including retrograde motions. The Earth was at the center. • There were 3 spheres each for the Sun and Moon, 1 for the stars. • There were 4 spheres each for the 5 known planets. • Each sphere provided one motion, such as diurnal motion and the eastward motion relative to the stars. • The major flaw was an inability to explain brightness changes. Euxodus’ Model Aristotle Aristotle was the most revered scholar in science, mathematics, and philosophy for almost two millennia. Over that time, his writings dominated Western thought. To break with Aristotle was tantamount to heresy. At the same time, many of his ideas in physics and astronomy were incorrect and actually held back progress in some areas. Aristotle’s Physics Aristotle believed that the logic and reason were the best way to figure out how the Universe worked. Experiments or observations were not as trustworthy, because the senses could be fooled. • Aristotle extended the four elements established by Empedocles, earth, air, fire, and water, to include a fih one found only in the heavens: aether. • He believed that there were two basic types of motion. Forced motion occurred when something was physically disturbed, like throwing a rock. Natural motion occured as an object sought its “proper” place in the Universe. • The forced motion stopped as soon as the cause of it was removed. Aristotle’s Physics • Earth and water moved down. Fire and air moved up. Aether, which made up objects in the heavens, underwent circular motion. • Aristotle thought that since everything in the heavens must be perfect, and the circle was the most perfect shape, everything in the heavens must undergo uniform, circular motion. • This idea became so entrenched that it was not seriously questioned until aer Copernicus. • Aristotle thought that it was impossible to have a vacuum. One argument was that an object’s motion could increase to infinity with nothing to slow it down. Aristotle’s Astronomy Aristotle refined the models of the celestial spheres by adding counter-rotation spheres in-between to decouple the motions. The mechanics of the celestial spheres concerned him. Contrary to popular belief, it was known since ancient times that the Earth is round. Aristotle was one of many who provided proofs of this. • There were stars that could be seen at some latitudes, but not others. • The Earth always cast a round shadow during a lunar eclipse. • Wherever you are, objects always fall straight down. However, he thought it impossible that Earth itself might be moving, or spinning. Aristarchus of Samos Aristarchus of Samos was one of the first to make an estimate of the relative sizes of the Earth, Sun, and Moon, and the distances to the Sun and Moon. He measured the angle between the Moon and Sun at first and last quarter, when they formed a right triangle with the Earth, to get the distance to the Moon and Sun. Since the Sun and Moon are about the same angular size, their distances and sizes must be proportional. He used the size of the Earth’s shadow at the Moon’s location during a lunar eclipse to get the size of the Moon in Earth diameters. This also gave him the size of the Sun. Aristarchus’s Measurements Aristarchus’s Measurements His technique was sound, but very diicult to carry out. The real angle is 89◦ 500. His measurement was about 87◦. So he estimated the distance to the Sun was 19x the distance to the Moon. The real value is closer to 390X. However, he did find that the Sun was much larger than the Earth. So, he thought, the smaller object should orbit the larger. Aristarchus, not Copernicus, was the first astronomer, to advocate for a heliocentric model of the solar system. He further wrote that the Earth spun on its axis and that the stars were distant suns that were much farther away than anyone had imagined. No one took these ideas seriously until aer Copernicus. Apollonius of Perga Apollonius was best known for his work on conic sections. His mathematical descriptions of the parabola, ellipse, and hyperbola are still basically what we use today. Many historians give him credit for the idea that solved the biggest problem with geocentric models - the retrograde motion of the planets. Apollonius reputedly showed that by combining two circular motions he could replicate retrograde motion. He moved the planet onto a smaller, the epicycle the center of which moved around the Earth on an orbit called the deferent. Apollonius of Perga Some kind of epicycle would be used in all models of the solar system for the next 1500 years, until Kepler devised his laws of planetary motion. Eratosthenes of Cyrene Eratosthenes excelled in many areas, particularly mathematics. This won him the position of Director of the Library of Alexandria. Like Aristarchus, Eratosthenes also determined the distance to the Sun and Moon, and the size of the Sun. His value was 27 times the diameter of the Earth. The real answer is 109 times. He may have been o, but he got even closer and verified that the Sun was much larger than the Earth. Eratosthenes of Cyrene One of his many other accomplishments was the invention of the armillary sphere. In time the armillary became the pre- eminent astronomical instrument for de- termining positions, until the invention of the telescope. It should be noted that Chinese astronomers independently in- vented this device. Size of the Earth Eratosthenes was most well known for making the first accurate estimation of the size of the Earth. He did this with a lile simple geometry. (Since we can’t be sure of his units, it’s hard to say how close he got.) It was known that at a certain day of the year the Sun was directly overhead at Syene (modern Aswan, Egypt). It could be seen at noon at the boom of a well and objects there cast no shadow. Eratosthenes showed that if on that day he measured the angle from the vertical for the Sun in Alexandria, the ratio of that angle to 360◦ is equal to the ratio of the distance between the two cities to the circumference of the Earth.
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