Chapter 3.docx

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Department
Astronomy & Astrophysics
Course
AST101H1
Professor
Ian Shelton
Semester
Summer

Description
Chapter 3: The Science of Astronomy 3.1 The ancient roots of science How did astronomical observations benefit ancient societies?  Modern measures of time come directly from ancient observations of motion in the sky. The length of our day is the time it takes the Sun to make one full circuit of the sky  The length of a month comes from the Moon’s cycle of phases and our year is based on cycle of the seasons  The 7 days of the week were named after the seven naked eye objects that appear to move among the constellations: the Sun, the Moon, and the five planets recognized in ancient times What did ancient civilizations achieve in astronomy? Determining the Time of Day?  In the daytime, ancient peoples could tell time by observing the Sun’s path through the sky  The ancient Egyptians built huge obelisks, often decorated in homage to the Sun, that probably also served as simple clocks  The Egyptians also divided the night into 12 equal parts. Egyptian star clocks, often found painted on the coffin lids of Egyptian pharaohs, catalogued where particular stars appeared in the sky at particular times of night and particular times of year. By knowing the date from their calendar and observing the positions of particular stars in the sky, the Egyptians could use the star clocks to estimate the time of night  Water clocks had the advantage of working even when the sky was cloudy  Despite availability of other types of clocks, sundials were common throughout ancient times and remains popular today both for their decorative value and as reminders that the sun and stars were once our only guides to time Marking the Seasons  Many ancient cultures built structures to help them mark the seasons. One of the oldest standing human-made structures served such a purpose: Stonehenge –it was both an astronomical device for keeping track of the seasons and a social and religious gathering place Lunar Calendars  A basic lunar calendar has 12 months, with some months lasting 29 days and others lasting 30 days; the lengths are chosen to make the average agree with the approximately 29 ½ day lunar cycle  A 12 month lunar calendar therefore has only 354 or 355 days, or about 11 days fewer than a calendar based on the sun  This 19-year cycle on which the dates of lunar phases repeat is called the Metonic cycle because the Greek astronomer Meton recognized it in 432 B.C.  Lunar phases repeat with the Metonic cycle because 19 solar years is almost precisely 253 lunar months  A lunar calendar will therefore remain roughly in sync with a solar calendar if it has 235 months in each 19 year period Ancient Structures and Archeo astronomy  The study of ancient structures in search of astronomical connections is called archeoastronomy  The Big Horn Medicine Wheel in Wyoming. A study once claimed that its spokes have astronomically significant alignments, but later research showed the claim was in error –making this a good example of how science adapts as new data come to light 3.2 Ancient Greek Science Why does modern science trace its roots to the Greeks?  Greek philosophers developed at least three major innovations that helped pave the way for modern science o 1 they developed a tradition of trying to understand nature without relying on supernatural explanations and of working communally to debate and challenge each other’s ideas o 2 the greeks used mathematics to give precision to their ideas, which allowed them to explore the implications of new ideas in much greater depth than would have otherwise been possible o 3 while much of their philosophical activity consisted of subtle debates grounded only in Greeks also saw the power of reasoning from observations. They understood that an explanation could not be right if it disagreed with observed facts  Perhaps the greatest Greek contribution to science came from the way they synthesized all three innovations into the idea of creating models of nature, a practice that is central to modern science; a scientific model is conceptual representation created to explain and predict observed phenomena  Greek models discussed of nature sought to explain things such as the properties of matter and the motions of the stars, sun, moon, and planets  Many of the scientific ideas discussed in this book originated with the greeks How did the greeks explain planetary motion? - Ancient Greek philosophers assumed that earth must reside at the center of the universe - The greeks therefore developed a sophisticated geocentric model of the cosmos Early development of the geocentric model - Trace the orgin of greek science to the philosopher Thales - Thales predication of solar eclipse o Thales was the first person to have addressed the question “What is the universe made of?” without resorting to supernatural explanations o His own guess that the universe fundamentally consists of water and that the earth is a flat disk floating in an infinite ocean was not widely accepted even in his own time - A more sophisticated idea followed soon after proposed by Thales student Anaximander o Anaximander suggested that earth floats in empty space surrounded by a sphere of stars and two separate along which the sun and moon travel o We therefore credit him with inventing the idea of a celestial sphere o He also imagined earth itself to be cylindrical rather than spherical shape o He chose this shape because he knew earth had to be curved in a north –south direction to explain changes in the constellation with latitude because the visible constellations do not change with longitude he saw no need for curvature in the east – west direction - Greeks found the earth is round about 500 b.c by the famous mathematician Pythagoras o The Pythagoreans had a mystical interest in mathematical perfection and they considered a sphere to be geometrically perfect o More than a century later Aristotle cited observations of earth’s curved shadow on the moon during lunar eclipses as evidence for a spherical earth - The supposed heavenly perfection of spheres influenced models of the cosmos for many centuries - Plato asserted that all heavenly objects move in perfect circles at constant speed and therefore must reside on huge spheres encircling earth o Greeks found this method problematic: the apparent retrograde motion of the planets already well known by that time clearly showed that planets do not move at constant speeds around the earth - An ingenious solution come from plato’s colleague Eudoxus who created a model in which the sun, the moon, and the planets all had their own spheres nested within several other spheres o Individually the nested spheres turned in perfect circles - Whether Eudoxus and his followers thought of the nested spheres as real physical object is not clear but Aristotle did - In Aristotle’s model all the spheres responsible for celestial motion were transparent and interconnected like the gears of a giant machine o Earths position at the center was explained as a natural consequence of gravity o Aristotle argued that gravity pulled heavy things toward the center of the universe (and allowed lighter things to float toward the heavens thereby causing all the dirt, rock and water of the universe to collect at the centre and form the spherical earth o We now know that Aristotle was wrong about both gravity and earth’s location o However largely because of his persuasive arguments for an earth centered universe, the geocentric view dominated western thought for almost 2000 years Ptolemy’s Synthesis of Geocentric model - Greek modeling of the cosmos culminated in the work of Claudius Ptolemy - Ptolemy placed earth at the center of the universe but it differed in significant ways from the nested spheres of Eudoxus and Aristotle - Ptolemy geocentric model as the Ptolemaic model to distinguish it from earlier geocentric models - To explain the retrograde motion of the planets the Ptolemaic model applied an idea first suggested by Apollonius o Idea held that each planet moves around earth on a small circle that turns upon a larger circle (the small circle called epicycle and large called deferent) o A planet following the circle upon circle motion traces a loop as seen from earth with the backward portion of the loop mimicking apparent retrograde motion - Ptolemy also relied heavily on the work of Hipparchus; considered one of the greatest greek astronomer - Hipparchus developed the circle upon circle idea of Apollonius into a model that could predict planetary positions o To do this he had to add several features to the basic idea, for example, he included even smaller circles that moved upon the original set of small circles and he positioned the large circles slightly off center from the earth - Ptolemy great accomplishment was to adapt and synthesize earlier ideas into single system that agreed quite well with the astronomical observation available at the time - In the end he created and published a model that could correctly forecast future planetary position to within a few degrees of arc – roughly equivalent to the size of your hand viewed at arm length against the sky - His model work therefore remained in use for the next 1500 years - Arabic scholars translated the title into Almagest derived from the meaning the greatest compilation AD 800 How was Greek knowledge preserved through history? - Greek thought gained broad influence because they were adept at politics and war as they were at philosophy - Alexander the Great tutored Aristotle. ATG was interested in education and science - Alexander encouraged the pursuit of knowledge and respect for foreign culture - On the nile Delta in Egypt he founded the city Alexandria which became the center of world culture and after his death it became a great library and research center called the Library of Alexandria which opened around 300 bc o It remained the world’ s preeminent center of research for some 700 years surviving well past the fall of the classical Greek empire and into heyday of the roman empire Destruction of the Library of Alexandria - The library of Alexandria long history remains unknown because books recorded was destroyed with the library - Female scholar Hypatia died as a result of the destruction of the library o She was the one of the last resident scholars of the library as well as the director of the observatory in Alexandria and one of the leading mathematician and astronomer of her time o She was murdered in ad 415 after rising sentiment of free inquiry - The final destruction of the library took place not long after her death - In commemoration of the ancient library, Egypt built a New Library of Alexandria (bibliotheca Alexandrina opened 2003) with hope that it will Alexandria a global center for scientific research Islamic Preservation of Science - Much of the greek knowledge was lost with the destruction of the library of Alexandria - That which survived was preserved primarily thanks to the rise of a new center of intellectual inquiry in Baghdad, while European civilization fell into a period of intellectual decline known as the Dark Ages - New religion of Islam sought knowledge of mathematician and astronomy in hopes of better understanding the wisdom of Allah - During the 8 and 9 centuries scholars working in the Muslim Empire translated and thereby saved many ancient Greeks works - Around AD 800 the Islamic leader Al Mamun established a House of Wisdom in Baghdad with a mission much like that of the destroyed Library of Alexandria - Using the translat
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