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The Origin of Modern Astronomy

The Origin of Modern Astronomy. Why did people look up?. Religion Navigation Time keeping (calendar, clock) Food (planting, hunting, breeding). 0. The Roots of Astronomy. Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky.

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The Origin of Modern Astronomy

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  1. The Origin of Modern Astronomy

  2. Why did people look up? • Religion • Navigation • Time keeping (calendar, clock) • Food (planting, hunting, breeding)

  3. 0 The Roots of Astronomy • Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky. • Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance. • Those monuments were probably used as calendars or even to predict eclipses.

  4. Stonehenge • Constructed: 3000 – 1800 B.C. Summer solstice Heelstone • Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices • Probably used as calendar.

  5. Other Examples All Over the World Big Horn Medicine Wheel (Wyoming)

  6. 0 The Roots of Astronomy Newgrange, Ireland, built around 3200 B.C.: Sunlight shining down a passageway into the central chamber of the mount indicates the day of winter solstice.

  7. Other Examples All Over the World (2) Caracol (Maya culture, approx. A.D. 1000)

  8. Other Examples All Around the World 0 Chaco Canyon, New Mexico Slit in the rock formation produces a sunlit “dagger” shape, indicating the day of summer solstice

  9. 0 Other Examples All Around the World (2) Mammoth tusk found at Gontzi, Ukraine: Inscriptions probably describing astronomical events

  10. Ancient Greek Astronomers (1) • Unfortunately, there are no written documents about the significance of stone and bronze age monuments. • First preserved written documents about ancient astronomy are from ancient Greek philosophy. • Greeks tried to understand the motions of the sky and describe them in terms of mathematical (not physical!) models.

  11. Ancient Greek Astronomers (2) Models were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned: • Geocentric Universe: Earth at the Center of the Universe. • “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

  12. Thales of Miletus lived from about 624 BC to about 547 BC Founder of Greek Science Suggested that supernatural explanations were not necessary to understand what the universe was made of. Suggested that the world was inherently understandable and not just the result of arbitrary or incomprehensible events.

  13. Thales’ Cosmos water Air water earth Thales’ believed the universe consisted fundamentally of water with Earth as a flat disk on an infinite ocean. This was not widely accepted.

  14. Anaximanderof Miletus610-c. 547 BC Student of Thales. Suggested that the heavens must form a complete sphere around Earth (to explain the sky turning around the north star). Based on how the sky changes with travel north and south, he concluded that Earth must not be flat. Because the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction.

  15. Anaximander’s Cosmos Ring of Fire air and clouds Horizon Horizon Earth (a cylinder) Underground home of the heavenly bodies Because the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction.

  16. Pythagoras of Samos lived from about 569 BC to about 475 BC Taught that Earth was a sphere.

  17. Pythagorean Theorem C A B a2 + b2 = c2

  18. Democritus of Abdera “Nothing exists but atoms and empty space. Everything else is opinion.” lived from about 460 BC to about 370 BC

  19. Democritus was a student of Leucippus. Together they are considered “co-originators” of the belief that all matter is made up of atoms. He said that atoms were eternal, invisible, indivisible, and incompressible. Democritus believed the universe was made of an infinite number of atoms of the four elements. • He claimed the moon had mountains and valleys, the Milky Way was a vast group of individual stars, and that Earth and other worlds were created by random motions of infinite atoms. Other philosophers, including later Aristotle, argued against this. • Democritus was among the first to propose that the universe contains many worlds, some of them inhabited: • "In some worlds there is no Sun and Moon while in others they are larger than in our world and in others more numerous. In some parts there are more worlds, in others fewer (...); in some parts they are arising, in others failing. There are some worlds devoid of living creatures or plants or any moisture." • Because his theories do not give credit to a Creator, atomism became linked with atheism. This persisted into the mid-1800s. (In 17th century France you could be burned at the stake for believing in atoms.)

  20. Plato lived from 427 BC to 347 BC

  21. Plato in a Small Nutshell • The world cannot not be known through the senses (world view presented by the senses are like shadows on a cave wall) • The philosopher, through pure thought, can see through surface appearances to the ideal forms underneath. • The heavens, for example, are perfect and, therefore, move in uniform, circular motion because a circle is the perfect form. • Question for students: If the heavens move uniformly in perfect circles, then why do planets appear to make loops in the sky and speed up, then slow down?

  22. Ancient Greek Astronomers (3) • Eudoxus (409 – 356 B.C.): Model of 27 nested spheres • Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages: Universe can be divided in 2 parts: 1. Imperfect, changeable Earth, 2. Perfect Heavens (described by spheres) • He expanded Eudoxus’ Model to use 55 spheres.

  23. Eudoxus’ Cosmos (simplified) Axis of sun sphere Sphere of the stars Earth Sphere of the Sun Axis of stellar sphere

  24. Aristotle’s Cosmos (simplified) Saturn Moon Venus Earth Mars Mercury Sun Jupiter Sphere of Fixed Stars

  25. Aristotle’s Physics • What is the world made of? • Earth, Water, Air, and Fire • How do things move? • Natural Motion (towards the Earth) • Violent Motion (requires a force) • The Heavens are different from the Earth • Made of fifth substance (Quintessence) • Experience only circular motion • Other than repetitive circular motion, heavens experience no change • The Earth is Round • The moon revolves around Earth, giving us lunar phases

  26. N A B B S Aristotle’s argument (2): Observer at A never sees star B. However, if he travels south to position B, star B becomes visible. Therefore, the earth’s surface must be curved.

  27. Aristotle’s argument (1): Shadow cast by earth on the moon during an eclipse is always curved. The only geometric shape which always casts a circular shadow is a sphere

  28. Aristarchus of Samosc. 310 – 230 BC • Proposed a heliocentric system • Distance and size of the moon • Distance and size of the sun • Geometry of eclipses

  29. Aristarchus of Samos • Proposed a heliocentric system - • His belief in a heliocentric system was not popular. • Many argued against it. Arguments included: • If Earth is moving, why don’t we feel it? • If Earth is moving, why don’t we leave the moon behind? • If Earth is moving around the sun, why don’t we see stellar parallax?

  30. Parallax

  31. Aristarchus of Samos If Earth is moving around the sun, why don’t we see stellar parallax? Philosophers who did not believe in a heliocentric system argued that no stellar parallax meant Earth didn’t move and Aristarchus was wrong. Now we know Earth does move, so why don’t we see stellar parallax? Try putting your finger in front of your nose and looking at it with one eye and then the other. Now move your finger farther from your face and try again. Move it farther still, and try again. What do you see? We don’t see stellar parallax because the stars are so far from us. The Greeks did not consider this answer as their version of the universe was smaller than our solar system. We know can measure stellar parallax for a handful of stars that are close to us.

  32. Relative Distances of Sun and Moon Moon at first quarter Right angle Earth Angular separation between sun and moon when moon is at first quarter Sun Method of Aristarchus

  33. Problem with Aristarchus’ Method Moon at first quarter Right angle To Sun Angular separation between sun and moon when moon is at first quarter is so close to 90 (89.5) that it could not be reliably measured in ancient times Earth

  34. Eratosthenes • Circumference of Earth • Tilt of Earth Lived from 276 B.C. to 195 B.C.

  35. How big is Earth? • Start with a circle. • He heard tell of a town named Syene, where on a particular day of the year at noon there were no shadows on the water in the water well. To Sun • The Sun was overhead. To Sun Alexandria • He was in Alexandria. Syene • The sun was not overhead for him, but he could measure the angle between overhead and the Sun.

  36. How big is Earth? • He calculated the angle using shadows. It was approximately 7°. • We have 2 parallel lines, bisected by a third line 7° • What can we say about this angle? ? 7° To Sun 7° • It is the same! 7°! To Sun Alexandria Syene 7°

  37. How big is Earth? • Now we have a 7° “pie piece” of Earth. • A circle has 360°. • We can keep adding “pie pieces” until we get to 360°. • Then you can calculate the circumference. If you know the distance between the two towns, you just keep adding that distance all the way around the circle. To Sun 7° To Sun Alexandria D Syene 7° 360°

  38. To Sun 7° To Sun Alexandria D Syene 7° How big is Earth? • Did he get the right answer? • That depends on how well he measured the distance between towns (without an odometer or a GPS). • He measured the distance between Syene and Alexandria as ~ 5,000 stadia • We think he was off by a bit. Depending on the length of a stadia, he was off by 3-14%. Best estimate yet!

  39. Tilt of Earth • measured difference between noontime elevation of Sun in winter and summer • deduced that Earth's equator is tilted by 23.5 degrees • difference in height of Sun at different times gives latitude

  40. Tilt of Earth • We now know that the tilt (obliquity) varies over a 400,000 year cycle. • It ranges from 22.1 to 24.5.

  41. Hipparchus of Rhodes lived from 190 BC to 120 BC

  42. Achievements of Hipparchus • Trigonometry • 1st Large Star Catalog (about 3000 stars) • Invented latitude and longitude • Discovered Precession • Measured the length of a year to 6 minutes • Used Eccentrics to explain retrograde motion (moved Earth off exact center)

  43. 0 Precession (1) At left, gravity is pulling on a slanted top. => Wobbling around the vertical. The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

  44. Precession As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. Precession is caused by the gravitational effects of the sun and moon. The pole will be closest to Polaris ~ A.D. 2100. There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, the celestial north pole will be close to Vega in the constellation Lyra.

  45. North and South Precession

  46. 0 Later refinements (2nd century B.C.) • Hipparchus: Placing the Earth away from the centers of the “perfect spheres” • Ptolemy: Further refinements, including epicycles

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