1. Science 3210 001 : Introduction to Astronomy Lecture 2 : Visual Astronomy -- Stars and Planets Robert Fisher

2. Items qCourse adds q The course had been booked to capacity, but I will be adding as many people as the room can accommodate, in the order I have received requests. q People who attended last week’s lecture q Natasha Shah q Amanda Mayfield q Simon Spartalian q Add/drop day is February 6th. This means today is the last day where I am generally available to sign add/drop cards. qCourse webpage has been updated with first week’s lectures, and the first reading and homework assignment : q http://flash.uchicago.edu/~rfisher/saic.html qQuestions -- seek and ye shall find!

3. Review of Lecture 1 qHistory of Astronomy q Ancient Astronomy q Advent of Natural Philosophy q Medieval Astronomy in Arab World q Birth of Modern Science qScience Overview q Scales in the Cosmos q Cosmic Calendar q Powers of Ten Video

4. Overview of Lecture 2 qI. The Celestial Sphere qII. The Stars qIiI. The Motion of the Planets

5. Important Lessons to be Learned Today qBecause the stars are very distant, their motion on the sky is well- described as if they revolved around the Earth qThe motion of the planets is significantly more complex, and required elaborate geometrical constructions in the ancient geocentric system due to Ptolemy qNiklaus Copernicus simplified matters tremendously by putting the sun at the center of the universe -- even though he lacked the “smoking gun” evidence to prove his case

6. Motion of the Stars qThe foundation of all visual astronomy is a simple fact : the Earth is a Sphere qWhile common knowledge today, determination of the shape of the Earth was a significant challenge to ancient peoples qThe most convincing elementary argument comes from the fact that the Earth’s shadow (as seen in lunar eclipses) is always circular, as Aristotle correctly deduced Earth Image, Apollo 17 Crew

7. Lunar Eclipse

8. The Earth as the Center of the Universe qLooking up at the night sky, it appears as if the entire Universe revolves around the Earth.

9. Celestial Sphere, Zenith, Nadir, Horizon qThe distant stars appear to lie on a solid sphere, the celestial sphere. qThe zenith is the direction directly upwards. qThe nadir is the direction directly downwards. qThe horizon splits the celestial sphere in half along the zenith-nadir axis.

10. Zenith and Nadir Depend on Your Location qThe zenith and nadir directions depend on where one stands on the Earth.

11. Rotation of the Earth

12. Motion of the Celestial Sphere qThe rotation of the Earth causes the celestial sphere to appear to revolve. qThe north/south celestial poles correspond to the north/south poles of the Earth’s rotational axis.

13. The Motion of the Sun qAt a given location, the sun rises towards the east and sets towards the west. qA sundial gnomon casts a shadow away from the sun, towards the west. qThe invention of the gnomon is attributed to the ancient Greek philosopher Animaxander, successor to Thales

14. Determining North from the Sun’s Motion qAt noon, the sun reaches its highest point in the sky, directly north. qThis was a common method used by the ancients to determine North.

15. Clockwise

16. qImagine one wanted to read a sundial. We are facing south. qIn the morning, the sun rises in the east, casting a shadow in the west. qIn the afternoon, the sun begins to set in the west, casting a shadow to the east, following the same circular arc traced in the morning. qThe direction traced by the sun’s shadow in its arc, facing south, is clockwise. qWhen mechanical clocks with hands were first made, they were constructed so as to rotate in the same sense as the sundial -- clockwise -- not counter-clockwise.

17. Describing the Celestial Sphere -- The Great Circle A great circle on a sphere divides the sphere into two hemispheres. One can imagine the equator as an example of a great circle, but any circle dividing the sphere is a great circle. Great circle

18. Describing the Celestial Sphere -- Great Circles Great circles Any of the circles in the figure above are examples of great circles.

19. Angles qSeparation between two points on the celestial sphere are measured in terms of angles -- much like a clock. qA full circle is 360 degrees. qEach degree is 60 minutes. q The full moon is roughly one-half degree in width. q By remarkable circumstance, the width of the sun is also one-half degree. qEach minute is 60 seconds -- sometimes referred to as arcseconds.

20. The Meridian qThe great circle on the celestial sphere found by connecting north and south and passing through the zenith is referred to as the meridian. qWhen a celestial body crosses the meridian, it is said to transit. qWhen a body transits, it reaches its highest point from the horizon. qThe terms “AM” and “PM” derive their meaning from the meridian : q AM = Ante-Meridian q PM = Post-Meridian

21. The North Celestial Pole and Circumpolar Stars qLooking north from Chicago at night, one can see the North Celestial Pole. qThe North Celestial Pole is the direction along which the Earth’s axis is aligned. qThe stars which immediately surround the pole never set beneath the horizon. They are called circumpolar stars.

22. Star Trails Over Mauna Kea, Hawaii

23. Angles on a Familiar Sphere qBefore describing the celestial sphere in more detail, it helps to recall the layout of a more familiar sphere -- the Earth. qOn the Earth, angle north or south of the Equator is marked off by latitude. qAngle around the Earth from West to East is marked off by longitude.

24. Daily Motion of the Stars qThe daily motion of the stars Is very simple. qThe celestial sphere makes one full circle about the Earth, once per day. qThe circle is determined by only angle -- the declination, directly analogous to latitude on the Earth.

25. Question qIn the Northern hemisphere, the stars rise in the East, set in the West, and revolve counter-clockwise around the North celestial pole. In the southern hemisphere the stars rise in the q A) East, set in the West, and revolve counter-clockwise around the South celestial pole. q B) East, set in the West, and revolve clockwise around the South celestial pole. q C) West, set in the East, and revolve clockwise around the South celestial pole. q D) West, set in the East, and revolve counter-clockwise around the South celestial pole.

26. View from North Pole qAt the north pole, the zenith is the north celestial pole. qThe nadir is the south celestial pole. qThe horizon is the celestial equator. qPrecisely half of the celestial sphere is visible. qAll stars are circumpolar.

27. View from Equator qThe zenith is the celestial equator. qThe north celestial pole always appears directly north. qThe full sky is visible -- each star rises for 12 hours each day.

28. View from Chicago qThe altitude of the north celestial pole is equal to the latitude of your position on the Earth - roughly 42 degrees for Chicago. qStars within 42 degrees of the north celestial pole are circumpolar. qStars within 42 degrees of the south celestial pole are not visible.

29. Summary of Celestial Sphere Viewed fom Earth

30. Question qThe celestial equator is : q A) The path of the sun compared with the stars. q B) The path of the moon compared with the stars. q C) The average path of planets on the sky. q D) Always directly overhead at the Earth’s equator. q E) Always along the horizon at the Earth’s equator.

31. Constellations  Constellations are the “states” on maps of the celestial sphere.  Each region of the sky belongs to precisely one constellation.  Stars within each region are alphabetically named, starting with the brightest stars, by a greek letter followed by the constellation name -- eg, Polaris is Alpha Ursae Minoris.

32. The Ecliptic  The sun appears to move along a plane in the sky referred to as the ecliptic.  The other planets also appear to move close to the ecliptic.  Physically, the fact that all solar system bodies lie close to the ecliptic is because the entire solar system lies within a flattened disk.

33. The Plane of The Ecliptic From Fire Island

34. The Solstices and Equinoxes qThe solstices occur when the sun reaches a maximum (solstice = sol sistere or sun stops in Latin) distance away from the celestial equator -- roughly June 21 and December 21. qThe equinoxes occur when the sun intersects the celestial equator -- roughly March 21 and September 21. On this day, the sun appears directly above the equator, and every point on earth has equal day and night.

35. Earth on Equinoxes

36. Yearly Sky and Zodiac  As the sun moves through the ecliptic, different portions of the night sky become observable.  The ecliptic falls into 12 constellations over the year -- the zodiac.

37. Angle of Inclination of Earth qThe ecliptic makes an angle of 23.5 degrees with the celestial equator. qPhysically, this means the Earth’s rotational axis is tilted with respect to its orbit.

38. Angle of Inclination qAs the Earth orbits around the sun, the angle of inclination remains the same.

39. Origin of Seasons qThe angle of inclination causes seasonal variation on Earth.

40. Question qThe ecliptic makes its smallest angle with the southern hemisphere during the q A) Summer q B) Autumn q C) Winter q D) Spring

41. Private Universe

42. Lunar Phases qThe appearance of the moon varies over the course of the month.

43. Eclipses qThe lunar orbit is inclined by 5 degrees relative to that of the Earth/sun. qSolar eclipses can occur during the new moon, but only when the sun, moon, and Earth happen to line up. qSimilarly, lunar eclipses can occur during the full moon, but only when the sun, Earth, and moon happen to line up.

44. Lunar Eclipses qThe moon passes through the shadow of the Earth. qLight is fully blocked in the umbra, and only partially blocked in the penumbra.

45. Types of Lunar Eclipses qThree types of Lunar eclipses.

46. Lunar Eclipses

47. Question qWhat would a total lunar eclipse look like to an observer standing on the surface of the moon facing the Earth?

48. Lunar Eclipses from Moon

49. Solar Eclipses qSolar eclipses occur when the sun’s light is blocked by the moon. qIn a sense, they are completely serendipitous : the sun is 400 times larger than the moon, but is also 400 times further away. qHence, the apparent angular size of both the moon and the sun are nearly identical.

50. Solar Eclipses qThree types of solar eclispes can occur.

51. August 11, 1999 Eclipse Viewed from Mir

52. The Eclipse of May 28, 585 BC qThales of Miletus is said to have predicted a remarkable solar eclipse on May 28, 585 BC. qOf this occasion, Herodutus writes, q ‘On one occasion [the Medes and the Lydians] had an unexpected battle in the dark, an event which occurred after five years of indecisive warfare: the two armies had already engaged and the fight was in progress, when day was suddenly turned into night. This change from daylight to darkness had been foretold to the Ionians by Thales of Miletus, who fixed the date for it within the limits of the year in which it did, in fact, take place…The Medes and Lydians, when they observed the change, ceased fighting, and were alike anxious to have terms of peace agreed on.’ qOne must wonder -- how was it possible for Thales to predict the eclipse?

53. Solar Eclipses, 1999 - 2020  Both lunar and solar eclipses recur with a frequency of 18 years, 11 days, known as the Saros cycle.  The Saros cycle was known to the ancient Babylonians, and may have been used by Thales to predict the eclipse of May 28, 585 BC.

54. Why are Eclipses so Rare? qFor a total lunar or solar eclipse to occur, there must be a precise alignment of the Sun, Earth, and moon. qHowever, because of the inclination of the moon’s orbit with respect to the plane of the ecliptic, such alignments are rare.

55. The Planets

56. The Motion of Planets qLike the stars, the planets are generally seen to traverse the sky. qUnlike the stars, occasionally the planets are observed to stop and move from west-to-east in so-called retrograde motion. qThis behavior gave rise to the ancient greek name -- “planets” comes from a Greek root meaning “wanderer”. qA fully satisfactory explanation of this motion was not developed until Newton.

57. The Earth as the Center of the Universe qLooking up at the night sky, it appears as if the entire Universe revolves around the Earth.

58. Geocentric Model of the Universe This observation led the ancients to formulate a geocentric model of the universe, with the Earth at the center, and the Sun, planets, and stars all revolving around the Earth along spheres.

59. Cacophony in the Celestial Harmony -- The Problem of Retrograde Motion qThe geocentric model of the universe works very well for stars, but there is a major problem for planetary motion. qOccasionally, the outer planets will appear to slow down, stop, then reverse their direction on the night sky -- retrograde motion.

60. Retrograde Motion qThe mystery of retrograde motion can be simply explained in a model with the Sun at the center of the Solar system. qAn inner body (like the Earth) is moving more rapidly than an outer body (like Mars), and so will “pass” it much like a faster car on the expressway. qDuring this passing, the outer planet will execute retrograde motion.

61. Retrograde Motion in the Geocentric Model -- Epicyclic Motion qExplaining retrograde motion in the geocentric model of the universe, however, is almost impossible, unless one invents an additional circular motion which each planet executes, called epcicyclic motion.

62. Ptolemaic Model of the Solar System qThe ancient astronomer Ptolemy (90 - 168 AD) created the most complex version of the geocentric model of the system, which was used for almost one and a half millenia. qIn the Ptolemaic model, the moon, sun, and planets all revolved in circles, which themselves revolved around circles around the Earth. qAnd in fact, the Earth was not quite at the center of this model, either.

63. Why Did the Ancients Reject a Heliocentric Model of the Solar System? qIn the heliocentric model, due to the motion of the Earth about the sun, the motion of the nearest stars should appear to vary with respect to the more distant stars. qThis effect is called parallax. qThe ancients attempted to measure this effect, but failed. In fact, because the stars are so distant, it is only detectable with telescopic measurements.

64. The Heliocentric World View  Niklaus Copernicus was a 16th century scholar and cleric, who wrote treatises in a number of fields.  He is best remembered today for his revolutionary astronomical ideas. Niklaus Copernicus (1473-1543)

65. The Copernican Model qCopernicus summarized his model by the following bold (and remarkably valid) assumptions :  1.There is no one center of all the celestial circles or spheres.  2.The center of the earth is not the center of the universe, but only of gravity and of the lunar sphere.  3.All the spheres revolve about the sun as their mid-point, and therefore the sun is the center of the universe.  4….the distance from the earth to the sun is imperceptible in comparison with the height of the firmament.  5.Whatever motion appears in the firmament arises not from any motion of the firmament, but from the earth's motion.  6.What appear to us as motions of the sun arise not from its motion but from the motion of the earth and our sphere, with which we revolve about the sun like any other planet.  7.The apparent retrograde and direct motion of the planets arises not from their motion but from the earth's. The motion of the earth alone, therefore, suffices to explain so many apparent inequalities in the heavens.

66. Phases of Venus qIn 1610, Galileo used the telescope to observe the phases of Venus for the first time from the Earth. qThe phases only made sense if Venus orbited the Sun, not the Earth. qThis proved to be a “smoking gun” in favor of the heliocentric model.

67. Next Week qI) Planetary Motion q A) Tycho Brahe and Johannes Kepler q B) Kepler’s Laws qII) Physics of Motion q A) Galileo and the Physics of Kinematics q B) Newton and Newton’s Laws of Motion qII) Physics of Matter and Light