Astro Lecture #2: the Celestial Sphere (The Geocentric Model of the Universe)

Astro Lecture #2: the Celestial Sphere (The Geocentric Model of the Universe)

Astro Lecture #2: The Celestial Sphere (The Geocentric Model of the Universe)

  1. Introduction
  2. Problems?
  3. Connect Problems?
  4. HW is due on ______
  5. The Night Sky (Unit5)
  6. Intro
  7. Does anyone know the difference between a constellation and an asterism?
  8. Constellation is an officially designated area of the sky with well-defined boundaries in the celestial coordinates of Right Ascension and Declination. There are 88 official constellations.
  9. Important point: Stars follow their lines of declination
  10. An asterism is an easily recognized pattern of stars. e.g. The Big Dipper, Orion’s Belt, The Summer Triangle. There are hundreds, if not thousands, of asterisms.
  11. What function do you think constellations serve today?
  12. Help astronomers and observers find objects in the sky.
  13. Horoscopes
  14. Story telling
  15. What function do you think constellations served our ancient ancestors in pre-literate societies?
  16. Knowledge in pre-literate societies was passed on by oral transmission.
  17. They allowed stories to be imprinted upon them. Stories of origins, moral values, practical time-keeping or calendric purposes.
  18. The stars were a “book” that could never be lost, before books even existed.
  19. It should be of little surprise that stars still hold a spiritual attraction today since they were an essential part in the development of western civilization.
  20. The Celestial Sphere
  21. The stars appear to be sprinkled on a giant sphere that surrounds the Earth. We can only see a portion of the sphere of stars at night due to the solid Earth hiding the lower half of this celestial sphere.
  22. Stars are so far away, that only very sensitive measuring instruments can distinguish between near and far stars. To the naked eye all, stars appear equally far away.
  23. Use the UNL Rotating Sky to illustrate the celestial sphere.
  24. Even though the concept of the Celestial Sphere is not physically true, it is still a very useful conceptual tool in understanding the night sky. Read Kuhn – yellow book mark.
  25. Daily Apparent Motion
  26. Illustrate with the movie Stars over Greece from APOD
  27. Further illustrate with Starry Night
  28. Some simple rules
  29. The cause of the daily apparent motion of the stars is the rotation of the Earth eastward on its axis once every 23h 56m 4.09s (Demonstrate this with Starry Night) This time period is called the Sidereal Day (Unit 7)
  30. Two points on the sky do not appear to move. The NCP (which is very close to Polaris, the North Star) that is always above the horizon as seen from northern latitudes and the SCP which always below the horizon as seen by northern observers. The celestial poles are located directly above the Earth’s north and south geographic poles.
  31. Halfway between the celestial poles lies the Celestial Equator that intersects the horizon due (exactly) East and West for all observers. Stars near or on the Celestial Equator spend equal amount of time above and below the horizon (about 12 hours each).
  32. The altitude of Polaris (NCP really) equals the observer’s latitude. (See proof on Hot Tips)
  33. The details of the apparent motion of the stars depend on your latitude on the Earth. See Figure 5.7
  34. For Northern Hemisphere observers:
  35. Looking North, stars appear to move in counterclockwise circles around Polaris (NCP really). Some stars never set because they are so close to Polaris their daily circles never take them below the horizon (a.k.a. Circumpolar stars). Northern stars are above the horizon longer than they are below the horizon.
  36. Looking East (or West), stars appear to rise (or Set) along a slanted path toward (away from ) the South whose angle from the vertical equals the observers latitude.
  37. Looking South, stars appear to make shallow clockwise downward-curving arcs about the hidden SCP. Some stars never appear above the horizon because they are too close to the SCP for their daily circles to carry them above the horizon. Southern stars are above the horizon less than they are below the horizon.
  38. See Figure 5.5
  39. Latitude and Longitude
  40. The latitude of an observer determines what fraction of the entire celestial sphere he/she can see. Demo with the Rotating Sky modules.
  41. At the North Pole, the observer sees only ½ of all the celestial sphere. Same for the South Pole.
  42. At the Equator, the observer can see the entire celestial sphere in one single night – if we lived on a perfectly spherical smooth Earth with no atmosphere.
  43. The longitude of an observer only effects when (i.e. the time of day) an object is seen.
  44. Since the Earth turns at 15/hour, a star that passes through the zenith at one location, will pass through the zenith at another location, at the same latitude, after a time period proportional to the difference in longitude between the two locations. See class exercise.
  45. Summary
  46. The Review Questions at the end of each unit will be a source of exam questions, as well as some of the Quantitative Problems.
  47. Please write and answer the Review Questions out in your notes. The answers can be found in the reading of the Unit or you can search the web – be careful though on the web – no all sources are reliable. Wikipedia is pretty good for astronomy
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