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The sky lrg

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The sky lrg

  1. 1. © Grunt Productions 2000 The Sky A brief by Lance GrindleyA brief by Lance Grindley
  2. 2. © Grunt Productions 2000 Horizon and Zenith • It is sometimes useful to think of the sky as a great dome over our heads. – The horizon is where the dome meets the Earth. – The zenith is the point directly overhead. – As the Earth turns, this dome turns over our heads. It appears as if the sky is a large hollow sphere centered on the Earth.
  3. 3. © Grunt Productions 2000 Horizon and Zenith
  4. 4. © Grunt Productions 2000 Altitude and Azimuth • The height of a star above the horizon is called the altitude. • The direction to the star as measured from true north is called the azimuth. – Note: True north is not the same as magnetic north. The magnetic north pole is not located in the same place as the true north pole. • On maps, the legend will show you how to correct from magnetic north, as measured by a compass, to true north.
  5. 5. © Grunt Productions 2000
  6. 6. © Grunt Productions 2000 Altitude and Azimuth
  7. 7. © Grunt Productions 2000 Magnetic North Pole
  8. 8. © Grunt Productions 2000 Magnetic North
  9. 9. © Grunt Productions 2000 Correction from Magnetic North to True North
  10. 10. © Grunt Productions 2000 Altitude and Azimuth • The altitude and azimuth of a star change during the course of night as the star rises and sets. • Angles are measured using degrees, minutes of arc, and seconds of arc.
  11. 11. © Grunt Productions 2000 Measuring Angles in the Sky
  12. 12. © Grunt Productions 2000 Measuring Angles • Angles in astronomy are measured in degrees, arc minutes and arc seconds. – 1 degree = 60 arc minutes – 1 arc minute = 60 arc seconds – 1 degree = 3600 arc seconds – 1 arc second is the size of a 50p as seen from 5 km (3 miles) or a penny as seen from 2.2 miles.
  13. 13. © Grunt Productions 2000 Measuring Angles – The Sun and Moon appear to be about ½ degree in size. – Your finger held at arms length is about one degree across. – Your fist at arms length is about 10 degrees. – Your outstretched hand at arms length is about 20 degrees across.
  14. 14. © Grunt Productions 2000 Angular Sizes and Distances
  15. 15. © Grunt Productions 2000
  16. 16. © Grunt Productions 2000 The Celestial Sphere
  17. 17. © Grunt Productions 2000 The Celestial Sphere
  18. 18. © Grunt Productions 2000 The Celestial Sphere • North Celestial Pole: The point directly over the Earth’s true north pole. – The north star, Polaris, is near the North Celestial Pole, but not exactly at the pole. It is currently about 1 degree away from the pole. • South Celestial Pole: The point directly over the Earth’s true south pole. • Celestial Equator: The equator of the Earth projected onto the celestial sphere. • Meridian: A line from due north to due south that passes straight overhead.
  19. 19. © Grunt Productions 2000
  20. 20. © Grunt Productions 2000 Longitude and Latitude
  21. 21. © Grunt Productions 2000 Longitude and Latitude • Latitude: Your north-south position on Earth. The equator is defined to have a latitude of 0 o . The north pole is at 90 o N and the south pole at 90 o S. • Longitude: Your east-west position on Earth. An arbitrary point, the Prime Meridian in Greenwich, England marks a longitude of 0 o .
  22. 22. © Grunt Productions 2000 Rotation of the Earth
  23. 23. © Grunt Productions 2000 The stars move from east to west because the earth rotates from west to east.
  24. 24. © Grunt Productions 2000
  25. 25. © Grunt Productions 2000 The Motion of the Stars • Just like the Sun and Moon the stars rise and set due to the rotation of the Earth. – They rise in the east and set in the west because Earth rotates from west to east. – Stars near the celestial poles do not rise or set. Instead they circle the poles and are called circumpolar. • In the northern hemisphere, the stars circle the pole in a counterclockwise direction.
  26. 26. © Grunt Productions 2000 Daily (Diurnal) Motion of the Stars
  27. 27. © Grunt Productions 2000 Circumpolar Stars
  28. 28. © Grunt Productions 2000
  29. 29. © Grunt Productions 2000 Diurnal Paths of Stars
  30. 30. © Grunt Productions 2000 Diurnal Paths of Stars at Intermediate Latitude
  31. 31. © Grunt Productions 2000
  32. 32. © Grunt Productions 2000 Celestial Poles
  33. 33. © Grunt Productions 2000 North Celestial Pole • In the northern hemisphere, the altitude of the north celestial pole is equal to your latitude on Earth. • This is useful for navigation. – If you measure the altitude of the north celestial pole, you can determine your latitude on Earth. • In the southern hemisphere, it is difficult, but not impossible, to find the location of the south celestial pole.
  34. 34. © Grunt Productions 2000 Celestial Sphere • To find due north, drop straight down from the North Celestial Pole to the horizon. • The celestial equator meets the horizon at due east and due west.
  35. 35. © Grunt Productions 2000
  36. 36. © Grunt Productions 2000 Diurnal Paths of Stars
  37. 37. © Grunt Productions 2000 Celestial Sphere Movie http://brahms.phy.vanderbilt.edu/~rknop/astromovies/
  38. 38. © Grunt Productions 2000 Celestial Coordinates • Just as Earth has lines of longitude and latitude, the celestial sphere has a system of celestial coordinates: • Declination (dec): The north-south position of a star on the celestial sphere. Declination is measured in degrees, arc minutes, and arc seconds. The celestial equator is defined to have a declination of 0 o . • Right Ascension (RA): The east-west coordinates of an object on the celestial sphere. R.A. is measured in hours, minutes, and seconds of time. The vernal equinox is defined to be 0 h .
  39. 39. © Grunt Productions 2000 Celestial Coordinates
  40. 40. © Grunt Productions 2000 Celestial Coordinates Vega RA 18h 35m DEC +38o 44’
  41. 41. © Grunt Productions 2000 Celestial Coordinates
  42. 42. © Grunt Productions 2000
  43. 43. © Grunt Productions 2000 Celestial Coordinates
  44. 44. © Grunt Productions 2000 Celestial Coordinates
  45. 45. © Grunt Productions 2000 Exercise • Find Orion in the night sky. Is it on the equator, the ecliptic, both, or neither. • How long is Orion above the horizon each day? • Find Sagittarius. Is it north or south of the equator? • How long is Sagittarius above the horizon each day?
  46. 46. © Grunt Productions 2000

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