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Lecture 4 Astronomy 1 Instructor: Dr. Babar Ali

Lecture 4 Astronomy 1 Instructor: Dr. Babar Ali. Announcements. Please raise your hand if you are here to add the course. Announcements. Lectures and materials on LA mission college website. http://www.lamission.edu/~ alib. Announcements. Use my mission college e-mail alib@lamission.edu.

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Lecture 4 Astronomy 1 Instructor: Dr. Babar Ali

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  1. Lecture 4 Astronomy 1 Instructor: Dr. Babar Ali

  2. Announcements • Please raise your hand if you are here to add the course. Astro 1

  3. Announcements • Lectures and materials on LA mission college website. http://www.lamission.edu/~alib Astro 1

  4. Astro 1

  5. Astro 1

  6. Announcements • Use my mission college e-mail • alib@lamission.edu Astro 103

  7. Announcements • Quiz # 1 next week • Covers the first 4 lectures • In class, given at the end of the lecture • 20 minutes … 15-20 questions • Multiple choice format • Bring scantron sheets Astro 1

  8. Outline • Time on Earth • Observing the sky • Constellations Revisited • How the Greeks measured the size of Earth • Precession Astro 103

  9. Telling time. North Pole Noon Midnight 6 pm Side View

  10. Telling time. Noon Midnight 6 pm North Pole Top View

  11. Time on Earth 0 Earth’s rotation is causing the day/night cycle

  12. Review North Pole Where is Noon, Midnight, 6am and 6pm?

  13. 1-5: Astronomers use angles to denote the positions and apparent sizes of objects in the sky Astron 1

  14. Astron 1

  15. Astron 1

  16. Astron 1

  17. Estimating Angles with Your Hand Astron 1

  18. The Small Angle Formula Astron 1

  19. Earth’s Motion • Earth is also revolving around the Sun. • The time it takes to complete on revolution is called a year. • At any given time, only half the sky is visible at night. You can see the constellations on this side only. Astro 103

  20. Conversely • The sun appears in a constellation on the “other side” • Zodiac: Belt around sky, ~18° wide, centered on ecliptic, w/in which we find  & planets • Root of Zodiac same as Zoo, means collection of animals • Pattern of ’s w/in zodiac belt reminded ancients of animals Astro 103

  21. Eighty-eight constellations cover the entire celestial sphere Three Views of Orion Astron 1

  22. Constellations 0 In ancient times, constellations only referred to the brightest stars that appeared to form groups. Astron 1

  23. Constellations (2) 0 They were believed to represent great heroes and mythological figures. Each culture has its own set of constellations, usually pertaining to local beliefs. Astron 1

  24. Constellations (3) 0 Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions. Astron 1

  25. Constellations (4) 0 The stars of a constellation only appear to be close to one another. Usually, this is only a projection effect: The stars of a constellation may be located at very different distances from us. Astron 1

  26. Constellations (5) 0 Stars are named by a Greek letter (a, b, g) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuse = aOrionis Rigel = bOrionis Betelgeuse Rigel Astron 1

  27. Constellations (6) 0 Some examples of easily recognizable constellations and their brightest stars Astron 1

  28. Apparent Motion of The Celestial Sphere 0 Some constellations around the Celestial North Pole never set. These are called “circumpolar”. The circle on the celestial sphere containing the circumpolar constellations is called the “circumpolar circle”. Astron 1

  29. Eratosthenes (c. 276-194 B.C.) • If the Earth is round ... • How big is it? • In Syene (modern day Aswan, Egypt) the Sun casts no shadow at Summer Solstice. • In Alexandria, Egypt Sun is ~7o from Zenith. The angle a is 7 degrees Astron 1

  30. Astron 1

  31. Eratosthenes’ Findings • Distance between Alexandria and Syene ~ 5,000 stadia • Angular between Alexandria and Syene ~ 7 degrees • R ~ 40,000 stadia ….. probably within 15% • 7 5,000 • = • 360 2*p*R Circumference of a circle Astron 1

  32. Precession (1) 0 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.

  33. Precession (2) 0 As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. It 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.

  34. Review Topics • How are constellations used in modern astronomy? • How many constellations are recognized by modern astronomers? • Name three constellations. • What is the concept of ‘projection’? And, how does it relate to constellations? • What are circumpolar constellations? Astron 1

  35. Review Topics • Why do the constellations we see at night change throughout the year? • What is precession? What causes it? Astro 103

  36. Review For Quiz Astro 1

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