Miscellaneous announcements

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Miscellaneous announcements

• Course web site is up to date with all handouts
  and Homework 1 solutions.
• Start planning your two observing projects.
• Questions on course policies?

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Motions of the Stars

• 30 August 2006

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Course Outline

• Naked-eye astronomy
• Crash course in physics
• Our solar system
• The stars
• Structure and history of the universe

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Course Outline
You are here

• Naked-eye astronomy
• Crash course in physics
• Our solar system
• The stars
• Structure and history of the universe

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Today

• How the stars appear to move through our sky each
  night (facing N, S, E, W)
• How this motion depends on your location on earth
  (and how to measure the earths size using the
  stars)
• A simple (but incorrect) model to explain this
  motion

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Star trails in east
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Star trails in west
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Star trails in south
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Star trails in north
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Summary (for mid-northern latitudes)

• Most stars rise in east, set in west
• Rising stars head toward the south, setting stars
  come from the south
• In southern sky, stars move left to right
• In northern sky, stars make CCW circles around
  the north celestial pole
• Star patterns dont change in size or shape

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To remember all these facts

• Pretend that the stars are all pasted on the
  inside of a giant rigid sphere, with us at the
  center.
• The sphere is spinning around, about once every
  24 hours.
• The rotation axis points through us along a
  diagonal, up and to the north.

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Star trails in north
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Star trails in north
19 degrees
75 minutes / 19 degrees 4 minutes per
degree
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Rate of motion

• A full circle is 360º
• Stars move 1º in approximately 4 minutes
• 15º in 60 minutes (one hour)
• 360º in 24 hours (approximately)

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More precisely

• Stars actually move about 361º in 24 hours
• 360º takes only 23 hours, 56 minutes
• Therefore eastern stars appear a bit higher each
  night western stars appear lower (at any given
  time of night)
• But in 360 days (actually 365.24), these 1º
  offsets will add up to a full circle

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For practice

• www.skyviewcafe.com (linked from course web page)
• Skychart III (free download)
• Low-tech planisphere (purchase or download and
  print)