Question of the Day

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Question of the Day

• Friday, September 15, 2006
• If we know that the celestial sphere is so wrong,
  why do we continue to use it?

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Day-to-Day Changes
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Theres Not Just 1 Day

• There are 2 different types of days
• Solar Day
• Sidereal Day

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Solar Day

• The day we are used to
• The time it takes for the sun to be at its
  highest point in the sky to the next

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Sidereal Day

• The length of time for the stars to appear in the
  same position as the day before
• This day is about 3.9 minutes shorter than the
  solar day

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Why the Difference

• Because the Earth moves in 2 different ways
• It ROTATES on its axis
• It REVOLVES around the sun

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This Means

• As a result of the 2 movements of the Earth, it
  must rotate slightly more than 3600 before the
  sun will appear in the same position again

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Long-Term Changes
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2 Types of Years

• The Tropical Year
• The Sidereal Year

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Tropical Year

• The year that we use
• The time it takes to get from one vernal equinox
  to the next (365.242 solar days)

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Sidereal Year

• The time it takes for the Earth to complete
  exactly on orbit around the sun (365.256 solar
  days)
• 20 minutes longer than a tropical year

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Why is There a Difference?

• A slow shift in our orientation that makes it
  take just a little bit less time to get to the
  next vernal equinox
• It is called precession

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• It occurs because the Earth is spinning like a
  top
• The north pole is going to be pointing in a
  different direction as time goes on

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Precession Cycle

• A complete cycle takes 26,000 years
• So if we didnt use the tropical year, in 13,000
  years the summer solstice would take place in
  February

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• This is why the first day of seasons dont really
  move, but the star field will
• Orion will be in the summer sky in 13,000 years