Solar vs. Sidereal Day

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Solar vs. Sidereal Day
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An Earth Day

• Sidereal Day 23 hr 56 min 4 secMotion relative
  to background stars
• Mean Solar Day 24 hoursThe average time between
  meridian crossings of the Sun
• Apparent Solar Day variesThe actual time
  between the meridian crossings

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(No Transcript)
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Earth Orbits Sun
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The Ecliptic
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Tilt of Spin Axis
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Ecliptic Path of the Sun
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The Equinoxes and Solstices

• Tilt of the spin axis is related to the seasons
• Effect of tilt on climate is not instantaneous

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Ecliptic on the Celestial Sphere
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An Earth Year

• Based on the motion of the Sun on the sky or the
  motion of Earth around the Sun
• Julian Year 365.25 mean solar days. Used in
  calendars before 1582.
• Gregorian Year 365.2425 mean solar days. Used in
  calendars today.

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An Earth Year

• Sidereal Year 365.256363 mean solar days.
  Defined by the motion relative to stars.
• Anomalistic Year 365.259635 mean solar days.
  Defined as perihelion to perihelion.
• Tropical year 365.242190 mean solar days.
  Defined from equinox to equinox. Thus, related to
  the seasons.

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Sun Changes Declination
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The Arctic and Tropical Regions

• How are the arctic and tropical regions defined?
• Can you answer this without looking at your
  books?
• Have you ever been to these regions?

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Back to the North Pole
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Cold Arctic Regions
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Land of the Midnight Sun
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Back to the Equator
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Tropical Heat

• In the Tropics, Sun is always close to zenith
• In the Tropics, the sun reaches the Zenith at
  some point in the year
• Lots of heating due to sunlight