The Sun

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The Suns Motion and The Four Seasons
http//www.history-of-rock.com/four_seasons.htm
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Annual Motion of SunThe Ecliptic

• Sun appears to move east to west each day due to
  Earths rotation.
• Superimposed on this is a yearly motion towards
  the East that is a reflection of Earths orbital
  motion.
• Signs of Zodiac.
• This path is called the Ecliptic.

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The Ecliptic

• The Earths rotation axis is tilted 23.5 degrees
  to its orbital plane.
• The rotation axis points to the same place in the
  sky (toward the star Polaris) no matter where the
  Earth is located in its orbit.

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The Ecliptic

• The tilt causes the Suns path in the sky to be
  at a 23.5 degree angle to the Earths equator.
• This 23.5 degree angle is the cause of our
  seasons.

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Thought Question
TRUE OR FALSE? Earth is closer to the Sun in
summer and farther from the Sun in
winter.
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Why doesnt distance matter?

• Variation of Earth-Sun distance is small about
  3 this small variation is overwhelmed by the
  effects of axis tilt.

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Cause of the Seasons
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Cause of the Seasons

• Direct and indirect lighting by the Sun.
• Number of hours daylight to produce heating is
  changed.

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Cause of the Seasons

• These two causes occur because of the 23.5
  degree angle and the Earths orbital motion.

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What causes the seasons?
Seasons depend on how Earths axis affects the
directness of sunlight
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Direct light causes more heating.
12
Axis tilt changes directness of sunlight during
the year.
13
We can recognize solstices and equinoxes by Suns
path across sky
Summer solstice Highest path, rise and set at
most extreme north of due east. Winter solstice
Lowest path, rise and set at most extreme south
of due east. Equinoxes Sun rises precisely due
east and sets precisely due west.
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Suns altitude also changes with seasons
Suns position at noon in summer higher
altitude means more direct sunlight.
Suns position at noon in winter lower altitude
means less direct sunlight.
15
Seasonal changes are more extreme at high
latitudes
Path of the Sun on the summer solstice at the
Arctic Circle
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Summary The Real Reason for Seasons

• Earths axis points in the same direction (to
  Polaris) all year round, so its orientation
  relative to the Sun changes as Earth orbits the
  Sun.
• Summer occurs in your hemisphere when sunlight
  hits it more directly winter occurs when the
  sunlight is less direct.
• AXIS TILT is the key to the seasons without it,
  we would not have seasons on Earth.

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First Day of Spring

• Spring or Vernal Equinox
• March 21st
• Sun directly overhead at Earths Equator
• 12 hours of daylight everyone on the Earth.

18
First Day of Summer

• Summer Solstice
• June 21st
• Sun is directly overhead at 23.5 degrees north,
  the Tropic of Cancer.
• North Pole has 24 hours of sunlight, Land of the
  Midnight Sun.
• Antarctica has 24 hours of darkness.

19
First Day of Autumn

• Autumnal Equinox
• September 21st
• Sun directly overhead at Earths Equator.
• 12 hours of daylight for everyone

20
First Day of Winter

• Winter Solstice
• December 21st
• Sun is directly overhead at 23.5 degrees south,
  at the Tropic of Capricorn.
• South pole has 24 hours of daylight.
• North pole has 24 hours of darkness.

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The Seasons as Viewed From North Pole
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Seasons and Weather

• In summer the Sun is overhead north of equator.
• Intertropical conversion is north of equator.
• Subtropical jet north of 30 degrees.
• Polar jet north of 60 degrees.
• In the summer the tropical jet stream drives our
  weather patterns.

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Seasons and Weather

• In winter the Sun is overhead south of equator.
• Intertropical conversion is south of equator.
• Subtropical jet south of 30 degrees and near
  equator.
• Polar jet south of 60 degrees and it may be south
  of Atlanta when the Siberian Express occurs.
• In the winter the polar (can be split into north
  south branches) and tropical jet streams drive
  our weather patterns.

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Resources
Ahrens, (1999). Meteorology Today (6th
Ed.) Allen, George (translation, 1964). The
Flammarion Book of Astronomy (Book
VIII.) Chaisson McMillan, (2004). Astronomy
Today (5th Ed.) Bennett et al. (2004) The
Cosmic Perspective (2nd Ed.) Shipman, Wilson,
and Todd, (2003). An Introduction to Physical
Science (10th Edition).