Reasons for the Seasons

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Reasons for the Seasons
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• True or False?
• Seasons are caused by the variation in distance
  between the Sun and the Earth.
• FALSE

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• Note that summer and winter have nothing to do
  with how close Earth is to the Sun!

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The reasons for seasons

• We have seasons on Earth

because of the obliquity of the ecliptic
(specifically, the tilt of the spin axis to the
ecliptic)
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Apparent motion Earths orbit Seasons

• Earths axis is tipped by 23o

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Tilt of the earth on its axis.

• The earth is tilted 23.5º from perpendicular to
  the plane of the ecliptic.

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Apparent motion Earths orbit Seasons

• When your hemisphere is tipped towards the Sun,
  it is summer for you
• When your hemisphere is tipped away from the Sun,
  it is winter for you

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• The tilt of the Earths spin axis produces a
  geometrical heating effect
• Sun near Summer Solstice position
• long days with Sun high in the sky
• hence warm days
• Sun near Winter Solstice position
• short days with Sun low in the sky
• hence cold days

Note this is a northern hemisphere view point of
the seasons
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(No Transcript)
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The Earth is curved and this causes differing
angles of incidence.
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Intensity of incoming solar radiation
(insolation) is related to angle of incidence.
Higher angles higher intensity.
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SOLSTICES SUMMER, JUNE 21 WINTER, DECEMBER 21
Where are the overhead rays of the sun on these
days?
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EQUINOXES VERNAL, MARCH 21 AUTUMNAL, SEPTEMBER 21
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NCP
To Sun at Summer Solstice
Spin axis
T of Can.
23.5o
To Sun at Equinoxes (celestial equator)
23.5o
T of Cap.
Earth
To Sun at Winter Solstice
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Annual Changes in Daylength
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• Important points to understand
• Seasons are NOT due to variation in the Earth-Sun
  distance.
• We have seasons because Earths spin axis is
  inclined to the plane of Earths orbit around the
  Sun.
• This tilt (or obliquity) causes variation in the
  amount of daily sunshine any particular location
  receives throughout the course of a year.
• More sunshine more energy warmer time of year
  
• Less sunshine less energy cooler time of year
  

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• Seasons are reversed in northern and southern
  hemispheres.
• Regions near the pole can experience midnight
  sun near summer solstice days without sunrise
  near winter solstice.
• At intermediate latitudes (like ours), the Sun
  climbs higher in the sky in summer, lower in
  winter.

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www.glendale.edu/geo/reed_physical/
10120Powerpoint/c_seasons_and_time.ppt