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Reasons for the Seasons

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Note that summer and winter have nothing to do with how close Earth is to the Sun! ... VERNAL, MARCH 21. AUTUMNAL, SEPTEMBER 21. Earth. NCP. To Sun at Summer Solstice ... – PowerPoint PPT presentation

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Title: Reasons for the Seasons


1
Reasons for the Seasons
2
  • True or False?
  • Seasons are caused by the variation in distance
    between the Sun and the Earth.
  • FALSE

3
  • Note that summer and winter have nothing to do
    with how close Earth is to the Sun!

4
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)
5
Apparent motion Earths orbit Seasons
  • Earths axis is tipped by 23o

6
Tilt of the earth on its axis.
  • The earth is tilted 23.5º from perpendicular to
    the plane of the ecliptic.

7
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

8
  • 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
9
(No Transcript)
10
The Earth is curved and this causes differing
angles of incidence.
11
Intensity of incoming solar radiation
(insolation) is related to angle of incidence.
Higher angles higher intensity.
12
SOLSTICES SUMMER, JUNE 21 WINTER, DECEMBER 21
Where are the overhead rays of the sun on these
days?
13
EQUINOXES VERNAL, MARCH 21 AUTUMNAL, SEPTEMBER 21
14
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
15
Annual Changes in Daylength
16
  • 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

17
  • 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.

18
www.glendale.edu/geo/reed_physical/
10120Powerpoint/c_seasons_and_time.ppt
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