Chap-2, Part-2: The Seasons of the Year

1-28-2002



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The Seasons

Seasonality = changing lengths of day and night; Sun's changing position above horizon

  • Sun's altitude = angle of Sun above the horizon at local, solar noon

  • Sun's declination = the latitude of the subsolar point

  • Day length, night length => Changing exposure to Sun and solar heating

 

Factors influencing seasonal change

Revolution = Annual, yearly path of Earth in its orbit around Sun

Rotation = Daily turning of Earth on its axis alternately exposing parts of Earth's surface to solar energy and not

  

Tilt of Earth's Axis and the Plane of the Ecliptic

Tilt of Earth's axis

  • Plane of the ecliptic -- A plane (sheet) encompassing Earth, its orbit, Sun

  • Tilt of Earth's Axis from the Plane of the Ecliptic = 66-1/2 degrees

  • Tilt from a perpendicular to the plane = 23-1/2 degrees

  • Polaris -- North Star -- Earth's North Pole points to Polaris
     

Parallelism of Earth's axis -- Earth's axis always points to the same place in space, no matter the season of the year

 

Annual march of the seasons

Circle of illumination

  • The circle surrounding Earth, demarking the middle of the twilight zone that separates the "day" and "night" sides of Earth

  • The twilight zone surrounds Earth as an area in which Sun is not visible but it is "light", not the darkness of true night
     

Day length at Equator

  • Always 12 hours each for "day" and "night"  (Realize that day and night do not turn off and on light a room lights controlled by a switch)

    • If you choose to be very "tight" with definitions, the above is not correct by a few minutes.  The atmosphere bends Sun's rays like a lens, making Sun visible for a few additional minutes per day, before it rises above the horizon and after it sets.

 

The Annual (Yearly) March of the Seasons

Know the dates, northern hemisphere names and the location of the subsolar point!

Winter solstice

  • December solstice -- Dec 20 or 21

  • Tropic of Capricorn -- 23.5 S Lat = location for subsolar point

    • Sun angle and intensity -- Vertical at noon -- Intense!

    • Length of day and night

      • Longest day and shortest night of year in Southern Hemisphere

      • Shortest day and longest night of year in Northern Hemisphere
         

  • Arctic Circle -- Sun does not rise for one day

  • Antarctic Circle -- Sun does not set for one day

Vernal/Spring Equinox

  • March Equinox -- 20 or 21

  • Equator -- 0 Lat = location for Subsolar Point

    • Sun angle and intensity -- Vertical at noon at Equator

    • Length of day and night  -- day and night equal over all of Earth
       

  • North Pole -- Sunlight just "grazes" the surface

  • South Pole -- Sunlight just "grazes" the surface

Summer solstice

  • June solstice -- 20 or 21

  • Tropic of Cancer -- 23.5 N Lat = Location of Subsolar Point

    • Sun angle and intensity = Vertical at Noon -- Intense!
       

  • Length of day and night

    • Nights longest in Northern Hemisphere; shortest in Southern Hemisphere
       

  • Arctic Circle -- Sun does not set for a full day

  • Antarctic Circle -- Sun does not rise for a full day

Autumnal/Fall Equinox 

  • September Equinox -- 22 or 23

  • Equator -- 0 Lat = Location of Subsolar Point

    • Sun angle and intensity -- Vertical at noon -- Very intense!
       

  • Length of day and night -- Day and night equal over all of Earth

  • North Pole -- Sunlight just "grazes" the surface

  • South Pole -- Sunlight just "grazes" the surface

 

Seasonal observations

Above is for a Northern Hemisphere location

Know the locations of the following for Summer, Spring-Fall, and Winter

  • Altitude of Sun

  • Location for sunrise and sunset

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