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Seasons

Autumn to winter, winter into spring, Spring into summer, summer into fall,-- So rolls the changing year, and so we change; Motion so swift, we know not that we move.

Dinah Maria Mulock

 

Vocabulary to Know:

Rotation - the spinning of the Earth on its axis; one rotation takes twenty-four hours

Revolution - the orbit of the Earth around the sun; this takes 365 1/4 days to complete one cycle or year

Orbit - path that an Earth makes around the Sun

Tilt - the Earth is tipped 23.5 degrees

Axis - an imaginary line that goes through both of the planet's poles. The Earth rotates around this line.

Summer solstice - Approximately June 22 in the Northern Hemisphere the

• Sun is highest in the sky
• Directly overhead at latitude 23.5º N, the Tropic of Cancer.
• The Earth's axis is tilted most towards the sun
• The longest day (most hours of daylight
• The beginning of summer

Winter Solstice - Approximately December 22 in the Northern Hemisphere

• Sun is lowest in the sky
• Directly overhead at latitude 23.5ºS, the Tropic of Capricorn
• The Earth's axis is tilted most away from the sun
• The shortest day (fewest hours of daylight)
• The beginning of winter

Vernal Equinox - Approximately March 20 in the Northern Hemisphere

• Sun passes directly over the equator
• The Earth's axis is neither tilted toward nor away from the Sun
• The day is the same length (12 hours ) everywhere
• The beginning of spring

Autumnal Equinox - Approximate;y Septemebr 23 in the Northern Hemisphere

• Sun passes directly over the equator
• The Earth's axis is neither tilted toward nor away from the Sun
• The day is the same length (12 hours ) everywhere
• The beginning of fall

Seasons are caused by the tilt of the Earth's axis

• The Earth rotates around the axis every day
• Axis is tipped 23.5 degrees
• In summer, the northern hemisphere is tipped toward the sun.
  • The sun's rays beat down upon the earth from almost straight overhead. 
  • Direct rays or direct energy from the Sun
• In winter, the northern hemisphere is tipped away from the sun
  • The sun's rays beat down upon the earth from an angle (slanted)

• Indirect rays or energy from the Sun
  
  
  
  

The tilt of the earth's axis causes the number of daylight hours to vary during the year

• Longer days - more hours of sunlight in the summer
• Shorter days - less hours of sunlight in the winter

As the Earth revolves around the Sun, the part of the earth that is tilted toward the Sun changes

The angle of the tilt does not change

The amount of sunlight the earth receives is called insolation

• Two factors cause insolation to change.
  • Length of day (because more hours of daylight mean more insolation)
  • The angle that sunlight strikes the earth's surface

 

In Summer, the northern hemisphere is pointed toward the Sun, and the sun's rays are more direct and concentrated (more sunlight energy)

On June 21, the summer solstice, the top of the axis is tilted directly toward the sun. Areas north of the equator (the northern hemisphere) have longer days and shorter nights.

 

 

In Winter, the northern hemisphere tilts away from the Sun.  It spends more time in darkness (has longer nights) and the sun's rays are not as direct (less sunlight energy).

On December 21, the winter solstice, the top of earth's axis is tilted directly away from the sun. Areas north of the equator (the northern hemisphere) have shorter days and longer nights.

 

 

In Spring and Fall, the days are nearly equal in length for both the Northern and Southern Hemispheres.

Halfway in between the summer and winter solstices are the equinoxes. At these times the earth's axis is pointing neither toward nor away from the sun. On both equinoxes, all locations on earth receive exactly 12 hours of daylight and 12 hours of night.

Sunlight energy is spread evenly on the Earth.

  Spring

 

Fall

 

   

There is a common misconception that the seasons on the Earth are caused by varying distances of the Earth from the Sun on its orbit. Some people believe that we have summer because we are closer to the Sun during that time of year. This is not correct.

Think about it: when it is summer in the Northern hemisphere it is winter in the Southern hemisphere.

The rotation axis of the Earth remains pointed in the same direction as it moves around its orbit.

 

 

Original Graphics Sources

http://www.uwm.edu/~kahl/CoVis/Seasons/

http://www.morehead.unc.edu/Shows/EMS/image%20pages/seasons%20explained.htm

http://liftoff.msfc.nasa.gov/News/2001/News-AutumnalEquinox.asp