Chapter 13: Atmosphere and Hydrosphere

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Chapter 13 Atmosphere and Hydrosphere

• Alyssa Jean-Mary

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The Seasons The Orbit of the Earth

• Between the earth and the sun, the average
  distance is about 150 million km.
• Because the orbit of the earth around the sun is
  an ellipse instead of a circle, the distance
  between the earth and the sun actually varies
  during the year. It varies from about 2.4 million
  km closer to the sun than the average to about
  2.4 million km farther away from the sun than the
  average.
• In early January, the earth is nearest to the
  sun.
• In early July, the earth is farthest away from
  the sun.
• The seasons are not due to the shape of the orbit
  of the earth because the seasons are reversed in
  the northern hemisphere as compared to the
  southern hemisphere. Actually, the intensity of
  the sunlight that reaches the earth varies by too
  little between the two orbital extremes to give
  rise to the difference between summer and winter.
  This can be seen in that the orbit of the earth
  only differs from a perfect circle by 1.6.

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The Seasons

• The seasons are due to the 23.5 tilt of the axis
  of the earth. Because of this tilt, one
  hemisphere receives more direct sunlight than the
  other hemisphere for half of the year and
  receives less direct sunlight than the other
  hemisphere for the other half of the year. If a
  light beam arrives at an angle to the surface of
  the earth, it delivers less energy per m2 than if
  it arrives perpendicularly.
• In the northern hemisphere, at noon on about June
  22, the sun is at its highest in the sky. This is
  when the north pole is tilted most toward the
  sun. Thus, the period of daylight in the northern
  hemisphere is the longest on this day. Also, in
  the northern hemisphere, at noon on about
  December 22, about 6 months later, the sun is at
  its lowest in the sky. This is when the north
  pole is tilted the most away from the sun. Thus,
  the period of daylight in the northern hemisphere
  is the shortest on this day. These two times are
  called solstices. In the southern hemisphere, the
  situation is exactly reversed.
• At noon on about March 21 and September 23, the
  sun is directly overhead at the equator. Thus,
  the periods of daylight and darkness are equal
  everywhere on the earth. These two times are
  called the equinoxes.