Seasonal Cycle

1
Seasonal Cycle

• The Earths axis is tilted 23.5o to the vertical.
• This changes the
• angle of sun (higher in sky in summer)
• length of day (longer in summer)

2

• The tilt of Earths Axis defines
• Arctic Circle 66.5oN90o-23.5o
• Antarctic Circle 66. 5oS
• Tropic of Cancer 23.5oN
• Tropic of Capricorn 23.5oS

3
Earths Tilt Changes Over Time

• Obliquity refers to the tilt of the Earth's axis.
  
• The angle of Earth's tilt varies between 22.2 and
  24.5o (currently the tilt is 23.5o and
  decreasing).
• These variations are caused by the gravitational
  pull of large planets, including Jupiter.
• Earth's obliquity varies cyclically with a period
  of 41,000 years.

4
Life on Earth is lucky to have the Moon

• Moon causes lunar tides
• inland tidal pools may have concentrated first
  amino acids

• Moon stabilizes tilt of Earth's spin axis
• obliquity would vary from 0 to 85 degrees over 10
  million years otherwise, like other terrestrial
  planets
• stable seasonal changes

5
Eccentricity

• Earths orbit is not a perfect circle, but an
  ellipse.
• The eccentricity of Earths orbit is 0.017.
• The distance from Earth to the sun varies ?1.7
  from the mean over one orbit (one year).
• Earth is closest in Northern Hemisphere winter.

6
Effect of Eccentricity of Solar Flux

• Inverse square law says that the solar flux at
  Earth is inversely proportional to the square of
  the distance from Earth to the sun.
• Dminimum (1-0.017)Dmean 0.983Dmean
• Solar flux varies ?3.5 due eccentricity.

7
Eccentricity Changes Over Time

• Earth's eccentricity has varied over time between
  values of 0.005 and 0.0607 (currently 0.017).
• Eccentricity has a period of about 100,000 years.
• This changes solar flux at Earth ?0.2.

8
Earths Solar Constant

• The sun is a black body at 5800K.
• The flux at the suns surface is
• The average flux received at earth is
• This is Earths solar constant, Fs.