2.7 - The Seasons

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The Reason for the Seasons

Seasons result from variations in the Sun's altitude (the angle of the Sun with respect to the horizon) and the Sun's declination (the latitude of the perpendicular or vertical rays of the Sun) . These are created by the  annual revolution of the Earth around the Sun, its daily rotation around its axis and most importantly, the tilt of the Earth's axis. To understand the tilt of the Axis, you have to use your imagination. Imagine a plane (a flat surface) the runs through the middle of the Earth and the Sun. Instead of the Earth revolving around the Sun with a 90° angle to that plane, the Earth's axis is tilted 23.5°.  

During the Northern Hemisphere winter,  because of the Earth's axial tilt, the Northern  Hemisphere is tilted away from the Sun. During our Summer, the Northern Hemisphere is tilted toward the Sun. The opposite conditions exist in the Southern Hemisphere. On the Equinoxes, neither hemisphere has the advantage and the Sun is most intense at the Equator.

 

 

 

Seasons in the Northern and Southern Hemispheres

	Winter Solstice	Vernal or Spring Equinox	Summer Solstice	Fall or Autumnal Equinox
Northern Hemisphere	Dec 21st	March 21	June 21	Sept 21
Length of Day	24 hours of darkness 66.5°N (Arctic Circle ) to 90°N (N. Pole) Shortest day of the year in N. Hemp	Everywhere on Earth experiences a 12 hour day	24 hours of daylight 66.5°N (Arctic Circle ) to 90°N (N. Pole) Longest day of the year in N. Hemp	Everywhere on Earth experiences a 12 hour day
Southern Hemisphere	June 21st	Sept 21	Dec 21	March 21
Length of Day	24 hours of darkness 66.5°S (Antarctic Circle ) to 90°S (S. Pole) Shortest day of the year in S. Hemp	Everywhere on Earth experiences a 12 hour day	24 hours of daylight 66.5°S (Antarctic Circle ) to 90°S (S. Pole) Longest day of the year in N. Hemp	Everywhere on Earth experience a 12 hour day

 

The Earth's Seasons are the first building block for weather on Earth.  Only latitudes between 23.5°N and 23.5°S ever receive the direct rays of the Sun. This is part of a complex system that creates an imbalance in the solar energy received at different latitudes. The Tropics have a surplus of energy. The Poles have a deficit. Much of weather is driven by this imbalance (the winds, ocean currents etc). Here at 38°N, the balance between incoming and outgoing solar radiation is about even.