2.6 - Seasons
Instructors's Video Links to an external site.
The Reason for the Seasons
Seasons result from variations in the Sun's altitude (the angle of the Sun with respect to the horizon - how high the Sun is in the sky) and the Sun's declination (the latitude where the rays of the Sun are perpendicular or vertical) . These changes are created by the annual 
revolution of the Earth around the Sun, it's 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) that 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. Watch this video to help you visualize what is happening.
The Seasons Links to an external site.
| Winter Solstice | Vernal or Spring Equinox | Summer Solstice | Fall or Autumnal Equinox | |
|---|---|---|---|---|
| Northern Hemisphere | Dec 21 | 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 Northern Hemisphere | 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 Northern Hemisphere | Everywhere on Earth experiences a 12 hour day |
| Southern Hemisphere | June 21 | 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 Southern Hemisphere | 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 Northern Hemisphere | 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) and is nature's attempt to restore balance. Here at 38°N, the balance between incoming and outgoing solar radiation is about even.
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