Instructors's Video

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

Diagram showing Earth tilt seaonally

**Seasons in the Northern and Southern Hemispheres**
	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

diagram showing imbalance of solar radiation by latitude

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.

Just for Fun

Humans have a long history of celebrating the solstices but probably not in quite the same way that they do today....