The Tilt of the Earth and Seasons
Latitudes above the Tropic of Cancer and below the Tropic of Capricorn experience four seasons as the result of Earth's 23.5 degree tilt. In northern latitudes, summer begins on June 22 at the Summer Soltice, fall begins September 23 at the Autumnal Equinox, winter begins December 21 at the Winter Soltice, and spring begins March 21 at the Vernal Equinox. The seasons are reversed in the southern latitudes. Equinox means equal night and is an imaginary line where the Earth's ecliptic crosses the celestial ecliptic. The day is approximately the same length as the night at all points of the Earth on the equinox. The summer soltice is the longest day of the year and the winter soltice is the shortest day of the year.
As the result of the 23.5 degree tilt, during the Summer Soltice the North Pole experiences 24 hours of sunlight while the South Pole experiences 24 hours of darkness. This event is reverse during the Winter Soltice. The day length changes slightly each day as the Earth tilts. Between the Tropic of Cancer and Tropic of Capricorn, the day and night are nearly equal every day. At the equator, sunrise is at 6 A.M. and sunset is at 6 P.M. all year!
Starting in spring with the Earth's axis parallel to the Sun, the Sun is directly overhead at the Equator and slightly overhead between the Tropic of Cancer and Tropic of Capricorn. As the Earth revolves the axis tilts toward the Sun. The Tropic of Cancer is 23.5 degrees north of the Equator and the Sun will be overhead at noon on the first day of summer. The Earth continues it's revolution around the Sun until the axis is parallel to the Sun on the Autumnal Equinox. The Sun is directly overhead on the Tropic of Capricorn on the first day of winter, when the axis points away from the Sun. The Sun is only directly overhead in places between the Tropic of Cancer and the Tropic of Capricorn.
The amount of radiation that the Earth receives from the Sun depends on the degree of tilt. Although the distance between the Sun and Earth changes throughout the year as the Earth travels around the sun in an elliptical path, this has no effect on the seasons. Temperature changes are directly related to the degree of tilt and radiation received. This will be confirmed in Grade 8 science when you graph data for various locations around the Earth over a year's time.
As the Earth rotates and revoles it undergoes precession, like a top. This wobble changes the axis of Earth's rotation. Over time, the ecliptic intersection with the celestial ecliptic changes. This intersection determines the equinoxes. Today, the Vernal Equinox is located in the constellation Pisces. About 2000 years ago, the Vernal Equinox was located in Aries, the first sign of the Zodiac. Sometime around 2100, the Age of Aquarius will dawn with the Vernal Equinox shifting to Aquarius. The Zodiac constellations are the twelve that appear along the ecliptic.
Urban Legend: Folklore states that a raw egg can be made to stand on end on the first day of spring, the Vernal Equinox. Actually, if you can get the egg to stand on end at all, you can do it any day of the year. In fact if you spin a raw egg, it will stand on end all by itself. That's friction for you. There is nothing magical about the Vernal Equinox and the laws of gravity and balance.
Try the Enchanted Learning seasons page for an animated description of how the tilt of the Earth affects seasons.
For a more challenging discussion visit celestial sphere. There's a tremendous amount of information for the adventuresome learner.
