Tuesday, December 20, 2011

The Seasons

The Seasons

Seasonal weather patterns are shaped primarily by the 23.5-degree tilt of our planet's spin axis, not by Earth's elliptical orbit (see diagram attached here). During the Northern Hemisphere Winter the north pole is tilted away from the Sun. Seasons are reversed in the Southern Hemisphere. When the north pole is tilted away from the Sun, as it is now (December), the south pole is tilted toward it. Daylight is at a minimum at the Winter Solstice and is at a maximum on the Summer Solstice. It is interesting to note that the daylight begins to increase on the 2nd day of Winter (the day after the Winter Solstice), i.e. daylight increases during the entire Winter season all the way up to the Summer Solstice

Some people think that our seasons are caused by a variation in the distance of the earth from the sun during the year. However, the earth at its nearest point on the ellipse that is the earth's orbit around the sun, the earth is 91,445,000 miles (147,166,462 km) from the sun. This point in the earth's orbit is known as "perihelion" and it occurs on January 3. The earth is farthest away from the sun on July 4 when it is 94,555,000 miles (152,171,522 km) from the sun. This point in the earth's orbit is called "aphelion". So ironically during the Northern Hemisphere Winter, the earth is 3 million miles closer to the sun than during the Northern Hemisphere Summer.

Dates for the start of the next 5 seasons:

December 22, 12:30 A.M. EST (December 21, 11:30 PM, CST)

March 20, 1:14 A.M. EDT

June 20, 7:09 P.M. EDT

September 22, 10:49 A.M. EDT

December 21, 6:12 A.M. EST

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