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Old 06-26-2008, 01:23 PM   #154
alkemical
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http://www.livescience.com/environme...-solstice.html

With 8 inches of hail falling in parts of Nebraska this week and Arizona reaching triple digit temperatures last week, it may seem rather arbitrary to call June 20 the first day of the summer this year, aka the summer solstice. But scientists really do have a reason.

It's all about Earth's cockeyed leanings and some celestial configurations that even the ancients understood.

Our planet is tilted 23.5 degrees on its spin axis. On June 20 this year (some years it's June 21), the North Pole is pointing toward the sun as much as is possible.

Imagine Earth as an apple sitting on one side of a table, with the stem being the North Pole. Tilt the apple 23.5 degrees so the stem points toward a candle (the sun) at the center of the table. That's summer for the top half of the apple. Now keep the stem pointing in the same direction but move the apple to the other side of the table: Now the stem points away from the candle, and it's winter on the top half of the fruit.

The setup at June solstice puts the sun as high in our sky as it can go, yielding the longest day of the year in the Northern Hemisphere.

Scientists put the exact moment of the solstice at 8:00 p.m. ET (keep in mind that the sun is always up somewhere, and the gods don't favor the Eastern time zone).

As long ago as the fourth century B.C., ancient peoples in the Americas understood enough of this that they could create giant calendars driven by sunlight. They built observatories of stone to mark the solstices and other times important for planting or harvesting crops. Shrines and even tombs were also designed with the sun in mind.

The sun comes up each day (except at the poles) because our planet rotates once on its axis every 24 hours or so. It is Earth's tilt, and our 365-day orbit around the sun, that explain much about how our world changes during the year.
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