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Happy Spring (or Autumn, Depending on Where You Are)

Tuesday, March 20, 2007, 22:46 EDT Leave a comment Go to comments

Here in the northern hemisphere, astronomical spring officially begins today. This is as opposed to meteorological spring, which I learned from my father the meteorology buff actually began a few weeks ago.

Meteorological seasons have to do with the weather, as the name implies. Most of us probably think of the seasons in these terms. Meteorologically, spring is March, April, and May. Summer is June, July, and August. Autumn is September, October and November. Winter is December, January, and February. As Dan Robinson at Storm Highway writes:

While ‘official’ spring is based on the equinox, or the point at which the earth’s tilt is ‘halfway’ between solstices, ‘meteorological spring’ (which begins on March 1) marks the climatological start of the transition from winter to summer. Meteorological spring is when temperatures begin to warm, precipitation begins to increase and storms and severe weather begin to ramp up.

Astronomical seasons, on the other hand, are determined by the solstices and equinoxes. Spring begins at the vernal equinox, the day when the sun is over the equator at noon and the period from sunrise to sundown is exactly 12 hours all over the world. That happened today. Likewise, autumn begins at the autumnal equinox, the similar astronomical event in September. Summer and winter begin at the solstices, the periods when the sun at noon is over either the tropic of Cancer (in the northern hemisphere) or the tropic of Capricorn (in the southern hemisphere). The summer solstice has the greatest time from sunrise to sunset, whereas the winter solstice has the least. It all has to do with the fact that the earth’s axis is not perpendicular to its orbital plane around the sun, so depending on where in its orbit the planet is, its axis tilts toward or away from the sun.

The equator and the tropics of Cancer and Capricorn aren’t the only lines of longitude that are astronomically significant. The Arctic and Antarctic circles have some pretty funky properties at the solstices. Again, because of the tilt of the earth’s axis relative to the sun, the sun sets to but not below the horizon on the Arctic circle on the day of the summer solstice. On the day of the winter solstice, it rises to but not above the horizon. Ditto at the Antarctic circle, six months offset. Personally, I could handle the 24-hour day, but the long winter night would drive me nuts.

That concludes today’s peek inside The Den Mother’s Almanac. Enjoy your spring, everyone.

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Categories: weather & climate
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