Winter Solstice
Sun., Dec. 21,
2008, 7:04 AM EST (12:04 UT), marks the solstice—the beginning of
winter in the Northern Hemisphere and summer in the Southern Hemisphere
by Ann-Marie Imbornoni
In astronomy, the solstice is either of the two times a year
when the Sun is at its greatest distance from the celestial equator, the
great circle on the celestial sphere that is on the same plane as the
earth's equator. In the Northern Hemisphere, the winter solstice occurs
either December 21 or 22, when the sun shines directly over the tropic of
Capricorn; the summer solstice occurs either June 20 or 21, when the sun
shines directly over the tropic of Cancer. In the Southern Hemisphere, the
winter and summer solstices are reversed.
Reason for the
Seasons
The reason for the different seasons at opposite times of
the year in the two hemispheres is that while the earth rotates about the
sun, it also spins on its axis, which is tilted some 23.5 degrees towards
the plane of its rotation. Because of this tilt, the Northern Hemisphere
receives less direct sunlight (creating winter) while the Southern
Hemisphere receives more direct sunlight (creating summer). As the Earth
continues its orbit the hemisphere that is angled closest to the sun changes
and the seasons are reversed.
Longest Night of the Year
The
winter solstice marks the shortest day and the longest night of the year.
The sun appears at its lowest point in the sky, and its noontime elevation
appears to be the same for several days before and after the solstice. Hence
the origin of the word solstice, which comes from Latin solstitium, from
sol, “sun” and -stitium, “a stoppage.” Following the
winter solstice, the days begin to grow longer and the nights shorter.
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