Tuesday, December 22, 2009

Winter Solstice *

At 12:47 pm today, December 21, 2009, winter begins in the Northern Hemisphere. The first day of winter is called the Winter Solstice; likewise, the first day of summer is the Summer Solstice. The word solstice is derived from Latin and means “sun standing still.” On these two days of the year, the sun’s apparent position in the sky has reached its most southern or northern extreme. Today there will only be 9 hours and 33 minutes of daylight in Richmond, but tomorrow the number of daylight hours will once again begin to increase.
The solstices and the seasons occur because the Earth’s axis is tilted 23.5° relative to its plane of revolution around the sun (see illustration). In the Northern Hemisphere winter, the northern half of the globe is tilted away from the sun; therefore, the sun is low in the sky giving us shorter days and longer nights. At this oblique angle, the sun’s energy is spread over a larger area of the Earth’s surface and thus is weaker than if it was hitting the surface more directly. Also, the sun’s rays must travel through more atmosphere before they reach the Earth’s surface, and some of the solar energy is reflected back into space. In addition, there are less daylight hours to warm the Earth. With all these factors combined, is it any wonder that we have winter weather?
Contrary to what many believe, the Earth is not farther from the sun during the winter. Actually, the Earth is almost at its closest point to the sun at the time of the northern hemisphere’s Winter Solstice. This variation in the Earth’s distance from the sun is small and does not greatly affect the weather; however it does slightly modify the severity of the Northern Hemisphere winters and summers.
While we are experiencing winter, the southern hemisphere is experiencing summer. Our Winter Solstice is the southern hemisphere’s Summer Solstice. If you lived in Rio or Sydney, the winter months would be June through August and the summer months would be December through February.
Here’s some food for thought: what if the Earth’s axis was not tilted? Would we have seasons? What if the axis was tilted more than 23.5°? Or less than 23.5°? What would our seasons be like? How did the Earth get its tilt? All the planets in our solar system have some axial tilt, but they are all different. Why?

*Technical difficulties delayed the posting of this blog entry.


Rich Conti said...

Is the earth's precession increasing or decreasing?

George Hastings said...

Actually, I believe it's neither increasing nor decreasing. The direction the Earth's North Pole is pointing traces out a cone, completing one cycle every 25,920 years. (Which is why over 2,000 years since astrologers designated the signs of the zodiac that the Sun appeared in front of over the course of a year, the present REAL signs of the zodiac to not match those cited in astrological forecasts)

George Hastings said...

Incidentally, in addition ignoring the gross mismatches between actual apparent position of the Sun in front of the zodiacal constellations, those who believe that astrological forecasts and predictions have any validity are actually espousing the theory that it doesn't matter if yo are born smart or stupid, industrious or lazy, to nurturing or neglectful parents, are wealthy or poor, but that the position of distant planets at the moment of birth is a significant determinant of your personality and who you turn out to be, just like everybody ELSE born at the same time! Actually, the doctor who delivered you, the nursing staff, and even the equipment in the room has a stronger gravitational influence at your moment of birth.
SO, the next time someone asks you, "What's your sign?" a more accurate response might be "the EXIT sign!"

George Hastings said...

one more "by-the-way".....I see that your posting clock is slow by exactly three hours!

Liz Mebane said...

I noticed that too. We will have to fix that.