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Darkness creeps in on little cat feet

Thursday, December 22, 2011

Darkness comes like Carl Sandburg's fog, "on little cat feet."

With the sun slowly creeping south each day, the length of the darkness increases until suddenly we notice that at 4:30 in the afternoon the sun is gone from the sky and it is getting dark.

No wonder the ancients used the occasion of the winter solstice -- the day the sun starts back north -- as an occasion of celebration. Usually a celebration of light.

The sun reaches the Tropic of Capricorn, an imaginary line around the Earth, located at 23 and one-half degrees south of the equator. That is as far south as the sun appears to moves in its circuit each year.

Earth's axis is tilted 23 and one-half degrees thus leaning one hemisphere toward the sun for six months and leaning the other away. This "away" part is what we are beginning now.

When asked why it is cold in winter I have seen responses that range from "The Earth is located at its farthest from the sun at that time," to "Well, it is darker longer so we don't have as much sun," to one actual comment, "I think it is because the moon gets in the way and blocks some of the sunlight."

Actually, during our winter, Earth is at its closest point to the Sun in its orbit.

It all has to do with that pesky 23 and one-half degree tilt. As we in the north are tilted away from the Sun we get less direct sunlight, hence less warmth, and yes, the folks down south get more direct, warmer, sunlight.

That is why I like to see photographs of people in Australia celebrating Christmas Day at the beach with a picnic.

The Winter Solstice, the time the sun is over the Tropic of Capricorn, occurred at 11:30 p.m. CST yesterday, Dec. 21. From this time on the days will begin to lengthen and the sun will rise earlier and set later.

Some call the night of the solstice the "longest night of the year." While it is a long night, technically the amount of darkness is pretty much equal for each night from Dec. 20 to December 25 at about 15 hours and 20 minutes each day.

And there is nothing we astronomers love more than lots of dark. (If proper journalistic procedure allowed emoticons there would be a smiley face right here).

Now, for my Christmas favorite. On Christmas Eve, go outside at about 6:30 p.m. MST and look the northwest for one of our old friends from summer, Cygnus, the Northern Cross. At this time of year it is standing almost upright with Deneb, the tail star on top.

Next go outside at about 10:30 p.m. and look east for a very dim constellation, Cancer, the Crab. It is located below Gemini, the Twins, and above Leo, the Lion. Using binoculars locate the nice little star cluster M-44 very near the center of the constellation. M-44 is called the Beehive Cluster, but has another name, Praesepe, The Manger.

I find it an interesting occasion when a manger, the place where the infant Jesus was placed, is rising in the east when the place of his end, a cross, is setting in the west at the same time. Merry Christmas to you all.

SKY WATCH:

New moon, Dec. 24. Mercury is in the morning sky about a half hour after sunrise. Further up and right is Saturn located near the star Spica, and further up and right is the planet Mars just left of bright Regulus in Leo.

These planets all mark the location of the ecliptic, the path the Sun seems to follow across the sky. In the evening after sunset very bright Venus is shining away in the west. In the east bright Jupiter. Keep watching this pair as they draw closer together in anticipation of a spectacular conjunction in March of next year.

NEXT WEEK:

What you can look at with that new telescope you got for Christmas and more astronomical blathering.


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Vernon Whetstone
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