Thursday, December 20, 2007

The Sun Stands Still

Here in the Northern Hemisphere, Saturday, December 22 marks the 2007 winter solstice, a time of revelry for vampires, werewolves, and amateur astronomers. Why this motley party crowd, you ask? Because the winter solstice offers something irresistible to denizens of the dark: the longest night of the year, not to mention the shortest day.

Speaking of revelry, the marking of the winter solstice has been ritualized by many cultures throughout history, and some practices persist into modern times. A number of ceremonies involve the lighting of fires and candles to encourage--and then celebrate--the return of the Sun after that longest night.

A Scandinavian/European solstice tradition
Image from
The Book of Days

The longest night/shortest day phenomenon occurs because the Sun is at its lowest--its most southern--position in our sky. You can see from the following diagram that the Sun's low arc (shown in blue) shortens the distance it has to travel across the sky, thereby shortening the day length.

Diagram by the YPOP Solar Classroom

The winter solstice marks the official first day of winter in the Northern Hemisphere, after which the Sun begins to arc ever higher in the sky and the days grow ever longer. The word solstice, derived from Latin, means “sun stands still,” a reference to the halt of the Sun’s movement southward. By the way, the apparent movement of the Sun north and south in our sky over the course of a year is caused by the tilt of the Earth’s axis; I will cover this in depth in a future post.

Lest you think that contemporary folk don’t take their solstice rituals seriously, I call your attention to Newgrange in Ireland. Built around 3200 BCE, the mound-like stone structure was apparently oriented so as to be a winter solstice marker. For five days surrounding the solstice, sunlight enters a slot in the roof and fully illuminates an inner chamber for 17 minutes. Just ten visitors per day may witness the lighting of the chamber, and admission is obtained solely by lottery. Over 28,000 applications were received for the 2007 lottery! Only 50 lucky people will experience this unique event, provided the weather in Ireland cooperates.

Aerial image of Newgrange from knowth.com

Here’s some exciting news for those of you who didn’t win the lottery. Because it is the 40th anniversary of the discovery of the solstice phenomenon at Newgrange, the illumination of the chamber will be broadcast for the first time this year via webcast and satellite transmission, on both Friday, December 21 and Saturday the 22nd. Of course, the harsh reality of the time difference is that the event will occur in the wee hours of the morning for those of us in North America. No guts, no glory, so lay in a store of your favorite high-test java.

Here’s another way to celebrate the winter solstice, in your own time zone. You can participate in the 2007 Cassini Favorite Image Contest by voting for your favorite images of Saturn and its moons. The voting deadline is December 30, and you’ll have a chance to win a poster of the top color image.

Saturn image by CICLOPS

But what, you ask, does that have to do with the winter solstice? Well, the winter solstice festival in ancient Rome was the rather raucous Saturnalia, dedicated to the god of agriculture, none other than, yup you guessed it, Saturn. Historians believe that the Catholic Church selected December 25 as the date of the birth of Jesus in order to absorb the established traditions of the month-long pagan Saturnalia festival, which already encompassed that date.


Sun image by OSPAN/AFRLSVD/NSO

Astronomically speaking, the winter solstice represents a turning point in the Earth’s year: the ‘return’ of the Sun from its southern decline. So too we bounce back, with all the frantic optimism of our year-end resolutions, moving again toward spring, following the Sun.

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