By Night Sky Columnist
updated 12/26/2003 5:19:29 PM ET 2003-12-26T22:19:29

December is the month of the winter solstice, which a large part of humanity associates with such festivals as the Nativity. It is also prime time for stargazing.

The moment of the solstice occurred on Dec. 22 at 2:04 a.m. ET. The sun, appearing to travel along the ecliptic, reached that point in the sky where it is farthest south of the celestial equator. Among the many varied customs linked with this special season for thousands of years, the exchanging of gifts is almost universal.

Mother Nature herself offers the sky observer in north temperate latitudes the two gifts: the longest nights and a sky more transparent than usual.

One reason for the clarity of a winter’s night is that cold air cannot hold as much moisture as warm air can. On many nights in the summer, the warm, moisture-laden atmosphere causes the sky to appear hazier. By day it is a milky, washed-out blue, which in winter becomes a richer, deeper and darker shade of blue.

For people in northern climes, this adds more luster to the part of the night sky containing the beautiful wintertime constellations. Indeed, it is seemingly nature’s holiday decoration to commemorate the winter solstice and enlighten the long cold nights of winter.

Find the planets
Soon after sunset, we can enjoy the sight of the brightest planet, Venus, well up in our southwestern sky.

Higher up and farther toward the south is the fading Mars, which during this week recedes to 100 million miles from Earth and diminishes to magnitude 0.3. That’s still brighter than Procyon, in Canis Minor, the eighth brightest star in the sky, but only about one-nineteenth as bright as it was back in late August when it was only about one-third as far away.

Look for Mars riding well above the moon late on the night of Dec. 29.

Saturn is a bright yellowish-white interloper among the stars of Gemini the Twins and is visible all night long, with its rings wide-open for telescopic inspection, while Jupiter appears above the eastern horizon like a brilliant silvery "star" soon after 10:30 p.m.

High toward the south, also at around 10:30 p.m., we see what astronomy author Hans A. Rey (1898-1977) called a "Great Hexagon" of bright winter stars.

To the south and a little east lies Sirius; up to the west, Rigel. Still higher, reddish Aldebaran; then at the north end of the circle, Capella. South and slightly east, we come to Castor and Pollux, the heads of the Gemini twins. Finally, south again to Procyon. In all, seven bright stars in six constellations.

In the center of the hexagon, more or less, you have the ruddy star Betelgeuse, while above it and somewhat to its left is Saturn. This is the rich region that gives the winter sky its splendor.

If you plan to be outside for a long period of time on these frosty, cold nights, remember that enjoying the starry winter sky requires protection against the prevailing low temperatures. One of the best garments is a hooded ski parka, which is lightweight yet excellent insulation, and ski pants, which are better than ordinary trousers. And it is also important to remember your feet. While two pairs of warm socks in loose-fitting shoes are often adequate, for protracted observing on bitter-cold nights wear insulated boots.

Marvel at the moon
For those with binoculars and telescopes, the next week will offer the best views for observing the moon, one of the most fascinating of celestial bodies.

In fact, even with just small optical power we can see a wealth of detail on its surface. Around those times when the moon is half-lit or at gibbous phase, those features lying close to the terminator — a variable line between the illuminated portion and the part of the moon in shadow — stand out in sharp, clear relief.

In contrast, around the time of full phase, the moon appears flat and one-dimensional as well as dazzlingly bright.

The moon will arrive at first quarter phase on Dec. 30 at 5:03 a.m. ET, when its disk will be exactly 50 percent illuminated. How does it brightness compare at that moment with full? One would think it’s half as bright, but in reality astronomers tell us that the first-quarter moon is only one-eleventh as bright as full. And believe or not, it isn’t until just 2.4 days before full that the moon actually becomes half as bright as a full moon!

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester in New York.

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