December is the month of the winter solstice, which a large part of mankind associates with such celebrations as Nativity festivals.
The moment of the solstice occurred on Dec. 21 at 5:23 p.m EST (22:23 GMT): The sun, appearing to travel along the ecliptic, reached that point in the sky where it is farthest south of the celestial equator.
While a variety of customs have been linked with this special season for thousands of years, the exchanging of gifts is prevalent among many different cultures. Mother Nature herself offers two gifts to sky observers in northern, temperate latitudes: the longest nights and a sky more transparent than usual. [See the Christmas Comet 46P/Wirtanen! ]
One reason for the clarity of a winter's night is that cold air cannot hold as much moisture as warm air can. Hence, 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 us in northern climes, this only adds more luster to that part of the sky containing the beautiful wintertime constellations, as this week's sky chart shows. Indeed, the sky this time of year can be seen as nature's holiday decoration to commemorate the winter solstice and enlighten the long, cold nights of winter.
The Yuletide evening sky is especially rewarding.
The eastern sky is filled with brilliant stars and star patterns. Distinctive groupings of stars that form part of the recognized constellation outlines, or lying within their boundaries, are known as asterisms. Ranging in size from sprawling, naked-eye figures to minute stellar settings, they are found in every quarter of the sky and at all seasons of the year.
The larger asterisms — ones like the Big Dipper in Ursa Major and the Great Square of Pegasus — are often better-known than their host constellations. Here are some of the best asterisms to spot as the year approaches its end.
The Northern Cross
During these frosty evenings, one of the most famous asterisms is in the northwest. Known as the constellation Cygnus, the Swan, the brightest six stars of Cygnus make up an asterism that's more popularly called the Northern Cross. [The Best Telescopes of 2018!]
Bright Deneb decorates the top of the Cross. Albireo, at the foot of the Cross, is really a pair of stars of beautifully contrasting colors: a third-magnitude orange star and its fifth-magnitude blue companion are clearly visible in even a low-power telescope.
While it is usually regarded as a summertime pattern, the Cross is best-oriented for viewing now: It appears to stand majestically upright on the northwest horizon at around 8:30 p.m. local time, forming an apt Christmas symbol. Furthermore, just before dawn on Easter morning, that cross lies on its side in the eastern sky.
The Great Hexagon
High toward the south, at around 10:30 p.m. in your local time zone, we see what astronomy author Hans A. Rey (1898 to 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. This is the rich region that gives the winter sky its splendor. The dominate star pattern in this field of stars is Orion, the Mighty Hunter, composed of four bright stars forming his body and three stars arranged diagonally forming his belt. Speaking of Orion's belt, the legendary French astronomer Nicolas Camille Flammarion (1842 to 1925) referred to the three belt stars of Orion as "The Three Kings."
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 to wear is a hooded ski parka, which is lightweight yet provides excellent insulation, along with 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.
The International Space Station
The largest man-made space vehicle is currently making southwest-to-northeast passes across North America at dawn. To the unaided eye, the ISS appears as a very bright, non-twinkling "star" moving with a steady speed across the sky. To get the latest times and directions to look for it, your best bet is to go to this NASA website: https://spotthestation.nasa.gov/sightings/ which provides sighting information for thousands of locations worldwide. Early before sunrise on Christmas morning, why not take your kids outside and wave a holiday greeting to the people onboard the station as they fly over your neighborhood at 18,000 mph (29,000 km/h)!
The Shepherd's Star
If you are up before sunrise waiting for the Space Station, look toward the east-southeast to get a view of what Flammarion described as "The Shepherd's Star," the planet Venus. He wrote:
"She shines in the east in the morning, with a splendid brightness which eclipses that of all the stars. She is, without comparison, the most magnificent star of our sky; the star of sweet confidences."
Indeed, Venus is always bright, but this December the planet's brilliance and altitude are exceptional. This month, from mid-northern latitudes the lamp-like "Morning Star" rises nearly four hours before sunrise. It will remind those who arise early on Christmas morning of the Biblical "Star in the East."
Your clenched fist held at arm's length measures roughly 10-degrees and at sunrise Venus will reach a maximum altitude about 32-degrees (more than "three fists" up from the east-southeast horizon) on Christmas morning. On clear mornings you should have little trouble following Venus naked-eye through the moment of sunup.
And on New Year's morning, a slender crescent moon forms an eye-catching tableau with Venus in the morning sky; the slender lunar sliver will hover a half dozen degrees to Venus's upper right.
Lastly, for those who receive a telescope for a holiday gift, Venus will present a dazzling and wide crescent image. But there is another splendid planetary target to gaze at. That very bright "star" that you notice about 18 degrees to the lower left of Venus is Jupiter; a superb telescopic showpiece with clouds bands crossing its disk, as well as its retinue of four large moons. On Jan. 22nd, Venus and Jupiter will engage in a spectacular conjunction, shining just 2.4 degrees apart. Thereafter, Jupiter moves off to Venus' upper right.
On Christmas night, the moon will be three days past full and rises above the east-northeast horizon before 8:30 p.m. local time. A couple of hours later it will be well placed for observation for those who have just acquired binoculars or a telescope as a holiday gift.
Your best views will be along the line separating day and night on the lunar surface, called the terminator. It is there where craters will appear heavily shadowed and will stand out in sharp relief. The rest of the lunar disk will appear dazzling to the eye.
Take note of the crater Tycho near the moon's lower limb, appearing like a sunflower with brilliant rays emanating from it in all directions. To the left of the moon's center is another vivid crater, Copernicus.
On New Year's Eve morning, just before sunrise, Copernicus will lie right on the terminator; the moon by then having shrunk to a wide crescent.
And then there is Mars, which pops into view in the southern sky as darkness falls, a solitary bright lamp shining with an orange-yellow glow, but appears through most telescopes as nothing more than a tiny disk as it continues to recede from Earth. It will set before the stroke of midnight.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's Lower Hudson Valley.