What to Look For in the 2017 Christmas Sky

December 26, 2017

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 11:28 a.m. EST (0428 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. 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. Originally known simply as the “Bird” in ancient times, without any indication of what sort of bird it was supposed to represent, it later became the constellation Cygnus, the Swan. But the brightest six stars of Cygnus make up an asterism that’s more popularly called the Northern Cross. 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.

A Christmas package

Can you also see a large package in the sky, tied with a pretty bow across the middle? Four bright stars outline the package, while three that are close together and in a straight line form the decorative bow. Now you can see how our modern imagination might work, but tradition tells us that those seven stars formed a mighty hunter called Orion, the most brilliant of the constellations, which is visible from every inhabited part of Earth. Two stars mark his shoulders, two more his knees and three 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.”

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