December is the month of the winter solstice, which a large part of the world associates with such celebrations as Nativity festivals. The moment of the solstice occurred on Dec. 21 at 11:19 p.m EST (0419 GMT on Dec. 22): 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. 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 bright star 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.
A Christmas Package
At the same time, about halfway up in the southeast shines the constellation of Orion, the hunter. One of my mentors in astronomy was Dr. Fred Hess (1920-2007), a popular lecturer for over three decades at New York's Hayden Planetarium. Dr. Hess used to tell his audiences at Christmastime, that Orion resembled a large package in the sky, tied with a pretty bow across the middle. The bow was represented by three nearly equally bright and nearly equally spaced stars popularly known as Orion's belt. But I like to tell planetarium audiences that we could possibly also think of these three stars, whose names are Alnitak, Alnilam and Mintaka, as representing the Three Wise Men.
And if we were to consider these three stars as representing the Magi, then not too far away, to the east, within the faint zodiacal constellation of Cancer, the crab is the star cluster known as Praesepe, the manger. A manger is defined as a trough or open box in which feed for horses or cattle is placed. But the Book of St. Luke also tells us that the baby Jesus, wrapped in swaddling clothes, was set down in a manger because there was no room at the inn. In our current Christmas week evening sky, Praesepe represents the manger where Christ was born.
Just before midnight the star cluster is situated about halfway up in the eastern part of the sky. If you have binoculars, sweep across the region of the sky roughly midway between the bright stars Regulus (in Leo) and Pollux (in Gemini). You'll run across Praesepe appearing as a beautiful object to behold, appearing to contain a splattering of several dozen stars. Using his crude telescope, Galileo wrote in 1610 of seeing Praesepe not as one fuzzy star, but as "a mass of more than 40 small stars." If you have access to a dark, non-light polluted sky, the manger appears as a soft, fuzzy patch or dim glow to the unaided eye.
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.
Spot the International Space Station
The largest human-built 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, visit spotthestation.nasa.gov/sightings, which provides sighting information for thousands of locations worldwide.
So, early before sunrise on Christmas morning, why not take your kids outside and wave a holiday greeting to the people on board the station as they fly over your neighborhood at 17,500 miles (28,000 kilometers) per hour?
Christmas Star: 2019
Our current evening sky is especially rewarding. The eastern sky is filled with brilliant stars — I've always likened it to a sort of "celestial Christmas tree." Prominently placed in the west-southwest sky, after sundown, is the brilliant planet Venus, often referred to as the "Shepherd's Star" by the legendary French Astronomer, Nicolas Camille Flammarion (1842-1925). It currently shines like a silvery lantern for 2 hours after sunset, making for an eye-catching celestial ornament.
Some may wonder if the fabled Star of Bethlehem might have been something similar. Astronomers and biblical scholars have long pondered this question. Would the Magi, for instance, have put much significance in the planets? Presumably they were followers of Zoroaster and believed the planets indicated the actions of the gods and were important to the affairs of man. Such primitive religions were the origins of today's astrology and the ancestors of astronomy. Perhaps the Magi witnessed a planetary grouping of particular beauty; an exceptionally close conjunction of two planets or an eye-catching grouping of several planets. New knowledge of the old astrological beliefs and modern computer-based planetary tables may yet shed new light on this age-old question.
And three nights after Christmas, on Saturday (Dec. 28), a slender crescent moon forms an eye-catching tableau with Venus in the evening twilight; the slender lunar sliver will hover a few degrees below Venus.
- 'Ring of Fire' to Wreath the Sun in Last Eclipse of 2019
- The Top 10 Skywatching Events to Look for in 2020
- Christmas in Space! Astronauts Celebrate a Special Cosmic Holiday in Orbit
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. Follow us on Twitter @Spacedotcom and on Facebook.