Christmas week is here, and the night sky obliges by presenting a beautiful display of celestial sights, perfectly timed to coincide with the holiday.
In ancient times, Dec. 25 was the date of the lavish Roman festival of Saturnalia, a sort of bacchanalian thanksgiving to the god of agriculture, for whom the slowest moving of the then-known planets was named. Saturnalia was celebrated on the date of the winter solstice by the calendar then in use, and it also marked the point when the sun had stopped creeping southward in the noon sky and would thenceforth cross the meridian higher each day, warming the Earth and reawakening nature.
This holiday of the Romans was a version of similar celebrations by other early peoples. It has been said that early Christians chose to celebrate Christmas on Saturnalia in order to avoid attention and thus escape persecution. When the Roman emperor Constantine officially adopted Christianity in the fourth century, the date of Christmas remained Dec. 25. [Venus, Mars and More: Dec. 2016 Skywatching Video]
Cold and clear
Among the many varied customs linked with this special season for thousands of years, the exchanging of gifts is almost universal. Nature offers the two gifts of the longest nights and a sky more transparent than usual.
Newcomers to astronomy often assume that winter's especially dark skies mean all forms of observing are at their best. But this is far from so. The "crystal-clear" winter skies and "unusually bright stars" are due to two factors. First, the cold temperatures prevent the air from holding as much moisture as in summer, resulting in less haze and greater atmospheric transparency. This is also why daytime skies in winter are generally a deeper, crisper blue. Second, the stars that appear overhead at this time of year happen to be brighter than those of summer. They are among the most luminous beacons lining the local spiral arm of the Milky Way galaxy.
However, the other side of the coin is that cold, sparkling skies are often very unstable, with strong atmospheric turbulence that makes star images churn and fuzz when viewed through a high-power telescope. This turbulence also explains why on wintry nights the stars — especially the brightest ones — seem to scintillate dramatically; the charming "twinkle, twinkle, little star" of the nursery rhyme is also the most obvious (and unfortunate) evidence of an unstable atmosphere.
Christmas star: 2016
The current evening sky is especially rewarding. Brilliant stars fill the eastern sky; I've always likened it to a sort of "celestial Christmas tree." Prominently placed in the south-southwest sky, after sundown, is the brilliant planet Venus, referred to as the "Shepherd's Star" by the legendary French astronomer Nicolas Camille Flammarion (1842-1925). Venus shines like a silvery lantern for 3 and a half hours after sunset, making for an eye-catching celestial ornament. [Photos of Venus, the Mysterious Planet Next Door]
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 that the planets indicated the actions of the gods and were important to the affairs of man.
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.
A cross, three kings and a manger
Step outside around 8:30 p.m. local time and look low toward the northwest. There, you will find a remnant of warm summer nights: Cygnus, the swan.
In mid-August, when many eyes are looking skyward trying to view the Perseid meteor shower, Cygnus is directly overhead at around midnight and flying seemingly on a southwest trajectory. But now, on these frosty, early-winter evenings, the Swan is poised to fly straight down into the northwest horizon.
Distinctive groupings of stars that form part of the recognized constellation outlines — or lie within their boundaries — are known as asterisms. The brightest six stars of Cygnus compose an asterism more popularly called the Northern Cross. Bright, white 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. The pattern of the cross is best oriented for viewing now, appearing to stand majestically upright on the northwest horizon during this holiday season.
At the same time, about halfway up in the southeast shines Orion, the mighty 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 says 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, Praesepe 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). Praesepe is a beautiful object to behold, appearing to contain a smattering 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.
Happy holidays, and many clear and starry skies in the New Year!
Editor's note: If you have an amazing night sky photo you'd like to share with us and our news partners for a possible story or image gallery, please contact managing editor Tariq Malik at spacephotos@space.com.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.
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