Science

The Christmas Night Sky: A 2021 Christmas Stargazing Guide

Christmas is almost here, and the night sky compels presenting a beautiful display of heavenly sights, perfectly timed to coincide with the holiday.

In ancient times, December 25 was the date of the lavish Roman feast of Saturnalia, a kind of bacchanalian thanksgiving to the god of agriculture, after whom the slowest moving of the then-known planets was named. Saturnalia was celebrated on the winter solstice date by the calendar then in use, and also marked the fact that the sun had stopped crawling southward in the noon sky and would henceforth cross the highest meridian each day. , warming the Earth and awakening nature. .

This feast of the Romans was a version of similar celebrations of other primitive peoples. It has been said that the early Christians chose the date of Saturnalia to celebrate Christmas to avoid attention and thus escape persecution. When the Roman Emperor Constantine officially adopted Christianity in the 4th century, the date of Christmas was still December 25.

Related: The Brightest Visible Planets in the December Night Sky

This sky map shows the planets that will be visible after sunset on Christmas (December 25) in 2021. (Image credit: SkySafari app)

Christmas Star: 2021

Our current night sky is especially rewarding. The eastern sky is full of bright stars; I’ve always compared it to some kind of “heavenly Christmas tree.” Highlighted low in the southwestern sky after sunset is the bright planet Venus, often referred to as the “Shepherd’s Star” by the legendary French astronomer Nicolas Camille Flammarion (1842-1925). It glows like a silver lantern for about 90 minutes after sunset, making for an eye-catching heavenly ornament.

Some will wonder if the legendary Star of Bethlehem could have been something similar. Astronomers and biblical scholars have long pondered this question. Would magicians, for example, have given much importance to the planets? They were presumably followers of Zoroaster and believed that the planets indicated the actions of the gods and were important to the affairs of man. These early religions were the origin of today’s astrology and the ancestors of astronomy. Perhaps the magicians witnessed a planetary grouping of particular beauty; an exceptionally close conjunction of two planets or a striking grouping of several planets. New knowledge of old astrological beliefs and modern computer-based planetary charts may shed new light on this ancient question.

Other homeless

Along with Venus, there are other worlds that invite you to this Christmas season. Two planets can be found in the upper left of Venus: soft yellow Saturn and bright silver-white Jupiter. If you received a telescope for the holidays, be sure to see Saturn and its beautiful ring system and Jupiter with its retinue of bright satellites. On December 29, the three planets will be equally spaced, stretched out on a diagonal line measuring 36 degrees from the top left to the bottom right; Jupiter highest, Venus lowest, and Saturn in the middle.

And look carefully 5 degrees to the left of Venus and you will see a fourth planet: Mercury!

A heavenly cross

See the Northern Cross asterism in the constellation Cygnus the Swan on the western horizon after sunset. (Image credit: SkySafari app)

Depart around 8:30 pm local time and look northwest. There you will find a vestige of warm summer nights, the constellation of Cygnus, the swan. In mid-August, when many eyes look skyward trying to see the Perseid meteor shower, Cygnus is directly overhead around midnight and is apparently flying in a southwesterly trajectory. But now, on these frosty early winter afternoons, the swan is ready to fly straight to the northwestern horizon.

The distinctive groupings of stars that are part of the outlines of recognized constellations, or that lie within their boundaries, are known as asterisms, and the six brightest stars in Cygnus make up an asterism more popularly called the Northern Cross. Bright, white, the bright star Deneb decorates the top of the Cross. Albereo, at the foot of the Cross, is actually a pair of stars of beautiful contrasting colors: a third-magnitude orange star and its fifth-magnitude blue companion are clearly visible even in a low-power telescope. The pattern of the Cross is better oriented to view now, and it appears to stand majestically on the northwestern horizon during this Christmas season.

A large package (or the Three Kings)

The constellation Orion, the Hunter, shines above the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile's Atacama Desert in this image by European Southern Observatory photographic ambassador Yuri Beletsky.  Two of the 66 radio telescopes that make up the array are shown in this view.  Perched atop the 16,000-foot (5,000-meter) Chajnantor Plateau, ALMA's location provides the dark, dry skies that are crucial for observing the cosmos at millimeter and submillimeter wavelengths.

The constellation Orion, the Hunter, shines above the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile’s Atacama Desert in this image by European Southern Observatory photographic ambassador Yuri Beletsky. (Image credit: Y. Beletsky (LCO) / ESO)

At the same time, about halfway in the southeast, Orion, the hunter, shines. One of my mentors in astronomy was Dr. Fred Hess (1920-2007), a popular lecturer for more than three decades at the Hayden Planetarium in New York. Dr. Hess used to tell his audience at Christmas that Orion looked like a big package in the sky, tied with a nice bow in the middle. The arc was represented by three almost equally bright and almost equally spaced stars, popularly known as the belt of Orion.

But I like to tell the planetarium audience that we could possibly also think of these three stars, whose names are Alnitak, Alnilam and Mintaka, as representatives of the Magi.

The manger

Praesepe is also known as the group of hives. (Image credit: Shutterstock)

And if we were to consider these three stars as representatives of the Magi, then not far to the east, within the faint zodiacal constellation Cancer, the Crab is the star cluster known as Praesepe, the manger. A manger is defined as an open trough or box in which horse or cattle feed is placed. But the Book of Saint Luke also tells us that the baby Jesus, wrapped in swaddling clothes, was laid in a manger because there was no room in the inn. In our current night sky for Christmas week, Praesepe represents the manger where Christ was born.

Just before midnight, it is located midway in the eastern part of the sky. If you have binoculars, pan across the region of the sky roughly halfway between the bright stars Regulus (in Leo) and Pollux (in Gemini). You will come across Praesepe who looks like a beautiful object to behold, seeming to contain a splatter of several dozen stars. Using his crude telescope, Galileo wrote in 1610 that he saw Praesepe not as a diffuse star, but as “… a mass of more than 40 small stars.” If you have access to a dark sky without polluted light, the manger appears as a soft, diffuse patch or a faint glow to the naked eye.

Happy Holidays and lots of clear, starry skies in the New Year!

Joe Rao serves as an instructor and guest speaker at the Hayden Planetarium in New York. He writes about astronomy for Natural History magazine, Farmers’ Almanac, and other publications. Follow us on Twitter @Spacedotcom and on Facebook.

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