science

Stargazing in December: Behold, the Christmas Star!


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ASA images of Jupiter and Saturn would be the obvious one, or you could go for an image of the Star of Bethlehem.

This month we’re treated to a sight in the sky that hasn’t been seen literally for centuries, and which may hold the clue to that most enduring of astronomical mysteries, the Star of Bethlehem.

Even better, you don’t need a telescope to view this event. Just an unobstructed horizon to the south-west; and, on the critical date of 21 December, that most elusive of commodities, clear skies!

For months now, the brilliant planet Jupiter has been hanging above the south-western horizon in the early evening, with its fainter sidekick Saturn to its left – like a pair of planetary “eyes”. Keep track of these worlds during December, and you’ll see Jupiter gradually creeping up on Saturn.

On the evening of 21 December, the two planets line up almost exactly. At first glance, you may even think they have merged into a single dazzling “star”! Look closely, or through binoculars, and you’ll see Saturn lies above Jupiter by one-fifth of the moon’s diameter. Grab a telescope for a once-in-a-lifetime opportunity to see the globe of Jupiter and its four moons in the same field of view as Saturn’s glorious rings.

When I say “once in a lifetime”, that’s no exaggeration. The last time these planets were so close was in July 1623. Then, Jupiter and Saturn were even lower in the twilight, so any astronomers of the time would have needed to use the newly-invented telescope.

For a clear view of Jupiter and Saturn almost on top of each other, we have to go right back to March 1226. In the morning sky, they were even closer than they will be this month, and would certainly have looked as if they had merged. Sad to say, though, we have no records of anyone viewing either this Grand Conjunction or the similar event in 1623.

I’ve dangled the solution to the Star of Bethlehem mystery in my introduction, and it would be really exciting if I could reveal that Jupiter and Saturn had merged into a magnificent sight in front of the astonished eyes of the Biblical Magi.

But, alas, the giant planets didn’t have such a close encounter at that time. What did happen, though, was that Jupiter and Saturn took part in an unusual pas-de-deux in 7 BC. They approached and separated again, three times over. Even at their closest, though, the two planets were separated by twice the diameter of the full moon.

Though this Triple Conjunction wouldn’t have been spectacular to the casual stargazer, it would have been fraught with meaning for the Magi. The closest translation of their title was not so much “Wise Men” as “astrologers” (the word Magi is also at the root of our words magic and magician).

Jupiter was the king of the planets, and Saturn was the planet of destiny. And the Triple Conjunction took place against the backdrop of Pisces, long regarded as the constellation of the Jewish people. So, the event was clearly marking the birth of a new king of the Jews and the beginning of a new era. The Magi duly set out to find him.

This is just one of many theories swirling around the Star of Bethlehem. Others invoke a comet, a shower of meteors or an exploding star. After years of investigation, I must admit that I don’t find any of them particularly convincing.

But as you gaze at the sky just four days before Christmas, just think – it might have been a dance of these two planets that led the Magi across the desert to that lonely manger in Bethlehem.

Away from the excitement of Jupiter’s encounter with Saturn, another planet is lighting up our evening skies. Mars shines with a brilliant reddish hue in the southern sky throughout the evening.

To the left of the Red Planet, you’ll notice a sparkling cluster of stars. The Pleiades are often called the Seven Sisters, though people with good eyesight can see a dozen or more stars in this tight little group. According to old star charts, the Pleiades lie on the shoulder of Taurus, the bull. His head is depicted by a more dispersed star cluster, the Hyades, with bright orange Aldebaran marking the bull’s angry eye.

The night sky at around 10pm this month

(Author Provided)

Below and to the left, the distinctive constellation of Orion is now rising in the evening sky. Three bright stars form the great hunter’s belt, while the red giant Betelgeuse lies at one shoulder and bluish-white Rigel at the hem of his tunic.

To the left again, Castor and Pollux are the main stars of Gemini (the Twins). On the night of 13/14 December, we’re treated to a shower of meteors streaming from a point nearby. The Geminid shooting stars are grains of dust from an asteroid called Phaethon, burning up as they impact our atmosphere.

And this is one night it’s definitely worth braving the cold winter air! The Geminids are the most spectacular of the annual meteor showers, with a meteor every minute or so, many of them quite bright as they traverse the sky.

You’ll have to travel to see the final major astronomical event of the month, a total eclipse of the sun on 14 December. It’s only visible from the Antarctic Peninsula and the Southern Ocean to either side, though there’s a partial eclipse for people in much of South America.

Back home again, if you’re up before dawn, Venus is a dazzling Morning Star in the south-east, rising about 5.30 am.

3 December:   Moon near Castor and Pollux

6 December:   Moon near Regulus

8 December, 0.36am:  Last Quarter Moon

12 December (am):  Moon near Venus

13/14 December:  Maximum of Geminid meteor shower

14 December, 4.16pm: New Moon, total solar eclipse

17 December:   Moon near Jupiter and Saturn

21 December, 11.41pm: First Quarter Moon; Winter Solstice (10.02 am); Jupiter and Saturn extremely close

23 December:   Moon near Mars

26 December:   Moon near the Pleiades

27 December:   Moon between Aldebaran and the Pleiades

30 December, 3.28am: Full Moon near Castor and Pollux

31 December:   Moon near Castor and Pollux

Philip’s 2021 Stargazing (Philip’s £6.99) by Heather Couper and Nigel Henbest  reveals everything that’s going on in the sky next year.



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