Stargazing in December: A brilliant and unique comet
A astronomical event that will take decades to complete is starting this month, writes Nigel Henbest
This month, an astronomical "event of a lifetime" begins to unfold, at a spot five billion kilometres away in the constellation Hydra. Halley’s Comet, which has been speeding away from us for the past 37 years, is slowed to a standstill by the Sun’s far-reaching gravity on 9 December. It then begins to fall back towards the inner reaches of the solar system, to grace our earthly skies in 2061.
Most brilliant comets – like Hale-Bopp in 1997 – have orbits that last thousands of years or more, and we can’t predict their appearances. Comets in small, predictable orbits have generally been burnt out by multiple encounters with the Sun, and they are now too faint to be visible to the naked eye.
But Halley’s Comet is unique. It’s both bright enough to be a magnificent sky-sight, and follows an orbit small enough to bring it into view regularly and predictably, once every 76 years – closely synced with a human lifespan.
My lifetime’s Halley Comet event occurred early in 1986; and my first glimpse of the celestial visitor was from the flight deck of Concorde, travelling over the Indian Ocean at twice the speed of sound. The comet was faint as viewed from the northern hemisphere, and anyone in a British city had little chance of seeing it all. Halley’s Comet would appear brighter from the southern hemisphere, though, and I was lucky enough to be one of the astronomers on board the supersonic jet on a charter flight to view the comet in New Zealand.
From there, the comet appeared as a luminous patch high in the sky, moving slowly from night to night. Through a telescope, we could see the classic comet-shape: a huge luminous head and extended tail, formed of dust and gases that had been erupted from a solid icy nucleus too small to be seen directly from Earth.
And in March of that year the world was treated to the first close-up views of a cometary nucleus, when the European spacecraft Giotto flew straight through the gaseous head of Halley’s Comet. Up until then, astronomers had envisaged the core of a comet as a "dirty snowball," looking like a pile of greyish slush beside a busy road after a snowstorm.
But Giotto astonished the researchers by revealing that the surface of Halley’s nucleus is black – even darker than coal. Rather than resembling a dirty snowball, a comet’s nucleus is more like a choc-ice, with frozen water trapped inside a crust composed of dark organic molecules.
The 1986 appearance changed the scientific view of comets, and Halley’s previous apparitions have often been punctuation marks in history. Our first records come from Chinese astronomers, who in 240 BC saw a "broom star" sweeping across the sky. In AD 66, the comet hung like a portent of doom over Jerusalem – which was sacked by the Romans four years later.
In 1066, Halley’s Comet appeared in the sky just before the Battle of Hastings, when the Saxons lost control of England to the invading Normans. On a more positive note, in 1301 the Italian painter Giotto di Bondone was so impressed with the comet that he included it in his Nativity painting as the Star of Bethlehem – even though, in reality, the comet wasn’t around at the time Jesus Christ was born.
English astronomer Edmond Halley was on his honeymoon, in Islington, when he saw the eponymous comet in 1682. He wasn’t the first to discover this heavenly wraith, but the comet became inextricable linked to his name when Halley calculated that it was same celestial visitor that had previously been seen in 1531 and 1607 – showing that comets, like planets, pursue regular orbits around the Sun. He predicted its return in 1758; and when the comet duly appeared Halley’s fame was assured.
Its "once in a lifetime" tag was exemplified by the American author Mark Twain, who was born two weeks after the comet passed the Sun in 1835. "I came in with Halley’s Comet," he wrote many years later. "These two unaccountable freaks came in together; they must go out together." And he duly died in 1910, one day after the comet again zoomed by the Sun.
What can we expect on the next scheduled appearance, in 2061? Compared to 1986, the comet is much nearer to the Earth as it passes closest to the Sun, so we have a grandstand view. Halley will shine as brilliantly as the most prominent stars in the sky. And if you have another lifetime to spare, its 2134 apparition will be even more magnificent.
What’s Up
The sky is beginning to fill with the bright stars of winter stars, but they are overshadowed by the brilliant planet Jupiter, lording it over the evening sky near to the glittering Seven Sisters star cluster, known to astronomers as the Pleiades. The Moon is near to Jupiter on 22 December.
Well to Jupiter’s lower right, fainter Saturn is now setting around 10 pm among the dim stars of Aquarius. The crescent Moon passes below the ringed planet on 17 December.
To the lower left of Jupiter you’ll the familiar figure of Orion, the celestial hunter, with three stars marking his belt. Follow these stars downwards to locate Sirius, the brightest star in the sky. Above Orion lie Capella, the jewel in the constellation Auriga (the charioteer); Aldebaran, marking the red eye of Taurus (the bull); and the twin stars Castor and Pollux, in Gemini.
On the night of 13 December, watch out for bright slow shooting stars scattering outwards in all directions from Gemini. Each is a speck of dust from asteroid Phaeton, which orbits close to the Sun, and may be the cinder of a comet that’s lost all its ice.
The Geminid meteor shower is the most prolific of the year, and 2023 is a great time to observe these shooting stars because the moon is well out of the way, leaving the sky as dark as it gets.
Livening up the dawn skies is brilliant Venus, rising around 4 am. The crescent Moon forms a lovely partner to the Morning Star before sunrise on 9 and 10 December.
Finally, 22 December is the Winter Solstice. From then on, the nights grow shorter and the days longer as we head towards the New Year and the spring of 2024.
Dairy
9 December, early hours: Moon near Venus
10 December, early hours: Moon below Venus
12 December 11.32pm: New Moon
13 December: Maximum of Geminid meteor shower
17 December: Moon near Saturn
19 December, 6.39pm: First Quarter Moon
22 December, 3.27am: Winter Solstice; Moon near Jupiter
24 December: Moon near Aldebaran and the Pleiades
27 December, 0.33am: Full Moon
28 December: Moon near Castor and Pollux
30 December: Moon near Regulus
31 December: Moon near Regulus
Nigel Henbest’s latest book, ‘Stargazing 2024’ (Philip’s £6.99) is your monthly guide to everything that’s happening in the night sky next year
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