Culture

Comets Part 2

by Duncan Lunan 

Halley’s Comet and Meteors

Bayeux Tapestry scene 38 Halley’s Comet

The astronomical highlight of 1066 was the return of Halley’s Comet, though of course it didn’t have that name then.  It was a particularly spectacular apparition, featured in the Bayeux Tapestry with the caption ISTI MIRANT STELLAM – these men are wondering at the star.  The Anglo-Saxon king Harold of England had the problem that his army took it as an omen of disaster  (literally, a misfortune from the stars)  and William the Conqueror agreed with them:  it did indeed mean disaster for Harold and his army.  Harold had just defeated a Viking attack in the north of England, and managed to get his troops back to Hastings in time, but victory was beyond them.  Halley’s Comet returned in 1145, conspicuous enough for a drawing of it in the Eadwine Psalter.  In 1222, brighter than the Moon and very red, it passed through Boötes, Virgo, Libra and Scorpius, and disappeared near Antares.

The back cover of my book New Worlds for Old  (David & Charles, 1979)  reproduced an interior painting by the late Ed Buckley, showing the Earth passing through the tail of Halley’s Comet in 1910.  But Ed liked to include mysteries in his paintings, and there is a space probe in the foreground of this one – whose it is, you must decide for yourselves.

Halley’s comet 1910, by Ed Buckley for ‘New Worlds for Old’

Comets are usually named after their discoverers, but Halley’s Comet was named after him because he was the first to realise that some comets return.  These are the ‘families’ of comets which have been captured into the planetary system by the giant planets, particularly Jupiter.   Halley’s’ Comet, returning every 76 years, is a member of the Neptune family.  In 1986 Japanese and Soviet probes had close encounters with it, with NASA ICE probe up-Sun, while Europe Giotto probe penetrated the comet’s head and imaged the nucleus – more about that in Part 4.

The BBC’s series Space Odyssey featured an imagined visit to ‘Comet Yano-Moore…spotted by two amateur astronomers working independently in Japan and Europe’.  (Tim Haines & Christopher Riley, Space Odyssey, BBC, 2004.)  The same year, Sir Patrick Moore said,”Before I die, I would love to discover a bright comet. It would be great to see Moore’s comet blazing across the sky.” (New Scientist‘s book 100 Things to Do Before You Die, 2004).  It became less likely to happen when he lost the use of one arm and to give up practical observing.  Sadly, on 9th December 2012, his chances to make the discovery ran out in real life.  But Moore’s Comet had happened much earlier in the Daily ExpressJeff Hawke’ strip, in episodes H6004-6005 of ‘The Comet’s Tale’ (1973-74), even if he wasn’t aware of it.  As depicted, with its straight dust tail and broken gas one, the comet was virtually identical to a NASA artist’s impression of Halley’s Comet  (Carl Sagan and Ann Druyan, Comet, Michael Joseph, 1985, p.287)  though I haven’t located the original.

Comet Hale-Bopp

Comets and Meteors

On an average night, in watching the sky for an hour or more, it’s almost inevitable that you will see a meteor.  The standard reply to the question “Was that one?” is that it’s like a sneeze – if you have to ask, then it wasn’t one.  Meteors are still popularly called ‘shooting stars’ or ‘falling stars’, and that’s so accurate a visual description that only purists contest it.  The appearance is of a streak of fire with  (almost always)  a starlike dot ahead of it.  The trail lasts longer than the dot because the friction of the atmosphere has reduced the solid object to incandescent plasma and dust.  Noctilucent clouds, the glowing cirrus-like formations seen on the northern horizon from time to time in the British summer, may be formations of meteor dust collected by high-altitude winds.  It is certain that the descending dust has a lot to do with the formation of raindrops at lower levels.

Annual Showers

Most meteors are only specks of dust or small pebbles, and some of them at least come from collisions in the asteroid belt.  The other major source of meteors in the inner Solar System is the dust which escapes from comets as they pass the Sun – as witness the damage done to the Giotto spacecraft during its encounter with Halley’s Comet.  As each dust particle has a slightly different velocity from the comet’s, a band of dust forms along the comet’s orbit.  When the Earth crosses the band we have a meteor shower, in which perspective makes the parallel tracks seem to come from a point in the sky termed the radiant, named after the constellation which holds it – hence ‘the Lyrid meteors’, in April, for example.

Lyrids 2011 Nighted Vail by Brad Goldpaint Middle Falls Mt Shasta Calif..

Meteor showers are not uncommon:  the Earth encounters fifty or so every year.   Each time a comet passes close to the Sun, dust particles are driven off, sometimes forming a ‘dust tail’ which separates from the gaseous one because dust grains move more slowly under sunlight pressure than gaseous ions.  Comet Hale-Bopp, so prominent in 1997, had a strong dust tail.  From some angles diffracted sunlight can make dust tails appear red, and as the relative motions often put a curve on a comet’s tail, it didn’t take much mediaeval imagination to see a bloody scimitar in the sky and predict disaster for Christians in the Holy Land.  There were a lot of comets at the time, as I’ve emphasised in my book Children from the Sky  (Mutus Liber, 2012).

Sydney Jordan, Leonids, aurora and Moon, 1366, for ‘Children from the Sky’

Before and after dawn, particularly in the Tropics, the dust in the plane of Earth’s orbit can be seen as the Zodiacal Light, a glowing cone of dust particles above the Sun.  On the opposite side of the Earth from the Sun keen-eyed observers sometimes glimpse a related glow called the Gegenschein  (counter-glow).  The Zodiacal Light is faint and the Gegenschein much fainter;  one of the more remarkable things about the George Pal/Chesley Bonestell film The Conquest of Space (Paramount, 1955), is that you can see the solar disc and the Zodiacal Light at the same time!   Until the space age it wasn’t known whether the two effects were truly in interplanetary space or generated by a dust cloud surrounding the Earth, but the Pioneer 10 space probe found that both effects persisted as far out as the Asteroid Belt.  The dust comes partly from there, and part of it is released by comets passing through the inner Solar System, and it spirals towards the Sun due to the Poynting-Robertson effect, in which light from the Sun exercises a slight but significant braking effect.

Gegenschein, European Southern Observatory, Chile

Over time the dust spreads along the comet’s orbit, and if the Earth’s orbit intersects it, there’s a meteor shower when we cross the dust band.  When watching for them the trick is not to watch the constellation containing the radiant point  (see above), but the sky all round it – otherwise you’ll only see meteors which are coming straight towards you!  By the luck of the draw, probably the first meteors you see won’t seem to come from the radiant at all.  Meteors which are not part of any known shower are termed sporadic – some of them come from the Asteroid Belt, others from old meteor showers which have been dispersed by the gravitational pulls of the planets.  Very bright meteors  (usually clumps of dust)  are known as fireballs; and slow fireballs, burning with a pure white light, are usually re-entering artificial satellites.  You never know what you may see…

Quadrantids radiant, 3rd Jan

The Quadrantid meteors, from an old comet designated 2003EH1, are expected on 3rd January.  The meteors appear to radiate from the obsolete constellation Quadrans, between the Plough and the top of Boötes, the Herdsman.  In January 1986 I saw a spectacular display from Largs beach, while observing Halley’s Comet.  Halley’s Comet itself generates two meteor showers, the eta Aquarids in May and the Orionids in October.

Perseid meteor-shower, August

The meteors seen in August come from Comet Swift-Tuttle, the Great Comet of 1862, and are called the Perseids because their radiant lies in the constellation Perseus.  Perseus is in the north-east as darkness falls in August, rising up the sky as the night goes on, and followed up the sky by the bright star Capella in Auriga.  Capella is circumpolar at the latitude of Britain, skirting the northern horizon in summer but never setting.

To see the meteors, however, don’t watch Perseus and Capella.  As each dust particle races into the atmosphere, it has to travel some distance before it flares brightly enough to be seen from the ground.  Because of perspective, again, the meteors are therefore seen elsewhere in the sky, though seeming to come from Perseus, when their paths are traced back.  So there are more meteors to see in the small hours – partly because Perseus is then high in the sky, but also because the Earth’s rotation is then turning us into the oncoming dust stream.

The Taurid meteors occur during October, especially in the first two weeks – not to be confused with the much larger daylight shower of Beta Taurids in June.  They come from the short-period Comet Encke, possibly a survivor from the break-up of a super-comet around 3000 BC.  They’re not currently associated with major displays, keeping up “a drizzle through most of the month” – though from time to time that includes bright fireballs – but when the Earth crosses the June dust stream, the shower is much more intense.  The June meteors are invisible in daylight, but were responsible for many false radar reports of incoming V2 rockets in the later stages of the war in Europe, and in 1975 the sensors left on the Moon by the Apollo astronauts recorded an intense bombardment on the Farside.  Other fragments may have included the giant fireball object which exploded over Tunguska in Siberia on June 30th, 1908, and perhaps the multiple fragments which hit the Farside in 1178, though there are other oddities about that.

1799 Leonids – contemporary woodcut

The meteors from Comet Tempel-Tuttle are called the Leonids, and they now come in mid-November, due to planetary perturbations and to changes in the calendar – but in mediaeval times they were dated in October.  Techniques developed by Drs. David Asher of Armagh Observatory, and Robert McNaught of the UK Schmidt at Siding Springs in Australia  (originally from Prestwick)  now allow Leonid storms to be predicted very accurately, and they were dramatically proved right in the Leonid showers of 1999-2002, the last of which I saw from the Castle Rigg stone circle in the Lake District. 

Leonids radiant, November

The Leonids travel in narrow streams close to the comet and major displays occur at 33-year intervals when Tempel-Tuttle swings past the Sun.  In 1966 displays were seen from the USA before dawn, though nothing was seen in Britain  (I spent the night in Pollok Park, Glasgow, to no purpose).  There were signs that the next 33-year peak would be big – several satellites suffered minor damage in November 1997 – and in 1998 there was a major shower over Britain, a year and a day ahead of schedule.  Even from central Glasgow I saw ten fireballs, which was ten more than I’d seen 32 years before.  But on November 17th, 1999, from Dumfriesshire, I counted 226 meteors in an hour of a display lasting about 90 minutes.  On November 18th, 2000, when the Moon’s position virtually coincided with the Leonid radiant, conditions in Scotland were cloudy, but any meteors would have appeared to be coming from the Moon, as described in a chronicle of 1366.

Planetary perturbations have now moved the peak of the shower to November 17th, but in the 10th century Leonid showers would have been on October 20th.  In the mid-1300s they’d be expected on November 5th, but because the calendar hadn’t yet been reformed, they would be recorded on October 28th.   (In the 13th century Lammas Day, August 1st, was actually August 9th in the solar year.)    That fits the chronicle display to perfection;  the year might seem wrong, but the comet’s period isn’t exactly 33 years, and a Chinese sighting of it in 1366 was used by Joachim Schubart to calculate it precisely and locate the comet on its brief appearance in 1965.   

Geminid-radiant, December

The Geminid meteors are expected on 13th/14th December.  They seem to come from the constellation Gemini, but their origin is the asteroid Phaethon, interesting because it’s the only shower known to originate from an asteroid rather than a comet, although collisions between asteroids have been observed by the Hubble Space Telescope.  Phaeton approaches the Sun to within less than half Mercury’s closest distance to it, and the meteors travelling in the same orbit first appeared only in 1892.  The number of meteors seen can vary a great deal, but they often have a distinctive green colour, and I once saw a dramatic shower of Geminid meteors over Troon in my teens.

John Ashley Glacier National Park Montana early Geminid earthgrazer Dec 6 2018 above Dusty Star Mountain

(To be continued) See also: Comets

Check out the astronomy series in our archive., including The Sky Above You – December 2021

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