Culture

Astronomy Beginners Guide: How to Find the Constellations  (May)

By Duncan Lunan

There are many beginners’ guides to the sky – personally I learned the constellations from a 1959 reprint of “Guide to the Stars” by Hector MacPherson.  It was an old book, but the stars don’t move significantly over periods as short as decades, and there was little advanced in our knowledge of the planets between the early 20th century and the first space probes of the 1960s.  Our map which accompanies the guide to the planets each month is drawn for 9 pm  (21:00 hours)  in mid-month, in Greenwich Mean Time, nowadays called Universal Time, so in summer the map is accurate for 20:00 hrs BST in the middle of the month, half an hour earlier at the beginning of the month and half an hour later at the end.  A Planisphere, which we’ll demonstrate how to use, can be set to any date and time in the year to show the stars which are then above the horizon.

Above the centre of the map are the circumpolar stars, which neither rise nor set at the latitude of Britain because they lie close to the North Celestial Pole, whose height above the horizon is equal to the latitude of the observer.  Dubhe and Merak, the handle of the Plough in the Great Bear, can be used to find Polaris, the Pole Star, in the Little Bear.  Polaris lies very close to the North Celestial Pole at present and all the other stars appear to circle around it as the Earth turns.

In Jim Barker’s maps, a set of twelve which he drew for my monthly column ‘The Sky Above You’, the East and West compass points are reversed relative to a map of the Earth, which is looking down from the sky instead of up at it.   Like a planisphere, in theory the map should be held overhead.   In practise it’s easier to face north or south and hold the map in front of you!   When you face south, east is indeed to your left and west is to your right.   

In spring as darkness falls one of the first objects to become visible, high in the east, is the prominent orange star Arcturus in the constellation Boötes, the Herdsman.  To the right of it, overhead, are the stars of the Plough, which are part of the larger constellation of the Great Bear.  It’s an old-fashioned plough:  picture the  (invisible)  horses on the left, pulling the blade through the earth, while a man at the rear steers it with a tiller  (a highly skilled art, as Robert Burns knew).   The two stars of the tiller, Merak and Dubhe, point upwards roughly to Polaris, the Pole Star, at the tip of the Little Bear’s tail.   (In real life, bears don’t have tails!)

In Britain during summer, Arcturus is rivalled only by Vega, the brightest star in the Summer Triangle of Vega, Deneb and Altair, and by Capella, low down on the northern horizon.  The other first magnitude stars in the summer sky, such as Regulus and Spica, stand out more because of their positions with few other bright stars nearby.  Vega, in the constellation of the Lyre, is the brillaint blue-white star high overhead.  To the left of it is Deneb, the brightest star in Cygnus, the Swan, also known as the Northern Cross.  Deneb is the head of the cross, or the tail of the Swan, whichever shape you find easiest to see!  Vega and Deneb are the upper two stars of the Summer Triangle, above the third star, Altair, the brightest star in Aquila the Eagle.  The Eagle is flying up the Milky Way as the Swan is flying down.  

The bright star in the north is Capella in Auriga, the Charioteer, which will take the place of Arcturus high overhead as winter comes on.  Arcturus is one of the brightest stars in the sky, the very brightest north of the Ecliptic, and was accorded a special rôle in the astrology of the ancient world.  It’s one of the few stars mentioned in the Bible, and often was regarded not as part of the constellation but as an entity in its own right.

In Britain Arcturus is dominant in the sky during summer – especially in Scotland, where twilight lasts throughout the night in June and July.  Boötes, the Herdsman, guards the North Pole as Orion guards the Equator and the southern sky.  It’s so bright because it’s only 41 light-years away, and as a result it has one of the largest Proper Motions known, crossing the apparent diameter of the Full Moon in only 800 years.   In Punch, 1907, a poet wasn’t impressed by that – “For a star of your parts, you’re confoundedly slow” – but it’s one of the few stars to have changed its position in the sky significantly, in the 13,000 years since Vega in Lyra was the nearest bright star to the Pole, half-way round the processional cycle from when Polaris stood there last.   As a red giant 26 million miles across, Arcturus must already be frying its inner planets

Altair is the southernmost star in the figure of the Summer Triangle, making an isosceles triangle with Deneb in Cygnus and Vega in Lyra.  Many people suppose the Summer Triangle to be an ancient asterism, but it was actually devised by Patrick Moore in the 1950s, by analogy with the much older Winter Triangle of Procyon, Betelgeuse and Sirius.   Altair is just 16 light-years from us and featured in the film Forbidden Planet  (1956), based on The Tempest and set on the imaginary planet of Altair IV, starring Walter Pigeon and introducing Robbie the Robot.   One of the most interesting features of Forbidden Planet which most critics get wrong is that the rôles of Caliban and Ariel are reversed:  Robbie, as Caliban, is good and non-violent, while the disembodied sprite is a vengeful monster.   

But Altair is too massive and too short-lived for intelligent life to have evolved there.   It spins so quickly, about 300 kilometres per second at its equator, that its shape is distorted, and the star is a full 22 percent wider than it is tall.  In 2007 its disc was imaged by the Mount Wilson Observatory, confirming the oblong shape, but with surface temperature patterns slightly different from those predicted.    

Vega is the 5th brightest star in the sky, 26 light-years from us.  It’s an A0-type star like Sirius, 49 times brighter than our Sun, and like Sirius, it’s therefore too massive and too short-lived for intelligent life to evolve there;  in any case, because of its very high surface temperature, Vega emits intensely in ultraviolet and x-rays, enough to overwhelm the ozone layer of any planet like Earth.  Like Altair, Vega rotates so rapidly on its axis, every 12.5 hours, that its equator is 23% wider than its polar diameter.   As with the cloud tops of Jupiter, Vega’s poles are therefore much hotter than its equator.   Its stable lifetime would be only 400 million years.   In fact, it’s so young that it’s one of the stars around which proto-planetary discs were detected by the Infra-Red Astronomy Satellite, IRAS, in 1983.   The disc extends out to 80 Astronomical Units, twice the distance of Pluto from our Sun, but the area near Vega is clear and may already have been swept out to form planets.   Fomalhaut and Beta Pictoris have similar discs.

It’s curious that the sky in British latitudes should seem so balanced through the year, with Vega skirting the northern horizon when Capella is overhead in winter, and with the Summer Triangle climbing towards the zenith as the winter one disappears.  The relationships are coincidental:  when Stonehenge was built, and the pole star was Thuban in Draco, Capella was not circumpolar in Britain and disappeared below the northern horizon for part of the year.  Jim Barker’s star maps are drawn for Europe, a little south of the British view, but if he had drawn them ten degrees further south for the mid-USA, then even the Great Bear  (‘The Big Dipper’) would be partly off the map during the winter. In the ancient world, no relationships in the sky were coincidental: everything had to be significant in itself and in relation to the earth below.  If the winter sky was ruled by Orion, the Hunter, then he had to have his counterpart in summer, the Herdsman, and there had to be a mystical identity linking the two.  It couldn’t be chance that both constellations contained a bright red star – Betelgeuse in Orion, Arcturus in Boötes – and in ancient texts the names are sometimes interchanged.  Arcturus was the guardian of the pole, holding the bears in check: like Orion he has his hunting dogs, Canes Venatici, under the paws of the Great Bear and driving him ahead of the Herdsman throughout the summer

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