Thursday, 23 January 2014

In the sky with diamonds – observing the Pleiades

Returning home at midnight the other week to find beautifully clear skies I braved the freezing night air and got out the trusty  ‘scope. Wrapped in two fleeces and wearing a woolly hat which is slightly too small and makes me look like I might feature in Last of the Summer Wine, I cranked it skywards and directed my gaze towards the twinkling cluster of the Pleiades. As my view filled with bright young stars, burning with blue fire and swathed in ghostly nebulosity I reflected that they are perhaps the prettiest sight in the heavens and that if Lucy is indeed in the sky with diamonds, then she must live in the Pleiades.
 I was unsurprised to learn that the Pleiades have been regarded as a significant feature of the heavens from the earliest times but this story nevertheless begins in an astonishing place, on a German hilltop in approximately 1700 BC.
In 1999 two treasure seekers illegally unearthed an extraordinary object on the Bronze Age site of Mittelberg in the Ziegelroda Forest in Saxony. Being rascals, the pair sold the artefact, which has come to be known as the Nebra Sky Disk, on the black market. Three years later it was recovered along with the two splendid Bronze Age swords that had been found alongside it and its discoverers were jailed for looting.
The Nebra Sky Disk
The shield-like object is crafted from bronze and decorated with gold and depicts a starry firmament which features, it is believed, images of the sun and moon, the seven clustered stars of the Pleiades and the planets Venus, Mars and Mercury. On the sides of the disk, two golden bows, of which only one remains attached, corresponded to the horizon at an angle of 82 degrees and are believed to mark the rising and setting positions of the sun at the summer and winter solstices. The disk also depicts what is believed to be an image of a ‘solar boat’ similar to that featured in Ancient Egyptian belief which carries the sun through the hours of darkness.
Through analysis of lead isotopes in the bronze and of the formation of malachite in the patina on the surface of the disk, the object has been dated to between 2100 and 1700 BC. The arrangement of sun and moon, Pleiades and planets furthermore corresponds to their alignment during an eclipse of the sun which would have taken place on April 16th 1699 BC, as it would have appeared from the latitude of Mittelberg. So what we have in the Nebra Sky Disk is nothing less than a snapshot of the sky at a moment in distant history recorded for posterity by people whose names we will never know and for reasons we can only guess at.

The Sky Disk was originally thought to be a calendar due to the coincidence of the rising and setting of the Pleiades with the key times for sowing and reaping during the agricultural year. The idea is discredited however by the absence of evidence for an agrarian society in the forests of Bronze Age Germany.

Bust of Hesiod - British Museum

In Ancient Greece, on the other hand, the Pleiades enjoyed just this significance as is made clear in Hesiod’s Works and Days, a poem filled with practical advice set down in approximately 700 BC.

When the Pleiades, daughters of Atlas, are rising begin your harvest, and your ploughing when they are going to set. Forty nights and days they are hidden and appear again as the year moves round, when first you sharpen your sickle. This is the law of the plains, and of those who live near the sea, and who inhabit rich country, the glens and dingles far from the tossing sea. Strip to sow and strip to plough and strip to reap, if you wish to get in all Demeter's fruits in due season, and that each kind may grow in its season. 

The rising of the Pleiades also marked the beginning of the sailing season, when ships could put to sea in expectation of good conditions and not fear storms and shipwreck. Once the Pleiades had set, sailors set out at their peril.

But if desire for uncomfortable sea-faring seize you; when the Pleiades plunge into the misty sea to escape Orion's rude strength, then truly gales of all kinds rage. Then keep ships no longer on the sparkling sea, but bethink you to till the land as I bid you. Haul up your ship upon the land and pack it closely with stones all round to keep off the power of the winds which blow damply, and draw out the bilge-plug so that the rain of heaven may not rot it. 

In 1891 Victorian architect, keen amateur astronomer and man-with-shed Francis Penrose (pictured above) declared his belief that the Parthenon was aligned with the rising of the Pleiades. Penrose dedicated himself to the painstaking measurement of the Parthenon and was the first to discern the architectural trick of entasis deployed in the structure; the almost imperceptible bulging of the columns towards the base which serves to make them appear perfectly straight to the observer. That the great seafaring city of Athens should have chosen to align its greatest monument with the rising of the asterism that signalled the return of fair winds makes much sense, although Penrose’s theory is not universally accepted.

In myth the Pleiades were the daughters of Atlas; seven sisters transformed into stars, pursued endlessly through the heavens by the huntsman Orion. Seven sisters are named; Electra, Merope, Maia, Taygete, Alcyone, Celaeno and Sterope, but only six stars are now visible to the naked eye. This gives rise to the theory of the ‘lost Pleiad’; (imaginatively depicted left by William Bouguereau) a star which in antiquity was bright enough to be seen but which subsequently reduced in brightness. It is uncertain which individual stars in the Pleiades the ancients applied the names above to, so this may have been different from the modern application. Mythology suggests a couple of candidates for the ancient name of the lost Pleiad.  Electra is said to have covered her face in mourning at the fate of Troy whilst Merope was the only Pleiad to wed a mortal; Sisyphus, for which she was shunned by her sisters. The most likely star to be the missing Pleiad is the rapidly rotating star known today as Pleione; the mother of the seven. Pleione exhibits considerable variability in its brightness over time and thus may have been visible back in the days of Hesiod.

In the Second Century BC the philosopher Hipparchus of Nicaea, (imagined right by Raphael) who had toyed with the idea of a heliocentric universe a millennium and more before Copernicus before dismissing it in accordance with the ancient belief that the orbits of heavenly bodies must be perfectly circular, set out the earliest known catalogue of the positions of the stars in the western world. In 1718 Edmund Halley revisited Hipparchus’ observations and discovered that a number of stars had shifted in their positions relative to the solar system as they moved through space. This phenomenon, known as ‘proper motion’ was subsequently enthusiastically studied.
In 1846 the German astronomer Johann von Maedler, (above) best known for creating the first accurate map of the moon, concluded that as the stars of the Pleiades showed no discernible proper motion relative to each other, they must constitute an unmoving central point around which all the other stars turned and breathlessly posited the star Alcyone as the very centre of the universe!

Well perhaps they are not quite that significant but the Pleiades continue to intrigue and fascinate and remain a telescopic treat.
Pleiades photographed by Isaac Roberts 1888

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