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

Wednesday, 8 January 2014

Hunting the Hunter – Observing Orion

Continuing my efforts to grasp with my feeble mind the things that incredibly clever people have worked out using only a few basic instruments, in order to bring more meaning to my own blundering observations of the heavens, I now turn my attention to the constellation of Orion.
Orion Nebula as drawn by Charles Messier 1771

The great hunter currently bestrides the firmament just above the shed roof, bow in hand; one of  the most instantly recognisable features of the winter sky. As such it has always enjoyed a special significance in every culture that has pondered the stars and sought to discern their own destiny in those distant, glimmering points of light.

In Ancient Egypt the constellation of Orion was associated with Osiris; the resurrected lord of the underworld. Within the Great Pyramid of Khufu, two ‘star shafts’ are angled upwards from the main burial chamber leading to the outside of the pyramid. The southern shaft is aligned with the centre of the constellation of Orion, which is associated with Osiris, at the same time as the northern shaft is aligned with Alpha Draconis; the star which at the time that the pyramids were constructed would have constituted the celestial pole. It is perhaps coincidental but the function of these shafts may have been to allow the spirit of the dead pharaoh to set out on and return from its celestial journeys.
For the Maya too the constellation of Orion represented a major deity; Hunhunahpo, the Great Father, who is sacrificed following a celestial ball game and whose blood fertilises the earth. Like Osiris the myth of Hunhanahpo is bound up with the cycle of sowing and reaping and sowing anew; of life and death and resurrection. The Maya were keen observers of the heavens with structures at various Mayan sites tentatively identified as observatories. Within the constellation of Orion the Maya traced a triangle between the two stars which form  the ‘feet’ Rigel and Saiph and the first star of the belt Alnitak. This triangle was described as the celestial hearth and at its centre the Maya identified a feature that they referred to as the smoke of the hearth; making them the first people to record an observation of the Orion Nebula.
The Observatory at Tulum as drawn by Frederick Catherwood 1844
The Mayan civilisation was long fallen into ruin by the time that the first European observer recorded a sighting of the Orion Nebula. Nicolas Claude Fabri de Peiresc was the archetypal enlightenment gentleman amateur scholar with a fascination for everything from the fossils beneath the earth to the stars in the heavens. His sprawling country home near Toulon featured an enormous garden filled with exotic plants and a menagerie of animals. By the end of his life he had amassed a correspondence of over ten thousand letters with around five hundred intellectuals of his day from all over Europe. Inspired by the exploits of Galileo and keen to see for himself the moons of Jupiter described in Sidereus Nuncius, De Peiresc had an observatory constructed and obtained one of the new fangled telescopes. In 1610 De Peiresc made a discovery of his own when he beheld through his new telescope the Orion Nebula. Indeed he coined the term ‘nebula’ to describe the cloudy phenomenon.

Nicolas Claude Fabri de Peiresc
 Having established himself as an astronomer of note De Peiresc then turned his attention to one of the vexing problems of the age; the accurate calculation of longitude. Seizing upon the opportunity of a lunar eclipse in 1635, De Peiresc on his own initiative dispatched agents to Rome, Cairo and Aleppo in order to record simultaneous observations of the event. From his resulting calculations De Peiresc was able to provide a revised estimate of the length of the Mediterranean, reducing the previous figure by an astonishing 1000km, not that they had kilometres back then! In the following year he produced the first recorded map of the surface of the moon.

The nebula that De Peiresc had discovered attracted the fascination of other notable astronomers. Christiaan Huygens produced a diagram of the nebula in 1656 and it came to the attention of the great cataloguer of nebulous objects Charles Messier in 1769. Messier's drawing of the nebula, which he designated M42, appears at the top of the post. Messier was primarily concerned with the search for comets and his cataloguing of other deep sky objects was intended to assist in this enterprise. The question remained however; just what were these nebulae made of? In 1814 the invention of the spectroscope by the expert lens maker Joseph von Fraunhofer presented the means to investigate the nature of nebulae.

Joseph von Fraunhofer demonstrates the spectroscope
Combining the spectroscope with a telescope allowed the emission spectra; that is the emission of light by hot gasses at specific wavelengths corresponding to their chemical composition, of various celestial bodies to be studied. In 1865 English astronomer William Huggins, working alongside his wife Margaret, used this method to study the Orion nebula and declared it to be composed of ‘luminous gas'. Huggins was puzzled by the emission spectra that he obtained from his studies of nebulae as some of the emission lines did not correspond to any known element. He therefore proposed the existence of an entirely new one – Nebulium. Sadly it was later shown that the mysterious spectra corresponded to doubly ionised oxygen and that nebulas are not made of nebulium. Sometimes the truth can be so disappointing. In the course of his studies Huggins also observed the phenomenon of ‘doppler shift’ in the spectral lines of celestial bodies either towards the red or blue end of the spectrum depending on whether they were moving closer to or further away from the earth, which would later be seized upon as evidence of the fact that the universe was expanding.

Sir William Huggins
                The constellation of Orion also boasts another notable feature in the red giant star Betelgeuse. This behemoth of a star became in 1920 the first star, other than the sun, to have its size successfully determined. Now maths was never my strongpoint and years of reliance on spread sheets and calculators have caused my mathematical ability to regress to the point that I would probably fail a maths test for ten year olds, so I am not going to get into this too much but suffice to say some very clever chaps worked it all out.

                The measurement was achieved through the use of an interferometer; a device which allows a light source to be split into separate beams and then re-converged in a single image. This allowed the observers to overcome the spurious interference which blurred and distorted the image of the star in the telescope.

Hooker Telescope with Michelson interferometer 1920
Making use of our old friend the Hooker Telescope at Mount Wilson Observatory California, Albert Michelson and Francis Pease mounted a twenty foot wide beam onto the front of the telescope upon which four six inch mirrors were arranged; two in front of the aperture angled at 45 degrees and two placed parallel opposite them at either end of beam. By observing Betelgeuse through the telescope and adjusting the distance between the two outlying mirrors until the image generated was free of interference, Michelson and Pease were able to accurately measure the angular diameter of its photosphere. From this, knowing its distance from the earth, they were able to determine its actual size at some 240 million miles across. See the links below for a full explanation. To put that in perspective, as the earth is 93 million miles from the sun if Betelgeuse was at the centre of the solar system it would engulf Mercury, Venus, Earth and Mars and extend almost to Jupiter. Blimey!

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