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Photographs of our Double Sunset

If you plan to visit Leek on the summer solstice to see the double sunset for yourself, make sure to bring your camera to get some great photos for your scrapbooks or photo books. You can even try photo books online to share your double sunset photos with family and friends. 

 

Dr. Plot and the Amazing Double Sunset.

 

In the Autumn of 1998, an amateur astronomer from Bollington, Kevin Kilburn, investigated the curious double sunset visible on Midsummer's Day from the churchyard of Leek's Parish Church, St.Edward the Confessor. His initial findings were published as a paper regarding historical astronomy in the February 1999 edition of Astronomy & Geophysics, the Journal of the Royal Astronomical Society. Since then, in January 2000, a Radio 4 interview generated additional public interest and in May 2001, the double sunset was featured in 'Peak District Magazine' and on BBC Radio Stoke, Friday, 20 June 2003. The material presented below was originally three separate articles written since 1999, but has recently been updated.

(C) Leekonline

Dr.Plot and the Double Sunset.

 

The first and perhaps best account of the double sunset is found in Dr.Robert Plot's book, 'The Natural History of Staffordshire', published in 1686. (There is a reprint in Leek Public Library.) Dr.Plot was the first Keeper of the Ashmolean Museum and Professor of Chymistry at Oxford. He described how for a day or two either side of Midsummer's Day, the sun was observed from the churchyard to set behind Bosley Cloud and then reappear in the hollow of the hill's almost vertical northern side before setting again. Plot illustrated his comments with a woodcut showing the hill and the emerging sun. In his illustration the northern aspect of hill appears much as it does today, allowing for a little artistic license that drastically truncates the Cloud on its leftward side.Another woodcut illustrating the double sunset appeared in the 'Gentleman's Magazine', in July 1738. In this, perhaps the most famous historical illustration of the phenomenon, artistic license runs amok with the hill being shown much too pointed and the setting sun describing a low inclination path that in reality is very much steeper. However, the text accompanying the illustration is quite valid... "...This hill is so situated with respect to the Churchyard of Leek, that a spectator standing there of an evening three or four days before the 10th of June, beholds the disk of the sun gradually emerging from beyond the northward side of the hill, which is nearly perpendicular..."It is worth noting that 1738 is before the introduction of the Gregorian calendar in 1752, which added eleven days to the date of the summer solstice, now 21st June. Curiously, even the adoption of the Catholic Gregorian calendar in the UK is directly attributable to a descendent of a notable Leek family. Thomas Parker was born in Leek in 1666 and went on to become Lord Chief Justice of England, the first Earl of Macclesfield, and then Lord Chancellor. He financed the building of Leek Grammar School in 1723. This was shortly before being found guilty of financial misconduct and his forced resignation. He later spent some time in the Tower of London for failing to pay a £30.000 fine! His son, George Parker, the 2nd Earl of Macclesfield, was an amateur astronomer and a friend of the Astronomer Royal. George Parker was largely responsible for pushing through parliament the 'Bill for regulating the Commencement of the Year'. This had the effect of not only introducing the Gregorian calendar to the UK in 1752 but also of moving the start of the financial year from 26th March to the present 7thApril.From the church, the Cloud presents a very distinctive profile but probably not quite the same as that seen by Dr Plot, although much closer to his illustration than that of 1738. Quarrying in the early nineteenth century certainly demolished a considerable portion of the northernmost side of the hill including a tall column known locally as 'Bully Thrumble'. Its stone was used in the construction of the Macclesfield Canal and for the aqueduct over the River Dane. The net effect was probably to 'round off' the hill contours but the overall profile is still recognisably that drawn in Plot's book.Of astronomical significance, from the historical perspective, Robert Plot proposed using the Leek sunset to measure changes in the angle of the Earth's tilt, the obliquity of the ecliptic. This is presently about 23.5 degrees but varies with a cyclical period of about 41,000 years and is currently reducing by about 47 arc-seconds (47/3600 of a degree) per century. However this tiny change is having a long-term effect. In prehistoric times the midsummer sun was higher in the sky and set somewhat northward of its present position and before about 500BC, in Iron Age times, the double sunset could be seen twice each year from the position of the church, firstly as the setting sun moved northwards along the horizon towards the summer standstill and then as it reversed. Since then, because of the reducing tilt angle, the solstitial sunset has slipped southwards along the horizon and will continue to do so for thousands of years. By using a telescope fitted with an eyepiece micrometer, Plot proposed that measurements of the amount of the solar disk reappearing in successive years would allow the long term variation in the setting point to be measured and hence the change to the obliquity of the ecliptic to be quantified.In the late 17th century the double sunset was relatively easy to see, weather permitting, from any position eastwards of roughly midway along the back wall of the churchyard. Indeed, the small outbuilding on this wall would have been in an ideal position from where to carry out the proposed observations. But since then, the ongoing reduction in the obliquity has caused the observing point for the reappearance to move towards the extreme northeastern corner of the churchyard, the so-called Doctors' Corner. Nevertheless its elusive reappearance has been occasionally reported down the centuries to within the past twenty five years. The last time the reappearance was seen was in 1977.During the 20th century the double sunset has rarely been seen, almost certainly due to inclement weather or local atmospheric conditions in the line of sight, yet as recently as 1977 the double sunset, including the brief reappearance, was still observable from the churchyard. Simple geometry shows that those observations of twenty odd years ago could still be repeated if the observer was to move a scant four feet eastwards. There is still a bit of room to maneuver before the vicarage garden wall prevents further pursuance of the elusive conclusion to this unique phenomenon.Many visitors go to St.Edward's on Midsummer's Day to watch for the double sunset but since the late 1970s trees have obscured the view. At the summer solstice in 1999, blessed by good weather and astro-geometrically ideal, a concerted effort was made to quantify the present geometry of the double sunset from its classical observation point, the churchyard of St. Edward's. Members of the Manchester Astronomical Society, Macclesfield AS and at least one astronomer from Jodrell Bank endeavoured to document the famous sunset. Photographs and video footage were taken from the churchyard, the church tower and from the field immediately north of the church. After several months of analysis, the evidence collectively confirmed that although the reappearance was still, theoretically, visible from the extreme NE corner of the upper churchyard, trees in the lower cemetery, allegedly planted in the 1960s to deliberately obscure a 'pagan' sightline, now prevent the reappearance from being seen. Nevertheless, it has been possible to fully justify and quantify Dr Plot's astronomical predictions.Eventually, within perhaps as little as twenty five years, the geometry dictates that from its classical observation ground the sun will not reappear beyond the Cloud having once set upon the summit. Sadly the double sunset will then cease to be visible from the churchyard for well over twenty thousand years. There is still time to see it, provided the obscuring tree is trimmed.It has been pointed out that a double sunset against Bosley Cloud is visible from elsewhere in Leek. This is to be expected from any reasonable vantage point along the solstitial sight-line. But the temporary hiding of a sunset by a hill or building or tree is certainly not exceptional. What makes Dr. Plot's Amazing Double Sunset so unique is that it can (or could) be seen from such an ancient place of worship.

(C) Kevin J Kilburn, FRAS.
June 2003

 

Map showing viewing point and car parks

The map shows where to park and where to go to see and photograph the
summer solstice double sunset for your photo books.

Dr Plot's woodcut showing the distant hill and the emerging sun.

Woodcut from the Gentleman's Magazine showing the double sunset.
The Astronomy of Leek's Double Sunset

 

Dr.Plot suggested in 1686 that the double sunset, or more particularly the partial reappearance of the sun from behind the northern end of Bosley Cloud, could be used to measure the changing obliquity of the ecliptic.

The earth's axial tilt varies between about 24.5 degrees and 22.5 degrees. This is because Earth is somewhat pear-shape, with the southern hemisphere being slightly bigger than the northern one. Gravitational attraction of the sun and planets results in an uneven pull on the earth's northern and southern hemispheres and results in a slow wobble of the rotation axis that takes about 41,000 years to complete a cycle. The Moon's gravitational attraction tends to dampen out that of the other solar system bodies and attenuates the changing obliquity.

The earth is presently tilted at an angle of 23.438 degrees but this has not always been the case. The angle has been decreasing since prehistoric times. In 2000BC it was 23.924 degrees and in AD 6000 it will be about 22.951 degrees. This changing angle is now fairly well understood and can be determined as being about 47.7 arc seconds per century. In Dr.Plot's day it was a relatively new discovery and quite difficult to measure.

Changing obliquity must be considered separately from the generally more familiar 'Precession of the Equinoxes'. The Platonic Year or Great Year takes 25,800 years to complete a full rotation. It is the largest of the Earth's 'wobbles' in which each end of the Earth's spin axis sweeps out a cone, exactly like a spinning gyroscope or toy top. Axis precession is quite noticeable and has been known since classical times. Precession rotates the direction of the earth's axis with respect to the stars and changes the seasons by changing the apparent position of the Vernal Equinox, when the sun moves from the southern celestial hemisphere to the north, now on or around 21st March.

Seen against the background of zodiacal stars, the sun's position at the Vernal equinox moves steadily in reducing Right Ascension at a rate of 502.9 arc seconds per century. This also slowly changes the dates of the solstices and equinoxes. The summer solstice, when the sun is highest in the noonday sky, is now on 21st June but in AD1500 it was a day later, 22nd June and in AD2500 it will be a day earlier, 20th June. This effect accumulates as one goes further back or forward in time.

Precession also determines the position of the 'pole star'. For practical purposes, this is the star that happens to be closest to the projected northern end of the rotation axis, the North Celestial Pole, around which all the others appear to rotate during the course of an evening. Nowadays it is the brightish star, alpha Ursae Minoris, more commonly known as Polaris. Mariners have used Polaris for estimating North for many centuries because it can be seen from anywhere north of the earth's equator.

Polaris is presently very close to the NCP and is a reliable indicator of North. A thousand years ago, Polaris was six degrees away from true north and five thousand years ago, it was nowhere near the NCP. The star, Thuban, in the constellation of Draco, was the northern pole star during the Bronze Age, ca 2100BC, at the time of the Egyptian Dynasties. Thirteen thousand years from now the Pole Star will be the bright, blue-white star, Vega. There is currently no bright pole star in the southern hemisphere.

In 1675, just six years before Dr. Plot visited Leek, Charles II founded the Royal Observatory at Greenwich. This was in the age when Britain ruled the waves and long voyages of exploration and trade were commonplace to the Americas, to the Orient and to the Pacific. Accurate celestial navigation was of supreme British national importance and could certainly be regarded as the 17th century equivalent of the modern strategically important American GPS (global positioning system) used to target cruise missiles! The Rev.d John Flamsteed, the 28 year old son of a Derby brewer, from the village of Denby, near Belper, became the Royal Observatory's first director and the first Astronomer Royal. He was charged with the task of accurately measuring the position of the naked-eye stars so that they could be used for navigation. A by-product of this research was to refine and more accurately quantify both precession and changes to the Earth's obliquity.

Flamsteed and Dr.Plot were both Fellows of the Royal Society (Plot became its Secretary in 1682) and they almost certainly knew one another. Robert Plot clearly knew his astronomy and was pretty much up to date with a broad spectrum of contemporary science. He was also a trained surveyor and was aware that if an experiment was set up in the churchyard to annually measure the setting sun cutting the edge of the hill... "the sun's greatest northern declination (which astronomers say is less now than heretofore) may be gradually adjusted, and at length perhaps limited".

Consider his reasoning. The most direct way of measuring the angle of the earth's axial tilt is to measure the maximum height of the sun in the noon sky at the solstice. But there are practical difficulties. It is only possible to measure the sun's position with respect to a known datum and at Greenwich this was with respect to the horizon or celestial pole using a graduated metal arc equipped with telescopic sights and capable of being swung vertically on the side of a masonry pier fixed in the plane of the meridian and accurately set up using plumb bobs and distant horizon markers. By 1689, Flamsteed's latest and greatest 7-foot mural arc (paid for out of his own pocket, when he inherited money at the time of his father's death) could be read to less than a second of arc, 1/3600th of a degree. But there was another other difficulty, the sun is a large object, half a degree in diameter and blindingly bright. It is very difficult to see where its edge lies for measurement purposes and for Flamsteed to work to the accuracy needed it was nigh on impossible.

However, Robert Plot proposed that from the Leek churchyard, a trained observer could measure the maximum northerly setting point of the sun, using the near-vertical north face of the Cloud as a datum line, with a suitable instrument equipped with an eyepiece micrometer to measure the projecting portion of the solar disk beyond the hill. He would be observing the edge of the sun whose brilliance was dimmed by passage through thicker layers of the atmosphere. As over the years the reduction of obliquity would cause the setting point to regress southwards, it would eventually allow calculation of the change to the tilt. Of course Plot also understood that so close to the horizon atmospheric refraction would have to be taken into account. So would air pressure but it is not known if he was aware of this important effect.

As far as is known, Robert Plot's idea to measure the changing obliquity from the churchyard at Leek was never carried out. In all likelihood, the weather dependent solstitial reappearance of the sun was simply too hit and miss. He was right in one thing though, it would have taken many years of painstaking observation, bedevilled by bad weather (the English climate was particularly rainy in the late 17th century), to even begin to gain meaningful results.

But in order to attempt to quantify the phenomenon at Leek, let's examine the geometry of the reappearance in more detail. The northern aspect of the hill is not vertical, it is largely concave and a line drawn across this hollow closely parallels the setting sun angle. By definition, to reappear, the sun must first disappear and then reappear within this concavity. The depth of this concavity dictates the time span during which the disappearance-reappearance phenomenon can be observed from any fixed observation point.

Think of it another way: if the solstitial sunset could be 'frozen' just as the sun's disk was reappearing an observer at the churchyard could, by moving along a line drawn diagonally across the churchyard 'control' what he saw. If the sun was just reappearing, by moving NE the observer would see a larger and larger portion of the disk revealed from the concavity until a point was reached where the sun's edge would not have actually disappeared on setting. Conversely, if he moved SW the sun's disk would be increasingly hidden from view until it was no longer seen to reappear. This applies constraints and defines an observing 'window' in which the double sunset phenomenon, disappearance and then partial reappearance, is possible.

The depth of the hill's concavity is about 6 arc minutes (360 arc seconds) and the obliquity is reducing by 47.7 arc seconds per century at right angles to it. So the double sunset lasts for about 750 years from any fixed place in the churchyard [360/47.7]. At 10.5km from the hill, this observing 'window', from the NE leading edge to the SW trailing edge, is about 35 meters (110ft) wide and owing to the changing obliquity of the ecliptic it has scanned slowly north-eastwards across the churchyard since the Iron Age.

About two thousand years ago, the midsummer sunset cast for the first time the double-edged shadow of the Cloud upon what is now the southwest corner of the churchyard. A thousand years ago, the shadow fell on the newly built church itself. In Dr Plot's day, three hundred years ago, the shadow had moved northeastwards due to the changing obliquity of the ecliptic that he wanted to measure, and by the end of the second millennium the trailing edge had reached the extreme northeastern corner of the churchyard. Only from the Doctor's Corner is the reappearance still, theoretically, just visible. Yet to duplicate the geometry of 1975, an observer in AD2001 has to step only 0.8m (2ft6in) north-eastwards, at right angles to the line of sight.

Negative observations can often be as important as positive ones. At least the 1999 solstice gave an opportunity to record the present geometry from several places in and around the site. Although not definitive, it has allowed a better understanding of the phenomenon and has opened up opportunities for further study.

One interesting observation was made in June 2000. The setting sun was sinking behind distant clouds level with the top of the hill. In high power binoculars (20x60), an aircraft contrail (but not the aircraft) was seen near the sun at the same elevation. It then was realized that considering that aircraft typically cruise at some 30-35,000 feet, the sun would be obscured by any cloud lower than this, a realistic cloud ceiling. Subsequent calculations showed that at the elevation of Bosely Cloud, ¾ degree as seen from Leek church, it was necessary for there to be clear sky below 35,000 feet along a line-of-sight roughly 200 miles long. That is, no cloud between Leek and a point roughly between The Isle of Man and Stranraer. This could explain why the double sunset was so rarely seen during the 19th and 20th centuries when air pollution from the Lancashire mill towns could easily have affected visibility of the sunset.

Archaeoastronomy

 

The church dates to the early 14th century but there are references to a building in pre-Norman times that was destroyed by fire in 1297. Although there is no direct evidence in the form of artifacts, because the immediate area has never been excavated, the hilltop on which St. Edward the Confessor stands may have been a place of worship for five thousand years. This argument may be strengthened by the name of an old house nearby, called Foxlowe. The place name, 'low', a hill, is commonly used in the moorlands and is often associated with prehistoric sites set on hilltops.

The precision of St.Edward's solstitial alignment to the Cloud is better than the famous midsummer sunrise alignment above the Heel Stone at Stonehenge on the same day of the year. It also matches the deliberate orientation, towards the midsummer sunset, of Arbor Low, the "Stonehenge of the North", nine miles south of Buxton. Judging from the orientation of many Neolithic tombs and early Bronze Age monuments, our prehistoric ancestors had a basic appreciation of the cycles of the sun and moon that was apparently associated with their religious beliefs. In particular, the period between about 3200BC and 2500BC saw the construction of monuments within so-called 'sacred landscapes' in which astronomical alignments often featured. The location of the church may indicate one such site from where in prehistoric times, the sun set directly behind the tip of the distinctive silhouette of the hill.

In 1862, John Sleigh, in his book, 'History of the Ancient Parish of Leek, in Staffordshire', recorded that an ancient burial mound, six yards high and forty yards in diameter, was excavated on what is now the Westwood estate. On 29th December 1851, cremated human bones were found together with pieces of a burial urn. Fragments of the urn are still kept at Leek's Nicholson Institute, in a room above the Public Library. More work needs to be done, but it has been suggested that this part of town was once occupied by a Neolithic or Bronze Age settlement of which Cock Low was its last remaining evidence, flattened a hundred years ago by urban development. Cock Low is long gone but was at the eastern end of the playground in Spring Gardens. From there the double sunset was never visible, but in the early Bronze Age, the midsummer sun did set above a shallow depression on the Cloud, the Bridestones 'notch', in which sits an even older megalithic long barrow.

Since June 2001, discoveries of other solstitial alignments further support the idea of archeo-astronomical activity in the area. Overlooked four years ago was the fact that extending a line from Cock Low through the Bridestones, roughly intersects the village of Marton, on the A34 northwest of Congleton. The idea of 'ley-lines' connecting places of archeological interest is nearly a century old but has no recognised archaeological basis. Nevertheless, the Leek-Bridestones-Marton line can be drawn and was worth investigating.

From the churchyard of St. James & St. Paul, Marton, the mid-winter sun now rises over Bosley Cloud on 21st December. More particularly, it can be calculated that from the site of that beautiful half-timbered14th century church, standing on a low mound of uncertain origin, the winter solstitial sunrise ca 2500BC was over the very obvious 'notch' in the Cloud in which the Bridestones lie. 

In recent e-mail communication from Dr. Jill Collens, Archaeological Officer at the Cheshire County Sites and Monuments Record office (CSMR), two Bronze Age finds have been identified within 2km of Marton but there are dozens of other sites and finds within a10km radius of Bridestones. This, together with the Cock Low alignment, strongly supports the idea of Bridestones being the focus of early Bronze Age solstitial solar alignments.

Further evidence was forthcoming later in the autumn of 2001 when it was pointed out that the prominent hill, Mow Cop, also thought to be a prehistoric site (although no evidence to support this), is visible from Bridestones. More particularly, calculations showed that on mid-winter's day the sun would set on or very close to the summit of the hill when viewed from the stone circle or forecourt of the Bridestones monument. This demanded further investigation as it had been of some puzzlement as to why the Bridestones are sited where they are, facing westwards below the summit of the saddle between the Cloud and Long Edge and yet placed awkwardly on rising ground on the north side of the dip.

3D computer maps, capable of being rotated in different directions, were constructed of the area around Bridestones and Mow Cop and used to study the landscape geometry. Together with basic trigonometry and planetarium software to allow calculation of prehistoric sunset points, and assuming that visibility of the winter sunset was important to the prehistoric builders, the positioning of Bridestones now makes sense and fits into a broader archaeoastronomical context. 

On mid-winter's day the sun does indeed set on Mow Cop as seen from the Bridestones forecourt. About five thousand years ago, when Bridestones was constructed, the sunset point was about two solar diameters left of the present point and closer to the summit of the hill. But the siting of the monument is more refined, as any visitor to the site can now appreciate. Leaving your car on Dial Lane and walking northwards along the driveway of Bridestones Farm towards the gate to Bridestones, Mow Cop comes into view when only halfway along the driveway. Before then, Mow Cop and the solstitial sunset is hidden by the nearby hill to the south. This imposes a constraining NE-SW sightline on the landscape southeast of which Mow Cop cannot be seen. A similar constraining sightline, running SE-NW and perhaps not so tight, extends towards the summer solstitial sunset. This is more difficult to calculate nowadays because of the high trees immediately N of the monument blocking the view. In the 18th century these trees were not present and in Neolithic times could also have been cut back. Archaeological evidence elsewhere suggests that localised deforestation was practiced and from Bridestones trees on more distant horizons would have had a negligible effect on solar alignments. 

Bridestones certainly commands a good view of the Cheshire plain and the midwinter sunset on Mow Cop. It is argued that the summer solstice was also important to the Neolithic builders but complications would have arisen if the monument had been placed further north on rising ground towards the Cloud (or lower down the slope, westwards from where the midwinter alignment would have been lost). On the midsummer azimuth, nearly a degree further north in 3000BC than at present, a projecting ridge running SW from the summit of the Cloud and overlooking Timbersbrook would have blocked the view of the midsummer sunset. The Mow Cop alignment is again affected. Yet sited just where they are, Bridestones gave their builders an unobstructed view of both solstitial sunsets and all sunsets in between, throughout the year.

Archaeoastronomy is still a contentious subject and it is difficult to prove that astronomical alignments were deliberate, yet this is apparently what we see regarding those at Bridestones. It should be made clear that the monument was not a prehistoric 'observatory' but simply a ritualistic burial site incorporating astronomical alignments. There is another site, a late Neolithic stone circle at Brickbank Farm, Henbury near Macclesfield, which also had a winter solstitial sunrise alignment against Sutton Common. This was pointed out nearly thirty years ago by a local archaeologist, Gorgon Rowley.

There remains one question, why? It is likely that a small hunter-farming community ca 3000BC constructed Bridstones overlooking the Cheshire plain to serve as a resting place for their dead and perhaps to mark a boundary of their territory. It may also have been located away from their main habitation as a special place of ritual to which they processed at certain times of the year. One thing is certain: At a time when the total population of Cheshire was probably no more than 5000 people and considering that a cairn allegedly100m long,10m wide and perhaps 3-4m high contained thousands of tonnes of stones quarried nearby on the Cloud, very many man-hours went into its building. It was a very important place. Elsewhere in Britain and Ireland there is growing evidence to suggest that the setting sun was significant in prehistoric burial rites and may have represented the end of life with the prospect of re-birth. We simply don't know for sure. In Ireland, at Nowlth and at Newgrange, and in Orkney at Maeshowe, the sun was the focus of interest in prehistoric times predating Bridestones by nearly a thousand years. Here on the Cheshire-Staffordshire border, Bridestones also appears to have been deliberately positioned to view the sunsets from midwinter to midsummer and back again and was also aligned very precisely towards the equinoctial sunset. However, without the builders bisecting the angle between the solstitial sunsets (which they may have been able to do) this very accurate E-W orientation may be a chance alignment.

Bridestones was probably in active use for hundreds of years in the late Neolithic and was apparently still used as a sightline to the winter solstitial sunrise from Marton and the summer solstitial sunset from Cock Low well into the Bronze age. By then, society had changed from a community-centred culture to an hierarchical one in which group burials were superceded by single burials of important individuals, in circular burial mounds. We know that such a mound existed at Cock Low in Leek, and we may yet find that Marton church is located on another.

Much more work needs to be done but the Bridestones legacy, the sunset phenomenon from the site of St. Edwards, Leek, may have been of secondary importance until well into the Christian era and latterly into recent historical times when, for its curiosity value and post-renaissance scientific interest, the double sunset has taken priority.

 

Kevin J Kilburn, FRAS. January 2002.

 


Arbor Low, plan of stones.


Arbor Low, solstitial sunset.

The Bridestones.

Marton Sunrise 2002


Bridestones winter sunset 2001

Alignment map
(C) Kevin J Kilburn, FRAS. January 2002

Photographs of our Double Sunset

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