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Explanation about the how the stone rows work as a sun calendar - CLICK HERE
Summary photographs - CLICK HERE
This is the eastern blocking stone of the southern double stone row. Great Mis Tor and Little Mis Tor on the skyline ..... the time is uncertain due to the clock being incorrect in the camera:.however, note that the shadow is LEFT of the stone's vertical mid-line at 23 days before the solstice .......
Closer view of the above. There is a similar photograph below taken on 28th December., where the shadow is RIGHT of the stone's midline at 7 days after the solstice. .
Shadow cast from the Row 3 blocking stone (at bottom edge of the photograph) towards the eastern blocking stones, photo at 3.54 pm (25 minutes before 4.19 pm sunset) ...... in view of this being some time before sunset, perhaps the shadow would move across to the other blocking stone? The job needs a team of observers .....
Shadow cast from the large cist south of the southern double row towards the blocking stone of the northern row (digitally highlighted). The blocking stone of the southern row is at the extreme right edge of the photograph. (digitally highlighted). Taken at 4.04 pm (15 minutes before 4.19 pm sunset).
Zoomed view, details as for the photograph above, again at 4.04 pm .....
SunCalc image showing the minute of sunset on Dec. 21, winter solstice, from the large cist beside the southern row with a shadow casting directly towards the east blocking stone of the northern row (inside the white circle). This is a clear winter solstice marker. .
Shadow cast onto the eastern blocking stone of the south row. Taken at 4.17 pm (2 minutes before 4.19 pm sunset). Note that the shadow (now 7 days after the solstice) is slightly to the RIGHT of the blocking stone's vertical mid-line. The light becomes very pink at this time of day. Four Winds car park in the background.
The centre of the notch in the stone casting the shadow is to the RIGHT of the blocking stone.
The menhir (enhanced) south of the stone circle aligned with a certain leaning slab, described in "Dartmoor Sun" showing sunset. This is illustrated in the book as an older technology for marking the winter solstice. Photo taken at 4.08 pm before sunset at 4.20 pm. , 7 days after solstice. I seem not to have captured it very demonstrably.
After a quick trot, a view from the western end of the northern row to the western blocking stones of the southern row - the sun is setting over a clump of rushes with the blocking stones of the far row almost hidden - see next photograph .....
Similar photograph to the above taken at dawn on 28th Dec. to show the western blocking stones of the southern row. It indicates that in the preceding photograph the sun set directly over the prominent stone. A week after solstice, this is quite an impressive alignment of stones that were put here 3,000-5,000 years ago to foretell the seasons for the early farmers .....
Pre-sunrise, 3rd January 2017, 8.33 am. The near stone is the eastern blocking stone of the northern row; the enhanced small stone in the distance is the eastern blocking stone of the southern row. This view is looking noticeably uphill so that the "ideal horizon" required for the predicted times of sunrise and this must affect the time of the observed rise, making it later than predicted. .
The free computer program, Stellarium, set for the Lat / Long and Elevation of the Merrivale stone rows, with the atmospheric and ground effects included, gave a first time for sunrise of 8.18:12 am and the second time (for complete sunrise) of 8.22:30 am - the sun takes just over 4 minutes to rise, from first to last position below and above the horizon. Why the first part of sunrise below was at 8.36 am, I cannot say.
As a point of interest, the sun is rising directly over the stone that casts its winter solstice shadow onto the eastern blocking stone, above. This may be significant.
8.36 am - the sun is just showing a rim of light. It appears to be over the large stone that casts the sunset shadow on the adjacent blocking stone at this time of year. Yellowmead Farm is in the distance, towards the left. I would have expected the position of the sun to have been slightly left i.e. north of this as it starts its progression towards summer, if this was a perfect alignment, unless this is the result of a delayed sunrise time due to the elevated horizon? The ground beyond Yellowmead Farm is about 460 metres (1510 feet) above mean sea level, although the east blocking stones are about 345 metres (1130 feet) elevation so this may explain some discrepancy. But, higher elevations should see the sun sooner?
For curiosity: the Stellarium program shows that sunrise, from first touch behind the horizon, to just above the horizon, takes 4:15 minutes for Plymouth, elevation 54 metres on this date, 3rd January, with an ideal horizon and no ground or atmosphere displayed. The effect of showing the atmosphere is about 4 minutes later, due to refraction. Try with Merrivale Lat/Long Elev 345 metres.
8.37 am - the sun is half above the horizon, glare reduced digitally in the image. .
Feb. 6th sunrise. Note that to observe the sun rise over the east blocking stone, southern row, it is necessary to have moved to the right (i.e. west) of the position in the sunrise above (Jan 3rd). The original position was at the east blocking stone, northern row, that is seen in this photograph directly below the tv mast. At top left is Hollow Tor. Just left of the far blocking stone with the sun above it is Yellowmead Farm. In front of the yellow farmhouse can be seen one of the old packhorse TA marker stones (Tavistock to Ashburton track).
8.37 am - just a general view of the rows in "dawn" sunlight. The big stone is the eastern blocking stone of the northern row. Pew Tor is seen left of centre.
Web site for sunrise and sunset times: TimeAndDate.com .
The question has arisen during this exercise as to exactly when are sunrise and sunset? Some information is available on Wikipedia - Sunrise and USNO - Rise, Set & Twilight Definitions. - this includes definitions of Astronomical, Nautical and Civil twilights.
Diagram from Wikipedia - Sunrise
One definition is "Sunrise is defined as the instant in the morning under ideal meteorological conditions, with standard refraction of the Sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon.", from Australian Government Geoscience Australia - Astronomical Definitions. If it coincident with an ideal horizon, can you see it or is it still hidden, and if so, how long before it appears?
Answers please, on a postcard .....
Dawn is rarely mentioned but it is the first appearance of light before sunrise.
SunCalc.net gives all the relevant times - click "More detailed" at top right.
SunCalc.org gives the left panel of details including dawn and dusk.
Postscript 2 - Spring Equinox
I visited Merrivale on Tuesday 21st March - the day after the Northern Hemisphere Spring Equinox to see what transpired .....
This spring equinox photograph was taken at 6.29 - 630 am (being a composite exposure merge image of 9 sequentially taken photographs of different exposures). Published time of sunrise was 6.18 am. The sun is not actually visible. There is a straight line alignment from the end stone in the foreground (highlighted), west end, north row, to the terminal stones, east end, south row (smaller highlighted area), and to the brightening in the sky at the horizon. This seems to be a quite accurate predictor of the equinoctial sunrise position. The exercise was terminated by a very heavy hail shower that lasted some time. It ended with a gentle fall of small snow flakes as I got back into the car!
Postscript 3 - Summer Solstice
I visited Merrivale for sunset on 18th and 20th June - the midsummer solstice was on the 21st, but the hot weather broke that day .....
One point of interest is that sunset time and direction for "Plymouth" on www.timeanddate.com are shown as 21:31 hrs and 310° from June 20-27th - so that the sun sets at the same place on the horizon for a week. Also, sunset seems to be about 10-12 minutes earlier at Merrivale, due in part to the high ground of Middle and Great Staple Tors, to the west.
Sunset on Sun. 18th June, seen from the east end of the southern row. The blocking stone of the northern row is marked by the darkened ellipse. The bubble effects are from dust on the camera lens ......
The same scene as above two days later, on June 20th. The sky was clearer and the mukti-frame merging is made from fewer darker images .....
The same as the previous photo but with an insert from the Middle to Great Staple Tors section from the first (18th) photograph. It is difficult see any shift in the sun northwards along the horizon - this fits in with the published times for sunset being the same for a week.
As with the mid-winter sunrise photos above, the sun hasn't quite reached the blocking stone alignment. We assume that this would be the intended design, although perhaps not, because the equinox alignment seems to be accurate (using the end stones).
Another possibility is that "the sun has moved", or rather, something has
changed since the rows were built 4,000-4,500 years ago. Milutin Milankovitch
(1879-1958) postulated that there are three cycles that describe the Earth's
movement around the Sun. These are
(1) Eccentricity i.e. a ellipsoidal orbit that also varies in it's plane, essentially a 100,000 year cycle
(2) Obliquity i.e. the tilt in the rotational axis of the Earth, this varies from 21.1 to 24.5° over 41,000 years
(3) Precession i.e. wobble in the tilted axis so that over many years, the axis will not point to Polaris over a 26,000 year cycle.
Working from this figure in the Wikipedia-Axial Tilt page, I have calculated that Obliquity has shifted by 0.471° over the last 4,000 years. This may be relevant as also Precession - but I leave that for a mathematician to calculate! .