Dioptre at Newgrange
In the chapter titled ‘Astronomy of Easter’ I have discussed how astronomical data hidden in Columbanus’ poetry could be related, inter alia, to dimensional data at the megalithic tomb at Newgrange, Co Meath. And in the chapter on ‘Other Cultures’ similar data relating to the Sun and Moon’s dimensions is seen to have been embedded by Plato in the Atlantis legend.
It is very likely that those who built Newgrange 5,500 years ago had included a means for measuring the change in the angular diameters of the Sun and Moon caused by the eccentricity of the orbit of the Earth around the Sun, and of the Moon around the Earth. They were able to observe these angles very precisely by marking the change in the distance from the Light Box down the passageway and into the chamber from where the Sun or Moon disc could be seen to be precisely framed at the Light Box. While the Sun can be seen in this way from the central chamber at Newgrange through its Light Box for just a few days each year around the Winter Solstice, that is not so with the Moon. The Moon rises more frequently, and on different days, around the same Azimuth of about 130 degrees on the Newgrange horizon than the Sun does, and while some of these risings are in daylight and will not be easily seen, many will indeed occur during darkness and would have been seen during the epoch of its use from within the Newgrange chamber in the same way as the Sun in seen on the Winter Solstice.
There are many images and plans of the Newgrange tomb available on the internet to illustrate this piece. One of the better series can be found at: http://www.newgrange.com/newgrange-plans.htm
Dr Frank Prendergast FSCSI FRICS of Dublin Institute of Technology has studied the Newgrange passage and alignments both archaeologiclly and archaeoastronomically. While he strongly argues against the proposition that the design of a monument such as Newgrange 1 was precise in the astronomical/calendrical sense, he goes on to state in an email (5 Jan 2013):
“There has been much interest in lunar rise at Newgrange over the years. Culturally, however, the link between the orientation of the tomb and the sun is a far more likely and meaningful phenomenon than is the case for the moon. This argument is strongly supported by extensive archaeoastronomical and archaeological research here and elsewhere. …. the greater 'engineering/construction' challenge was in the positioning of the roofbox structure and its role in channelling the light into the chamber. For me, the vertical/sectional relationships between the passage and space through which the light is admitted are a far more impressive a feat than the horizontal alignment.”
He also very kindly provided the following data from his own measured dimensions:
“The lengths from the roofbox to the front/end recess backstone of the chamber are 18.54 m / 20.64 m (and see Figure 17 in O'Kelly 1982 p. 94). In relation to the roofbox, …… while it is difficult to assert how it may [have] been altered during/by the restoration process, the aperture is unlikely to have changed before/after restoration by more a small few cms at most. ….. The slope from left to right is evident and pronounced, as is the lack of parallelism, and this can also be seen in the historical images of the entrance recorded before 1900. It may well have been built as such. Internally, the box measures 0.96 m in width and thus subtends c. 2°.7 at the backstone of the end recess. Now, only c. 0°.5 horizontally can now be seen from the chamber, due to structural deformation on the passage orthostats. Vertically, the left/right sides of the box measure 0.15 m and 0.28 m respectively. Angularly, this is equivalent to 0° 26' and 0° 46'. This demonstrates that while the solar disc is framed by the roofbox, it is certainly not the case that it was engineered to precisely do so. Consequentially, I argue that using the roofbox to determine the fractional change in the apparent max/min diameter of the sun (from 32'.6 to 29'.4), or the moon (from 33'.3 to 29.4) not to be tenable because of its irregular profile.”
However, Dr Prendergast’s data does indeed allow for accurately measuring the fractional changes in angular diameters of both the Sun and the Moon, which could have been done by selecting a fixed vertical point for the Sun/Moon discs at the Light Box, even if the lack of parallelism in the structure were not smoothed out by, say, the insertion of an appropriate adaptor or shim. The measurements could be made by marking the change in the distance into the chamber from where the observer saw the Sun/Moon upper and lower limbs just touching the top and bottom of the box at the fixed vertical point selected. If the minimum angular diameter of the Moon (29.23' = 0.4897deg) was marked as it’s upper and lower limbs were seen touching the top and bottom of the Light Box aperture from the maximum 20.64m distance which is at the back stone in the recess at the farther end of the chamber, then the fixed point for all subsequent measurements to be selected at the Light Box would be at that point, and the height of the aperture at that point would have to be 0.176m (i.e. tan(0.4897)*20.64). Because the top and bottom are not parallel there has to be at least one point where it is of that height, and therefore the irregularity in height does then not matter. The angular diameter of the Sun on the 21 Dec (32.5' = 0.5417deg) would be marked at 18.615m, a very obvious and measurable difference in distance of 2.025m which just happens to fall at the forward edge of the backstone recess. The fact that the distance marks for the angular diameter of the Sun at the Winter Solstice which is the monument’s principle alignment, and the extreme angular diameter of the Moon and as calculated here, fall respectively at the extreme front and back of the backstone recess provides very strong evidence that the Light Box, passage, and the backstone with its recess were constructed together to incorporate the proportions of these very precise angular measurements, as well as the already well known and dramatic Winter Solstice alignment.
Using the same procedure, the Moon's Max (33.53' = 0.5588deg) would have to be marked in the central chamber at a distance of 18.04m from the Light Box, another very easily measured change of 2.6m in distance from where the Moon’s Min angle was marked at the backstone, and has for it’s explanation the change in distance of the Moon from the Earth due to the eccentricity of the Moon’s orbit.
Of course by the time of the winter solstice the Earth has not as yet arrived at Perhelion in it's orbit around the Sun; that happens on the 3 Jan, when the Sun's angular diameter will be seen at it's absolute maximum, but not from within the Newgrange chamber. Nowadays, and for the past few centuries, sight reduction tables for the use of sextants by sailors have included the variation (from 16.3' at Perhelion to 15.7' at Aphelion, as per one old Nautical Almanac) in the semi diameter (SD) of the Sun. However for the period between the Solstice and Perhelion the change is not sufficiently significant to be cited for navigational purposes in these tables. Had the builders of Newgrange maintained their observations up to Perhelion they would have been able to mark the change in distance of an easily distinguishable 5.9 cms due to the angular diameter increasing from 32.5’ on the 21 Dec to 32.6’ on the 3 Jan, just 1/600th of a degree, a very fractional change indeed. And so, as it is the case that the disc of the Sun can be seen from a marked point in the chamber to be precisely framed in the Light Box, with upper and lower limbs just touching, then that arrangement could have allowed as sufficiently precise a measurement that is required today for accurate navigational purposes. Because of the much larger scale of Newgrange as an instrument, it is far more accurate than Hipparchus' handheld four cubit rod dioptre described by Ptolomey in 'The Almagest' about two thousand years ago.
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Codici Bobbiesi is a work published in Milan in 1907. There are two Volumes, the second has approx 100 plates of MSS brought from Columbans' monastery at Bobbio to Turin in the 17th cent. 175 copies of the Codici were printed, and it is now very rare indeed. Up until now, it has been the only place to see images of some these MSS.