Technical Facts Concerning The Observation of The Winter Solstice At Newgrange

Technical Facts Concerning The Observation of The Winter Solstice At Newgrange

The passage way built into The Ancient mound at Newgrange, Ireland, is well known for the fact that it is aligned so that sunlight lights the back end of the passage only at sunrise on the winter solstice.

However, while technically the winter solstice occurs only at an instant, and while most people think of the solstice as occurring on a single day, it is very likely that pre-historic man thought of the solstice as occurring during that period of days for which he could make reliable and repeatable observations that the sun had halted its southerly direction (Northern Hemisphere only) and was AbOUT to start moving back north, returning warmth to the earth.

The passageway into the Newgrange mound is a kind of solar observation mechanism; however, different observers have reported the accuracy of the resultant observations differently.

Most reports have stated that the back chamber is lighted for a period of about 17 days surrounding the solstice – giving an observation of a solstice period rather than an observation of its occurrence on a single day. There are also reports of how the light advances into the passageway, and then backs out again after a relatively short time period. That time period has been reported as short as 4 minutes, and as long as 17 minutes.

While different viewers may have different memories of the event, a detailed explanation of the lighting phenomena is easily made by analyzing the optics involved by the shadowing of the sun’s rays by the roof box at the entrance to the passageway. This shadowing is related to the change in azimuth (north-south movement) of the sun at sunrise on the multi-day period surrounding the day of the solstice, the changing elevation and azimuth or declination of the sun at sunrise (the sun does not rise perpendicularly to the horizon except at the equator), the angular diameter of the sun, the dimension and placement of the light box opening and the dimension of the illuminated area at the back of the passageway.

The following data was determined by use of The National Oceanic And Atmospheric Site Solar Position Calculator for the approximate Latitude (53 degrees 45 minutes north) and Longitude (7 degrees West) of the Newgrange site.

(December 12 and December 30 are given since they are 17 days apart.)

AZIMUTH CHANGE DURING SUNRISE The sun does not rise perpendicular to the horizon except at the equator. At the latitude of Newgrange the following shift in azimuth occurs during the 10 minutes following the first perception of sunrise.

By comparison the angular diameter of the sun is approximately 32 arc minutes.

Additionally, while the height of the light box is about 5 feet, its width is several feet. The height and width of the illuminated area at the back end of the passageway is constrained by the height and width of the tunnel part of the way through. This has been described as just barely big enough to let a person pass by hunching down. (Not enough room for two people to pass.) Further, pictures of the light beam entering the passage way and impinging on the wall suggest that the light beam is about 2 feet in height.

The walls of the passageway are irregular, made by the placement of large stones, so that the exact alignment of the passageway is determined by the most shadow constraining rocks – not just by the entryway.

Lastly, numerous artist sketches show that the floor of the passageway is graded slightly uphill passing into the interior (presumably important to keep out water).

While some observations report the entering light moving along the floor towards the rear with first light, many of the available pictures show that the right wall is first illuminated and then as the sun rises further, the light appears on the floor. How this light appears to move on the floor must be related to both the altitude of the sun and the levelness of the floor. Uphill portions will be lighted first, with lower portions being lighted as the sun rises further so as to expose them. Only with an inward down slope would the light appear to move further inward with increased elevation of the sun. (Does not seem to be the case.) Further, the dim light that occurs as just the first sliver of sun becomes visible may lead observers to confusion as to just where the light strikes since some light will bounce off of the walls and illuminate the floor.

The observation that the light on the floor moves outward in the direction of the inlet as the sun rises higher is quite consistent with the geometry of an up sloping floor, and a bright enough light to be sure that that is what is being observed.

The amount of time that the rear wall is illuminated on any one day, and the number of days that it is illuminated, are also a clues relative to the orientation of the passageway. Most reports suggest 17 minutes of illumination for 17 days.

From the above data it may be seen that the azimuth of the sun will shift by 0.75 degrees southward during the 9 days preceding the solstice and 0.51 degrees back northward during the 9 days after the solstice. (That these numbers are not equal is probably due to a limit on the accuracy in the source tables.)

However, using an average value of 0.6 degrees, for which the sine is 0.01047, the width of the light illuminating the rear wall some 60 feet distant can be determined to be approximately 7.5 inches, extending from one edge when first lighted by a sliver of the 32 minute diameter sun and widening to the right as the azimuth of the sun approaches the solstice. Then reversing after the solstice has occurred until it is dark again. This dimension is consistent with reports that there is a several day period during which the rear of the passageway is lighted.

The duration of the light period can be determined in the same manner since when the sun rises to a certain point the top of the roof box will shadow the rear face, and the light will start to be withdrawn. One difference is that the sun has an angular diameter and that the back face will be lighted from the time that the top edge of the sun becomes visible over the bottom edge of the roof box and continue to be lighted until the bottom edge of the sun has risen above the top edge of the roof box.

If that light shines on the rear wall at an elevation of from 2 to 4 feet then it can move downward approximately 2 feet before it starts retreating along the floor towards the entrance. The sine of the angle formed by this movement is 2/60 (equals 0.033), or the angle of movement is 54 arc minutes. Added to this is the angular diameter of the sun of 32 arc minutes, giving a total of 86 arc minutes, or approximately 1.5 degrees. From the above table the amount of time it takes the sun to rise 1.5 degrees above the horizon is the expected 17 minutes, certainly not just 4 minutes.

These calculations are not quite correct since the azimuth, or declination, of the sun is also changing as the sun is rising, causing a slanted motion of the leading edge of the light beam to also move to the right rather than just in a vertical motion, which could perhaps cause the lighted period to be different depending on the shadowing that takes place in the internals of the passageway.

The next factor to consider is the accuracy of observing the solstice relative to any previously fixed marks on the rear wall with respect to which the sunbeam should shine.

The change in the azimuth of the sun for the single day before and the single day after the solstice is approximately 1.2 arc minutes, the sine of which is approximately 0.00031. Since the rear wall is 60 feet back from the roof box, the light beam should shift to the right by 0.2 inches from the day before, and then shift back 0.2 inches on the day after. While this could be determined by very careful observation of the light edge, the observation of that shadow edge will be clouded by the fact that a gray area exists between illumination from the full sun to the less illumination as less of the diameter of the sun is visible. It will also be more difficult due to the light diffraction that takes place across the edge of any sharp shadow – further causing the edge of the shadow to be fuzzy.

Lastly, due to the azimuth shift as the sun rises, the light beam will shift to the right on days prior to the solstice and to the left on days after the solstice. Importantly, the amount of this shift is on the order of 6 times greater than the daily shift that must be detected, requiring that any observation be made at the precise same time (same degree of azimuth shift) after the sun rises.

Thus, while an observation of the solstice season can easily be observed, it does not seem plausible that the exact date of the solstice could be determined by this method.

The above comments and calculations relate to the passageway as it reportedly stands to day. However, a historical record states that “After the excavation, the interior passage was straightened and enclosed by a second passage (now unseen) in order to relieve the pressure from the weight of the mound.” Also, while the current roof box appears to be about the same dimensions as it was in pictures of it taken in 1910 it has been replaceed by a more substantial modern structure. Additionally, internal pictures of the passage way show that the sun shines on the walls as well as the floor. Thus, it is possible that such straightening shifted the limiting shadow line to the right, thereby allowing a longer penetration of the light for the 17 days claimed by most current observers compared to the 4 day period that is apparently recorded in some of the historical documentation.

Unfortunately it is not now possible to recreate and test to determine if the accuracy of the original system was better than it is today since the exact dimensions and orientation of the light beam may have been slightly changed.

Links:

Newgrange

Observation of the solstice by amateurs

• Observe/Determine the solstice in your own back yard.