Observation of the solstice by amateurs
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Observation of the solstice by amateurs is difficult because the sun moves too slowly at either solstice to determine its specific day, let alone its instant.
The word solstice is derived from the Latin word solstitium where sol means sun and stitium (from consistere) means stand still. Thus, the solstices occur on the day when the sun stops rising, or descending, at its mid-day high point and stops changing its azimuth (angular degrees from south) at sunrise and sunset. Further, although we normally think of the solstice as occurring on a single day, it actually occurs at some specific instant during that day.
Thus, while many people observe the day of the solstice — when someone else has already told them when it will occur and where to look, it is more difficult to determine the day if you do not know it in advance. If fact, even those who observe it on the stated day probably could have done so 1–2 days earlier, or later, and would not have noticed the difference.
This potential error occurs because the angular daily movement of the sun's azimuth, and elevation, is very small for the few days on either side of the time when it is actually zero — the true instant of the solstice.
The angular diameter of the sun, the angle subtended by the lines extending from opposite edges of the sun to the viewer's eye, is related to the actual diameter of the sun and the distance of the earth from the sun. This varies slightly during the year since the distance of the earth from the sun varies as a result of the fact that the orbit of the earth around the sun is an ellipse rather than a circle. However, the result is that the angular diameter is approximately 32 arc minutes (60 arc minutes equals one degree, a full circle is 360 degrees).
The change in the sun's azimuth, if viewing the sun relative to the horizon at sunrise or sunset, and elevation, if viewing the sun at mid-day, is zero at the instant of the solstice and slowly increases with time on either side of the solstice. During the 24-hour period surrounding the solstice this change is only on the order of ½ arc minute, increasing to about 2 arc minutes two days before (after), and continuing to increase faster with increasing time removed from the actual instant.
Thus, it is not possible to detect the actual instant of the solstice (by definition, you can not observe that an object has stopped moving until you make a second observation in time showing that it has not moved further from the preceding spot, or that it has moved in the opposite direction). Further, if you wish to be precise to a single day you must be able to observe a change in azimuth or elevation less than or equal to about 1/60th of the angular diameter of the sun. Observing that it occurred within a two day period is easier – requiring an observation precision of only about 1/16th of the angular diameter of the sun.
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There is at least one place on earth where the day of the Winter Solstice can be observed and measured by ordinary people on the day and where the Winter Solstice may be accurately predicted in advance, without any equipment or training.
That place is "Newgrange": located at the River Boyne valley in Ireland about seven miles west of the East-coast port of Drogheda (pronounced 'Draw-Dah'). The measurement can and may be confirmed annually with one proviso: that the sun rises unobstructed by smoke or cloud on the day of the solstice at that place and that the observer be in position within the tumulus to observe before dawn. Each year a small number of [ordinary] people are invited to attend. A list exists for consideration but the waiting list is about ten years and irish weather is magically unpredictable.
Newgrange was developed between about 3700 and 3600BC, certainly more 1000 years before the construction od the Great Pyramid of Giza. Newgrange fell into disrepair at an unknown date but was meticulously repaired and restored to use by an Irish archaeological team led by Prof. O'Kelly over a twenty year period culminating in 1969.
Newgrange accurately measures the Winter Solstice by pin-pointing the azimuth of the sun at its moment of rising above the south-eastern horizon on the appropriate day. The mammoth structure of Newgrange contains some 200,000 tons of material including greywacke and other boulders weighing in at as much as 15 tonnes. The facility at Newgrange causes a ray of sunshine to route along a pitch black 'tunnel' so as to illuminate a stone marker some 60' in, four? minutes after 'nominal' sunrise.
The ray of light may be seen to travel along the floor of the passage in advance of the critical 'moment'. It is then seen to 'climb' and illuminate the 'plate' for about one minute of our time before retreating in the same manner. The complete cycle from start to finish is about six minutes. In the three days preceding, the ray progress only part of the way, but improving daily.
It is notable that the alignment of the light ray is indelibly marked on stones on opposite sides of the substantial tumulus approximately 180' apart. The alignment has been 'measured' by a geophysics team from Bolton Street College using state-of-the-art theodolites. No measurable error could be established and the team were obliged to find that the alignment could not have been improved on with their equipment.
[These notes are taken from an original manuscript the property its author and the author of this addendum who confirms that he vests copyright of this addendum upon the principals of Wikipedia for the greater enlightenment of the web community Southockendon 21:45, 14 January 2007 (UTC)]
The above notes relate an observation capability that is much more accurate than today's capability, following the re-alignment of the passageway that was conducted in restoring the facility in the 1960's. Click here to view a separate page on the Technical Facts Concerning The Observation of The Winter Solstice At Newgrange. Gene Dyer, A Solstice Amateur
