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Horizon Astronomy 3

Angular displacement

1.3.1   I would draw the reader's attention to experiences of night travel when the Moon, if it is low, appears to race over the countryside keeping pace with the observer.  This is the phenomenon of angular displacement and the apparent speed of the Moon is an illusion. We can see that this apparent angular movement is in direct proportion to the distance travelled by the moving observer. This simple but sensitive phenomenon was not only known by the ancient British astronomers but was developed into a high resolution technique for their sophisticated system of positional astronomy Using angular displacement methods of measurement the megalithic astronomers appear to have achieved such advanced feats of commensuration as observing the declination change of the Sun in the 24 hours before and after a solstice, and arriving at a close estimation of the extremes of Cyclic Perturbation of the lunar orbit which was not matched until the 19th Century of our era.

1.3.2  In his Introduction to` Megalithic Lunar Observatories ´(p.13) Thom introduces the concept of ´stepping aside´ to obtain finely measurable angular displacement of the Sun or Moon on the horizon.

1.3.3   Identifying the day of a solstice by stake positions. Working for perhaps one week before the solstice the astronomer would start finding, by lateral movement, the position from which the Sun's disc sits on the foresight whilst rising or setting. A wooden stake would be driven into the ground at this spot. The next day the astronomer would move about until the Sun sat once again on the same foresight and another stake driven. The distance between the first and second stake is in exact relationship to the movement of the Sun in the elapsed 24 hours. The procedure would be repeated daily and a line of stakes would develop. One day a stake would need to be placed behind the last one. That is- the direction of movement had reversed. Then the astronomer knew that thesolsticehad passed and the foremost stake marks it's position and date. 1.3.4  Unique astronomical places. This final position is an unique place in the landscape and, as it would function as a reliable time keeper for many years to come, would be of great value to the community. At the construction of a site this foremost stake would be replaced by a suitable menhir, (standing stone), of sufficient weight as to prevent it's displacement or damage by most forces. If the astronomer, when stepping to the side, also took a step back then the line of stakes would describe a parabolic curve on the ground. This would be, in effect, an accurate graph of the turn around of the Sun at a solstice or the Moon at a standstill. 1.3.3   Ballochroy. One of Thom's clearest examples of a high- resolution solstitial site identified by Alexander Thom is the fine stone row near Ballochroy, North Kintyre in the west of Scotland. This impressive backsight stands on the mainland bank of the Sound of Jura with a wide sea panorama and the mountainous island of Jura to the west. The central stone is a typical cleit which indicates, with both it's long faces, the descending flank of the most northern mountain on Jura to the north west across the sound. The position for the 24 hour stance appears to be still marked by another large stone a few yards to the north of the central aligned stone. See html page Ballochroy.

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