The calculated bearing of the more distant part of the axis of the avenue determined in the same manner by observations from station b is 49° 32′ 54″. The mean of the two, namely, 49° 35′ 51″, justifies the adoption of the value 49° 34′ 18″ as given by the Ordnance Survey for the straight line from Stonehenge to Sidbury Hill.
(3) Observation of Sunrise.—On the morning of June 25, 1901, sunrise was observed from station a, and a setting made as nearly as possible on the middle of the visible segment as soon as could be done after the Sun appeared.
The telescope was then set on the highest point of the Friar’s Heel, and the latter was found to be 8′ 40″ south of the Sun.
| Sun’s declination at time of observation | 23 | ° | 25 | ′ | 5 | ″ | |||||||
| Elevation of horizon at point of sunrise | 0 | 35 | 48 | ||||||||||
| Assuming 2′ vertical of Sun to have been visible at observation, we have apparent altitude ofSun’s upper limb | 0 | 37 | 48 | ||||||||||
| Refraction | - | 27 | ′ | 27 | ″ | } | -0 | 27 | 18 | ||||
| Parallax | + | 0 | 9 | ||||||||||
| True altitude of upper limb | 0 | 10 | 30 | ||||||||||
| Sun’s semi-diameter | 0 | 15 | 46 | ||||||||||
| True altitude of Sun’s centre | -0 | 5 | 16 | ||||||||||
| From this it results that the true azimuth of the Sun at the time of observation | = | N. | 50 | ° | 30 | ′ | 54 | ″ | E. | ||||
| And since azimuth of Friar’s Heel | = | 50 | 39 | 5 | |||||||||
| 2′ of sunrise should be N. of Friar’s Heel | 0 | 8 | 11 | ||||||||||
| Observed difference of azimuth | = | 0 | 8 | 40 | |||||||||
| Observed - calculated | = | 0 | 0 | 29 | |||||||||
The observation thus agrees with calculation, if we suppose about 2′ of the Sun’s limb to have been above the horizon when it was made, and therefore substantially confirms the azimuth above given of the Friar’s Heel and generally the data adopted.
II. Hints on Making, and Method of Reducing, the Field Observations.
It will probably be found useful if I give here a few hints as to the precautions which must be taken in making the field observations and an example of their reduction to an astronomical basis.
For the azimuths of the sight-lines the investigator of these monuments cannot do better than use the 25-inch, or 6-inch, maps published by the Ordnance Survey. Their accuracy is of a very high order and is not likely to be exceeded, even if approached, by any casual observer having to make his own special arrangements for correct time before he can begin his surveying work.
In some cases, however, it may be found that the Survey has not included every outstanding stone which may be found by an investigator on making a careful search; many of the stones are covered by gorse, &c., and are not, therefore, easily found.