I. Details of the Theodolite Observations at Stonehenge

The instrument chiefly employed was a six-inch transit theodolite by Cooke with verniers reading to 20″ in altitude and azimuth. Most of the observations were made at two points very near the axis, which may be designated by a, b. Station a was at a distance of 61 feet to the south-west of the centre of the temple, and b 364 feet to the north-east. The distance from the centre of Stonehenge to Salisbury Spire being 41,981 feet, the calculated corrections for parallax at the points of observation with reference to Salisbury Spire are:—

Station	a	+	4	′	12	″.
„	b	-	25	′	20	″.

(1) Relative Azimuths.—Theodolite at station a—

Salisbury Spire	0	°	0	′	0	″
N. side of opening in N.E. trilithon of the external ring	237		27		40
Tree in middle of clump on Sidbury Hill	237		40		20
Highest point of Friar’s Heel	239		47		25
S. side of opening in N.E. trilithon	240		14		40
Middle of opening in N.E. trilithon	238		51		10

(2) Absolute Azimuths.—All the azimuths were referred to that of Salisbury Spire, the azimuth of which was determined by observations of the Sun and Polaris.

(a) Observation of Sun, June 23, 1901, 3.30-3.40 P.M.

Mean of observed altitudes of Sun							41	°	26	′	35	″
Refraction	-	1	′	4	″	}	0		0		58
Parallax	+			6
True altitude of Sun’s centre							41		25		37

Latitude = 51° 10′ 42″. Sun’s declination = 23° 26′ 43″. Using the formula

cos² ¹⁄₂ A = sin ¹⁄₂(Δ + c - z) sin ¹⁄₂(Δ + z - c) sin c . sin z

where A = azimuth from south, Δ = polar distance,
c = co-latitude, and z = zenith distance,

we get

Azimuth of Sun	S.	75	°	30	′	30	″	W.
Mean circle reading on Sun		84		38		35
Azimuth of Salisbury Spire	S.	9		8		5		E.

(b) Observations of Polaris.—June 23, 1901. Time of greatest easterly elongation, calculated by formula cos h = tan φ cot δ, is G.M.T. 1.34 A.M.

Azimuth at greatest easterly elongation, calculated by the formula

sin A = cos δ sec φ

is 181° 57′ 0″ from south.

Observed maximum reading of circle		256	°	33	′	″
True azimuth of star		181		57
Meridian (S.) reading of circle		74		36
Circle reading on Salisbury Spire		65		28
Azimuth of Salisbury Spire	S.	9		8			E.

The mean of the two determinations gives for the azimuth of Salisbury Spire S. 9° 8′ 2″ E. This result agrees well with the value of the azimuth communicated by the Ordnance Survey Office, namely, 9° 4′ 8″ from the centre of the circle, which being corrected by +4′ 12″ for the position of station a, is increased to 9° 8′ 20″.

Hence, from the point of observation a, 9° 8′ 20″ has been adopted as the azimuth of Salisbury Spire.

We thus get the following absolute values of the principal azimuths from the point a:

Highest point of Friar’s Heel		239	°	47	′	25	″
		-9		8		20
		230		39		5
or	N.	50		39		5		E.
Middle of opening in N.E. trilithon		238		51		10
		-9		8		20
		229		42		50
or	N.	49		42		50		E.

The difference of 8¹⁄₂′ between this and the assumed axis 49° 34′ 18″ is so slight that considering the indirect method which has necessarily been employed in determining the axis of the temple from the position of the leaning stone, and the want of verticality, parallelism and straightness of the inner surfaces of the opening in the N.E. trilithon, we are justified in adopting the azimuth of the avenue as that of the temple.

Next, with regard to the determination of the azimuth of the avenue as indicated by the line of pegs to which reference is made on [p. 65]. The small angle between the nearest pegs A and B (which are supposed to be parallel to the axis of the avenue), observed from station a, was measured, and the corresponding calculated correction was applied to the ascertained true bearing of the more distant peg B.

Thus

True bearing of peg B =		238	°	35	′	0	″
Calculated correction to peg A =		0		12		8
True bearing of line AB		238		47		8
Bearing of Salisbury Spire		189		8		20
True bearing of a line parallel to the axis of near part of avenue	N.	49		38		48		E.

The mean of the three independent determinations by another observer was 49° 39′ 6″.

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		}	-0		27		18
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.