Alignment. Centre of circle to Longstone.
Az. (from 25″ Ordnance Map). N. 66° 38′ E.
Elevation of horizon (measured) 2° 10′.
Reference to the May-Sun curve, given on [p. 263], indicates that this is probably an alignment to the sunrise on May morning. Therefore, in determining the zenith distance, the correction for the sun’s semi-diameter (16′) must be taken into account, allowing that 2′ of the sun’s disc was above the horizon when the observation was made.
Zenith Distance:—
| Zenith | distance of | true | horizon | = | 90° | ||
| „ | „ | local | „ | = 90° - 2° 10′ | = | 87° | 50′ |
Bessel’s tables show that refraction, at altitude 2° 10′, raises sun 17′. If 2′ of sun’s limb is above horizon, sun’s centre is 14′ below.
∴ True zenith distance of sun’s centre = 87° 50′ + 17′ + 14′ = 88° 21′.
Declination:—
Having obtained the zenith distance, and the azimuth, the latitude being known, the N.P.D. (North Polar Distance) of the sun may be found by the following equations:—