Astronomy Principles and Practice Fourth Edition.pdf

76 The celestial sphere: coordinate systems
Figure 8.16. The azimuth limits of sunset.
The azimuth at sunrise, A r ,isgivenby
cos A r = sin δ
cos φ .
It is obvious that for northern latitudes, with positive declinations,
while for negative declinations,
0 ≤ A r ≤ 90 ◦
12 h ≤ H r ≤ 18 h
90 ◦ ≤ A r ≤ 180 ◦
18 h ≤ H r ≤ 24 h .
Thus, in figure 8.16 for a place in northern latitude φ, the sunset point on the western horizon
will move back and forth between B and C where FG and LJ are the most northerly and southerly
parallels of declination of the Sun respectively, and AF = AL = ε, the obliquity of the ecliptic.
A corresponding cycle in the azimuth at the sunrise point takes place along the eastern horizon.
8.12 Megalithic man and the Sun
The Sun, as god and giver of light, warmth and harvest, was all-important. He was a Being to be
worshipped and placated by ritual and sacrifice. On a more practical note, his movements provided
a calendar by which ritual and seed time and harvest could be regulated. Throughout the United
Kingdom, for example, between the middles of the third and second millennia BC, vast numbers of
solar observatories were built. Some were simple, consisting of a single alignment of stones; others
were much more complicated, involving multiple stone circles with outlying stones making ingenious
use of natural foresites along the horizon to increase their observational accuracy. A surprising number
of such megalithic sites still exist, the most famous of which are Stonehenge, in England, and Callanish,
in the Outer Hebrides, Scotland.
One example will be sufficient to illustrate megalithic man’s ingenuity. It exists at Ballochroy on
the west coast of the Mull of Kintyre, Scotland.