Fig. 5.—The four Astronomical Divisions of the year.
Fig. 6.—The various bearings of the sun risings and settings in a place with a N. latitude of 51°.
In order better to consider the problem as it was presented to the early astronomers who built observatories (temples) to mark these points, we may deal with the bearings of the points occupied by the sun on the horizon (either at rising or setting) at the times indicated. These points are defined, as we have seen, by their “amplitude” or their distance in degrees from the E. or W. points of the horizon. In the diagram ([Fig. 6]) I represent the conditions of our chief British sun-temple, Stonehenge, in latitude 51° N. approximately.
Taking the astronomical facts regarding the solstices and equinoxes for the first year (1901) of the present century, we find—
| Sun | enters | Aries, | Spring equinox, | March 21. |
| „ | „ | Gemini, | Summer solstice, | June 21. |
| „ | „ | Libra, | Autumn equinox, | September 23. |
| „ | „ | Sagittarius, | Winter solstice, | December 23. |
These points, then, are approximately ninety-one days apart (91 × 4 = 364).
In [Fig. 6] I deal with the “amplitudes” at Stonehenge, that is, the angular distance along the horizon from the E. and W. points, at which the sunrise and sunset are seen at the solstices; at the equinoxes they are seen at the E. and W. points. But as these amplitudes vary with the latitude and therefore depend upon the place of observation, a more general treatment is possible if we deal with the declination of the sun itself, that is, its angular distance from the equator.
The maximum declination depends upon the obliquity of the ecliptic, that is, the angle between the plane of the ecliptic and that of the equator at the time of observation. When the Stonehenge Sarsen Stones were erected this angle was, as I shall show later on, 23° 54′ 30″. Its mean value for the present year (1906) is 23° 27′ 5″; it is decreasing very slowly.
It will be obvious from [Fig. 6] that in temples built to observe the solstices or equinoxes, if they were open from end to end, looking in one direction we should see the sun rising at a solstice or equinox, and looking in the other we should see the sun setting at the opposite one. I shall show later on that this statement requires a slight modification.