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Moonrise

Minor standstill

§2.3.5 for the discussion of the Moon motions and of the phenomena of major and minor standstills in the 18y6 cycle). Figure 6.31 demonstrates the amplitudes of the Sun and Moon at the latitude of Stonehenge in this plan view of the astronomical horizon (for an "elevation" view, see Figure 2.17a) for a late epoch.

The declinations indicated by alignments among pairs of the four stations are given in Table 6.4, the data of which are taken from Hawkins (1965). Dibble (1976) noted that the rectangle made by the four Stations is composed of two triangles very nearly matching 5 x 12 x 13 Pythagorean triangles. Among themselves, the stations could not have provided high-precision alignments. In 1966, however, three postholes were discovered in a car-park to the northwest.

Figure 6.30. One of the two main outliers at Stonehenge: The Heelstone. (a) Distance view, photo by Marie C. Jack. (b) In closeup, revealing the cup markings. Photo courtesy of Sharon Hanna.

Figure 6.30. One of the two main outliers at Stonehenge: The Heelstone. (a) Distance view, photo by Marie C. Jack. (b) In closeup, revealing the cup markings. Photo courtesy of Sharon Hanna.

Their locations are marked in concrete on the road surface. Newham (1972) suggests that in combination with stations 91, 92, and 94, and the Heelstone, both solar (setting, summer solstice) and lunar (setting, northern major standstill; setting, northern minor standstill; and setting, midway between the standstills) alignments were achievable with posts set into these holes. They may have been erected in order to help in the layout of the Stations (Newall 1953/1959/1981, p. 23).

If, and only if, the full moon is considered, the directions 91-94 and 91-93 refer to winter solstice moonsets (that is, a moonset when the Sun is at the winter solstice) at the times of major and minor lunar standstills, respectively. Similarly, if only the full moon is considered, 93-92 and 93-91 refer to summer solstice moonrises at the time of major and minor lunar standstills, respectively. When Hawkins (1965) refers to "Midwinter Moonrise," for example, in connection with these alignments, he is referring to this phenomenon. It must be remembered that the Moon goes through its entire amplitude in a mere month, but that the amplitude varies in size from lunation to lunation, continuously changing over the period of 18.6 years of the nodal regression cycle. Thus, from night to night, the Moon will rise, and set, at a sequence of intermediate azimuths within its rising and setting amplitude.

It is this circumstance that suggests the use of the Aubrey Holes as an eclipse predictor. The ring of Aubrey Holes is ~250ft (86.87 m) in diameter (Wood 1978, p. 163, gives the circumference as 271.6 m and the radius as 43.2m). The holes are about 3.5 ft (1.07 m) in diameter and ~2.5 ft (0.76 m) deep with flat bottoms and were dug out of the chalk that underlies much of the region. They are filled with rubble, crema-torial remains, charcoal, and chalky debris. The organic material was used in the 14C dating of Stonehenge I and may well be later than the actual construction date. The works by Hawkins (1965) and later by Hoyle (1977) emphasize the importance of the number of Aubrey holes: 56. The mean of three intervals of 19, 19, and 18 years is 18y 67, an approximation to the nodal regression cycle, and the sum of these numbers is 56. There are three other potential tie-ins of this number to astronomical phenomena.

The number of the Aubrey holes, which, it must be remembered, were a prominent component of the earliest stage of Stonehenge, in a way already nearly encapsulate the motion of the Moon. Other possible tie-ins that have been suggested are the nearly 55 (542/3) nights in which the Moon has completed its motion among the stars twice (Psid ~ 27d3), and the period of the triple Saros cycle, 54y 33d, over which nearly identical conditions for an eclipse will recur. Thus, an earlier eclipse will be followed by another of similar duration, and seen at a similar range of longitudes on Earth. Finally, this number is serendipitously close to a triple Metonic cycle (19 x 3 = 57), which reconciles the lunar and solar calendars.

Hawkins's (1965) scheme for the use of Stonehenge as a "computer" (Schlosser et al. 1991/1994 suggest an "abacus" to be a better analogy), at its simplest, involved the use of six stones of alternate kinds (say, black and white stones), placed in Aubrey holes at intervals of 10, 9, 9, 10, 9, and 9 holes. Figure 6.32 demonstrates the arrangement. Each year,

Figure 6.31. A plan view of the horizon shows the northern and southern extreme rise points for the Sun and Moon for a flat horizon at latitude 51°N and, thus, the amplitudes of the major and minor standstills of the Moon and the solstices for those conditions. The set points are symmetrical. Drawing by E.F. Milone.

Figure 6.31. A plan view of the horizon shows the northern and southern extreme rise points for the Sun and Moon for a flat horizon at latitude 51°N and, thus, the amplitudes of the major and minor standstills of the Moon and the solstices for those conditions. The set points are symmetrical. Drawing by E.F. Milone.

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