Astronomical Dating of Artifacts and Cultures

There are two important astronomical quantities, the variation of which permits dating of artifacts and cultures to be carried out: the obliquity of the ecliptic, e, which results in the variation of the maximum and minimum declinations of the Sun, Moon, and planets, and the precession of the equinoxes, which causes variation in the right ascension and declination of all objects in the sky, but most particularly affects the positions of the stars and constellations with respect to the vernal equinox. Precession has been discussed in §3.1.6, and its discovery will be described in §7.3; so we note here only that one of its effects is to cause the declinations of stars to change; this effect means that the azimuths of rise and set in the sphere of the fixed stars will vary with time in a systematic way and can yield dates of structures (or streets, like the Avenue of the Dead in the ancient Mesoamerican city Teotihuacan—see §12.21) that were aligned to them at the time of construction.

The variation in the obliquity of the ecliptic is slow, but the change is significant over millennia. The value of e at any epoch is e = 23 ? 439291 - 0.0130042 x T -16 x 10-8 x T2

where T = (t - 2000.0)/100 = (JDN - 2,451,545.0/36,525), t is the Gregorian date in years, and JDN is the Julian day number and decimal corresponding to that date. The variation of e results in changes in the standstill declinations of both Sun and Moon. This in turn changes the amplitude of the azimuth variation (see §3.2.1). Therefore, if it can be assumed that the orientation of a certain structure was intended to be in line with the amplitude of the Sun, say, and if sufficient precision of construction can be assumed for the builders, then it is possible to provide a date for the construction. However, the variation in e is not rapid; as noted in §2, the change amounts to ~7600 years/degree, or -0°000 000 36/day. Lockyer (1894/1973, p. 113ff) had difficulty accommodating his alignment measurements to midsummer sunset with the range of dates assigned to the temple of Amon-Re at Karnak by the contemporary archaeologists, viz., 2400-3000 b.c., and preferred a date of 3700 b.c. Hawkins (1974, p. 161), however, notes that a rebuilding of the Hall of Festivals part of the temple at ~1480 b.c. by Thutmose III could accommodate an alignment with the sunrise of a midwinter Sun having a declination of -23°9 ± 0°2. We will return to the Egyptian alignments and the problems they raised again in §8.1.

The astronomical dating of structures is only one of several methods available to determine the dates of usage, as we noted in §4.3. Given the uncertainty in the motivation of the builders, it is most unwise to try to rewrite the archaeological record or the ethnological history of a people on the basis of alignments alone. On the other hand, when archaeoastron-omy can be used in conjunction with other techniques, it may often substantially increase the precision of a date.

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