Several scholars have posed to me the question of why the Mithraists would have focused on the shift of the spring equinox out of Taurus, which had occurred about 2,000 years before the origins of their cult, rather than focusing on the dramatically imminent shift of the equinox from Aries into the constellation of Pisces the Fishes, which, as is well known, occurred in the first century A.D., not long after the founding of the Mithraic religion. 
The answer to this question lies in the fact that although today we know that the spring equinox moved from Aries to Pisces around the first century A.D., this would not have been the opinion of astronomers and astrologers at the time Mithraism flourished. The reason for this is that in the late Hellenistic and Roman periods there was an almost universally accepted standard that the spring equinox was located at 8° Aries (a figure borrowed from Babylonian astronomy, specifically Babylonian "System B").  As Otto Neugebauer says, "we have ample evidence for Aries 8° as vernal point for the two centuries which straddle the beginning of our era,"  and Neugebauer goes on to present exhaustive "proof for the continued use of the Babylonian norm of 'system B' for the vernal point [i.e., 8° Aries] during the first five centuries A.D." 
Now, we know that Hipparchus estimated the rate of precession as roughly 1° per century.  Thus, any ancient astronomer who learned of Hipparchus's discovery of the precession and who held to the standard location of the equinox at 8° Aries could only have concluded that the Age of Aries still had 800 years remaining to it. Thus, the end of the Age of Aries was hardly so imminent that the Mithraists would have been impelled to focus on it rather than on what was at their time the most recent shift caused by the precession-- namely, the end of the Age of Taurus.
We should also note that among the Greeks and Romans it was generally felt that a deep spiritual potency was inherent in great antiquity (one need only think of the famous Greek and Roman myths of the "Golden Age"); thus the fact that the end of the Age of Taurus was a very ancient event, far from discouraging mythological speculations about it, would likely have given additional power and significance to such speculations. 
 This question was first raised by Jacques Duchesne-Guillemin in an
article in Le Ciel: Bulletin de la Société Astronomique
de Liège 52 (June, 1990) p. 207, and by the late Ioan Culianu
in his review of my book in The Journal of Religion 72.2 (April 1992)
p. 302. In private correspondence I referred Professor Duchesne-Guillemin
to the evidence that I will present here, and he responded (letter, Sept.
27, 1990) that I had answered his question to his complete satisfaction.
(Prof. Duchesne-Guillemin refers to this correspondence in a brief correction
to his article "Sur l'origine des mystères de Mithra,"
Académie des inscriptions et belles-lettres [Comptes Rendus]
January-March 1990, p. 284, n. 1.) Since Prof. Duchesne-Guillemin has changed
his position, a correction is also necessary in Mary Boyce and Frantz Grenet,
A History of Zoroastrianism: Volume Three (Leiden: E.J. Brill, 1991)
p. 469, n. 536, final sentence. Sadly, Professor Culianu's untimely death
prevented my discussing the matter with him.
 See Otto Neugebauer, A History of Ancient Mathematical Astronomy (New York, Heidelberg, Berlin: Springer-Verlag, 1975) pp. 594-8. On Babylonian "System B" see ibid., p. 368; on Greco-Roman borrowing of the 8° norm from Babylonian "System B" see ibid., p. 594.
 Ibid., p. 595.
 Ibid., pp. 596-7.
 Ptolemy, Almagest VII,2.
 Interestingly, in his one extant work, his Commentary on Aratus, Hipparchus says that he himself places the equinoxes at the beginnings of the signs-- i.e., 0° Aries and Libra (ed. Manitius, p. 48, lines 5-7). However, there exists good evidence that Hipparchus at other times used the traditional 8° Aries figure. In his De nuptiis Philologiae et Mercurii (8.823-4), Martianus Capella attributes the 8° figure to Hipparchus, and in A History of Ancient Mathematical Astronomy pp. 286-7, Otto Neugebauer notes the intimate relationship between the so-called "Liber Hermetis," which uses the 8° figure, and Hipparchus. In fact, it seems quite likely that it was Hipparchus himself who was responsible for the original introduction of the 8° figure into Greek astronomy: see Otto Neugebauer, The Exact Sciences in Antiquity (Providence: Brown Univeristy Press, 1957) p. 188, where he notes that the 8° figure entered Greek astronomy exactly at the time of Hipparchus, and also Neugebauer, History of Ancient Mathematical Astronomy p. 309, where he points out that it was most likely Hipparchus who first introduced Babylonian astronomical norms-- of which the 8° Aries equinox was one-- into Greek astronomy. Indeed, concerning the introduction of Babylonian astronomical ideas into Greece, G.J. Toomer has concluded that "the main channel through which they were funneled was Hipparchus" ("Hipparchus and Babylonian Astronomy" in E. Leichty, M. DeJ. Ellis, P. Gerardi eds., A Scientific Humanist: Studies in Memory of Abraham Sachs [Philadelphia: Occasional Publications of the Samuel Noah Kramer Fund #9, 1988] p. 362).
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