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where l is the old analog of the celestial longitude (ecliptic longitude in degrees from the beginning of the zodiacal sign), the zero point terms are in degrees and sexagesimal fractions thereof, n is the number of Julian years (of 365d) from the date 31 December 31 b.c. = JDN 1684898, and the denominators of the fractions are the periods of those terms. The 25,816 Julian year period is the secular term, i.e., the variation in longitude due to the gradual westward shift of the vernal equinox among the stars (the classical precession), and 1717y is the alleged period of the trepidation. The physical basis for the theory was in fact nil: the erroneous value of Ptolemy's precession value, 36"/y, but Ptolemy's reputation was such that this value went unchallenged, until Tycho Brahe. Table 7.15, from Swerdlow and Neugebauer

Table 7.15. Basis for Copernicus's trepidation.

Observer interval

Date

At A1b

0 0

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