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Hello Graham,
They are not meant to be read sidereally or tropically. The astrological decans are compared with a decan list to show concordance between the spacing of the honored decans and the later astrological decans of exaltations. We do not know when or where observations for the various decan lists were made. The oldest known list of decans that was absorbed into Hellenistic astrology is that of Hephaestion, whose list seems to have been derived from two different Egyptian lists, which themselves date centuries earlier, but not to the same centuries. It is difficult to "translate" any Egyptian decan to the correct later astrological decan or with a specific star.