Dear Bill,

You must be right when saying that "the 28-nakṣatra system may also
likely postdate the 27 one". Actually, the Vedāṅga-jyotiṣa (VJ), which
has only 27 n., uses the beginning of n.Śravişţhā and the middle of
n.Āśleşā as winter and summer solstices respectively, which places the
astronomical data of the VJ around 1150 BC. The Gargasaṃhitā and the
Paitāmahasiddhānta, summarized by Varāhamihira, are similar, but the
jain works Jyotiṣakaraņḍa and Sūryaprajñapti replace n.Śravişţā by
n.Śravaņa and add n.Abhijit (Vega) to the zodiac. On the other hand, the
Sūryaprajñapti puts the winter solstice at the beginning of n.Abhijit,
which makes a difference of 17,3° with the beginning of n. Śravişţhā
(VJ), that is 1246 years after VJ (see G.Thibaut, « On the
Sūryaprajñapti », JASB (1880), p.117).

Best,

Jean Michel Delire,
Lecturer on Science and Civilization of India - Sanskrit Texts at the
IHEB (University of Brussels)