Dear Jim,
> The Buddha was occasionally referred to as the "Kinsman of
> the Sun". Does anyone know how this phrase originated? It
> may just be a standard honorific, but apparently the planet
> mercury is referred to as "Budh" in Indian astrology, so
> there may be a play on words here.
I vaguely recall that the commentators do somewhere play
upon budho (wise), budho (Mercury), and buddho (awakened),
but this has no connection with "Kinsman of the Son"
(aadiccabandhuu).
In the Tipi.taka the only attempt to explain aadiccabandhuu
is in the two Niddesas, both of which attribute it to
Aaditi, the father of Suriya, being of the Gotama-gotta.
The commentators give two further grounds:
1) on account of the Buddha's noble birth: ta.m pa.ticca
tassa ariyaaya jaatiyaa jaatattaa (I think this might be
intended as a nirutta).
2) on account of Suriya, the sun deity, being his orasaputta
('breast-born son'), meaning his disciple.
Best wishes,
Dhammanando