Australis' name is also related to one of the stars (Kaus Australis) in the star constellation of Sagittarius, which is supposedly ruled by Jupiter. Will's occupation of Accountant was chosen because Jupiter is largely associated with finances. JupiterAstro's color was going to be purple, however, no such color exists in the sentai regime (only: Red, Green, Black, Blue, Yellow, White and Pink; the last four are the only allowable colors for female rangers). His ThunderBow is associated with the mythological beast of Sagittarius; the Centaur, whom were known for firing from bows and arrows.
It might be interesting to note that according to the rec.arts.martial-arts FAQ as contributed to by E. Clay Buchanan, the martial-art Kyudo involves archery. However, the bow as a weapon became extremely unpopular after firearms got popular until Honda Toshizane re-introduced kyudo. In 1953, the Zen Nihon Kyudo Federation (All Japan Kyudo Federation) was founded to instruct the art and the first seminar and promotion test was held in America in San Jose, California.
Anyway, Australis is more of a leader than Sheratan because Jupiter's character represents leadership and being of higher-mind. This is why he holds such a philosophical outlook on humans.