Mercury in Sagittarius

Detriment here is a matter of geometry. Sagittarius faces Gemini across the wheel, and since Gemini is Mercury's house, Lilly 1659 lists the opposite sign as the planet's detriment. The term names a position and describes nobody's mind.

In Ptolemy (Ashmand's 1822 translation) Sagittarius is a house of Jupiter and a bicorporeal sign, which gives the mind range and changeability. Lilly gives Mercury a face in the first ten degrees of this sign and a term from 15 to 20. Those small entries sit inside a sign where his tables otherwise have nothing for the planet.

Alan Leo 1899 describes Sagittarius as aiming at a single goal, and Lilly's Mercury searches into every science. A wide-ranging mind chasing one idea is the tension this pairing offers to anyone comparing the two books. Modern astrology usually calls the placement blunt and far-seeing.