Which preposition to use with gnomon

of Occurrences 3%

Mr. Bowman, F.L.S. says, "We arrested several of these little aëronauts in their flight, and placed them on the brass gnomon of the sundial, and had the gratification to see them prepare for, and recommence, their aerial voyage.

for Occurrences 1%

Some have maintained that the obelisks which the Egyptians erected served the purpose of gnomons for determining the obliquity of the ecliptic, the altitude of the pole, and the length of the tropical year.

Which preposition to use with  gnomon