Which preposition to use with cusps

of Occurrences 7%

This is the last day of my year, and should my engagement be unredeemed when the sun attains the cusp of that nethermost house of heaven which he is even now traversing, I must become an inmate of the infernal kingdom.

in Occurrences 1%

The details are poor (note the absence of cusps in alternate windows of nave), and the fan tracery (original in choir only) is exuberant.

Which preposition to use with  cusps