Parnassus moon was the sole natural satellite of the planet Parnassus Beta.
After attacking the USS Cayuga and the Parnassus Beta colony in 2259, the Gorn Hegemony transmitted a message to Starfleet demarcating a border through the Parnassus system that placed Parnassus Beta on their side but not its moon. With orders not to cross the line, the USS Enterprise held position near the moon while the crew attempted to extract survivors from the colony without inciting a wider conflict with the Gorn. (SNW: "Hegemony")