Luna is the most registered pet name in this catalogue, and it means the moon. That is not a fashion; it has been true across every register we count, in three countries, for as long as the data goes back.
Each one names something
The tag records which object or mission a name refers to, which is what makes it auditable — Nova for an exploding star, Apollo for the programme and the god before it. A name that merely sounds cosmic does not qualify.
Every count on this site is a tally of real registrations: pet licences and shelter intakes published as open data by New York, Seattle, Austin, Montgomery County, Cambridge, Zurich, Basel and three Australian councils. Popularity here is the average share a name holds across those registers, so a name common in a small city is not buried by one common in a large one. Nine of the eleven sources are North American, so read the ordering as naming in those places rather than worldwide.