Simplify the following text for a Grade 5 native English-speaking person.
Original: He named it simply Saturni Luna ( or Luna Saturni , Latin for `` Saturn 's moon '' ) , publishing in the 1655 tract De Saturni Luna Observatio Nova .
Simplified: At first he called it `` Luna Saturni '' , which means `` Saturn 's moon '' ( he did not know there was more than one ) .