Another small Saturnian moon, Calypso shares an orbit with Tethys, trailing its larger associate by sixty degrees. With Saturn at the center, if Tethys were at the 12 on a clock face, Calypso would be at 2, and Tethys’ other “Trojan” moon, Telesto, would be at 10. Picturing it another way, Saturn, Tethys, and Calypso form an equilateral triangle by their positions.
These positions are the most stable two of five points for a natural or artificial body to “associate” gravitationally in a sort of station-keeping position. The “Trojan” designation comes from the naming convention of those asteroids clustered in front of and behind Jupiter in its orbit around the sun. Those bodies are named for characters in Homer’s Iliad.
Mathematician Josef Lagrange discovered that at five points, the gravities of the earth and sun cancel out in such a way so as to provide a “resting spot” for a smaller body. Mars has the asteroid Eureka and maybe a few more Trojans, but Earth has only concentrations of dust at its L4 and L5 points. Read a bit more on them here.
Those of you who are astronomically savvy might recall the L5 Society, an organization that promotes the colonization of space. Actually L4 in the Earth-Moon system will have a more spectacular view, with the Orientale impact basin staring at you like an eye. (From earth, we just see the edges of the surrounding mountains and a hint of the center mare (or “sea.”))