Optical depth: A
measure of the transparency of a ring system or a cloud. The greater
the optical depth, the more light is blocked from passing through
the ring. In other words, a ring or cloud with a large optical
depth is less transparent than one with a smaller optical depth.
Resonance: A periodic
gravitational influence on one orbiting object by another that
affects its orbital path.
Shepherd moon: A
moon that limits the extent of a planetary ring through gravitational
Trojan moon: A moon
that orbits a planet 60 degrees ahead of or behind a large moon.