GLOSSARY

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 forces.

Trojan moon: A moon that orbits a planet 60 degrees ahead of or behind a large moon.