Iapetus's unusual visibility invites study

Observations of a small nearby object laid the groundwork for identifying an orbiting moon.

Titan's detection revealed that Saturn, like Jupiter, has a substantial family of orbiting objects.

Huygens's observational work linked improvements in telescopes with discoveries beyond Earth.

Cassini detected a moon later called Iapetus while observing Saturn.

Iapetus joined Titan among the known satellites of Saturn.

The discovery illustrates the expanding reach of systematic telescope observation.

The first edition laid out laws of motion and universal gravitation.

The new work used the same gravitational principle to explain motion on Earth and in the heavens.

Newton presented mathematical arguments for orbital motion and other celestial phenomena.

Publication supplied tools for calculating motions of planets and comets, though later work refined them.