Cassini discovers Saturn's moon Iapetus

Projected solar viewing made the planet's angular size measurable during the event.

Christiaan Huygens detected the first known moon of Saturn through a telescope.

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.

Iapetus joined Titan among the known satellites of Saturn.

Cassini's later observations found the moon much easier to see on one side of Saturn than the other.

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.