Instrument making helps reveal Titan

Horrocks's calculation enabled the observers to watch a transit missed by earlier forecasts.

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.

Cassini detected a moon later called Iapetus while observing Saturn.

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.