A companion appears beside Saturn

Jeremiah Horrocks watched Venus pass in front of the Sun from England.

William Crabtree also saw the transit from England; the two observers are the earliest documented witnesses.

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.

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.

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