Huygens discovers Titan

The 1631 Mercury transit showed how a projected solar image could support planetary measurements.

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.

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.