A predicted transit becomes visible

The observation provided a direct test of a calculated planetary transit.

The apparent size of Mercury on the Sun surprised observers and helped refine ideas about its dimensions.

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.

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.