Horrocks observes Venus in transit

A more powerful telescope made faint neighboring points visible; recognition of their orbits followed later.

Pierre Gassendi observed Mercury as a tiny dark dot moving across the solar disk.

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.

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.

Observations of a small nearby object laid the groundwork for identifying an orbiting moon.