A telescope records a rare alignment

His first dated observations began the sequence that established satellites orbiting Jupiter.

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.

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.