Gassendi observes Mercury cross the Sun

The 1604 outburst furnished another striking challenge to the doctrine of immutable heavens.

He recorded three small points near Jupiter, initially regarding them as background stars.

The positions of the lights were unusual enough to prompt repeated observations on later nights.

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.

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.