Galileo's observing notes mark a turning point

Kepler's dated observation became part of the record of the last supernova known to have been visible in our Milky Way.

Observers could see this extraordinary object without a telescope; the remnant remains a subject of study.

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.

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.