The Principia sets out planetary mechanics

Iapetus joined Titan among the known satellites of Saturn.

Cassini's later observations found the moon much easier to see on one side of Saturn than the other.

The discovery illustrates the expanding reach of systematic telescope observation.

The first edition laid out laws of motion and universal gravitation.

The new work used the same gravitational principle to explain motion on Earth and in the heavens.

Publication supplied tools for calculating motions of planets and comets, though later work refined them.

NASA's UTC catalog dates the transit to 6 June; some observers described it as 5 June in their local time.

Teams at widely separated sites recorded the transit to help estimate the Earth–Sun distance.

Astronomers sought to compare the instants when Venus crossed the solar limb across different locations.

The indistinct contact of Venus with the solar edge complicated the measurements made at this transit.