That the speed of planetary motion varies in different parts of the orbit in such a way that an imaginary line drawn from the sun to the planet--that is to say, the radius vector of the planet's orbit--always sweeps the same area in a given time.
These two laws Kepler published as early as 1609.

Many years more of patient investigation were required before he found out the secret of the relation between planetary distances and times of revolution which his third law expresses.

In 1618, however, he was able to formulate this relation also, as follows: 3.

The squares of the distance of the various planets from the sun are proportional to the cubes of their periods of revolution about the sun.
All these laws, it will be observed, take for granted the fact that the sun is the centre of the planetary orbits.

It must be understood, too, that the earth is constantly regarded, in accordance with the Copernican system, as being itself a member of the planetary system, subject to precisely the same laws as the other planets.