Time it takes planet #1 to orbit a star (such as the Sun).
Time a planet #2 needs to orbit the sun. So we need the data of two planets for the third Kepler law.
Planets travel around the star on elliptical orbits. The corresponding ellipse has a large semi-axis. It corresponds to half of the long axis of the ellipse. The traversed elliptic orbit is different for different planets. Here \(a_1\) is the semi-major axis of the elliptical orbit of planet #1.
Semi-major axis of another, second planet.