In day II, Galileo does not determine or prove that the earth rotates. However, neither does he disprove that the earth remains motionless. Through this section, he shows only that many, if not all, of the arguments against the earth rotating have absolutely no relevance to the theory that earth remaining motionless. Early in this section, Salviati reveals many concepts including that the earth's motions do not affect bodies on the earth. This concept is later addressed in an attempt to disprove the relevance of the various arguments. Although other concepts are discussed, this concept is shown to have the greatest effect on the arguments of those who do not believe the earth to rotate.
Relative motion is the concept that motion does not exist for objects that move equally. This principle allowed for many of Galileo's arguments as portrayed as Salviati. Salviati analyzes the results of several experiments and observations to prove the principal of relative motion, which serves to defeat many of the arguments against the rotation of the earth.
Most of the arguments against the rotation of the earth were in the form of experiments. Such as one experiment was that a ball, when dropped from a tower, would appear to follow a perpendicular path to the surface of the earth and land directly in line with the point from which the ball was released. This was taken to conclude that the earth was motionless, because it was believed that if the earth rotated, then the ball would land to the east of where it had been released. A second experiment involved a cannonball being shot directly up. This experiment was taken to conclude motionless of the earth by much the same conclusions as the prior experiment.
Another experiment, which had previously been thought to conclude that the earth didn't rotate, involved firing a cannonball in the four basic directions, North, South, East and West. The observations regardi