The orbits are elliptical, not circular. The period squared equals the mean distance from the Sun cubed. Kepler's laws are important because they are the first laws to mathematically describe the solar system very accurately. He also found the four moons of Jupiter. This is important because objects that are orbiting Jupiter cannot also be orbiting the Earth, and therefore the geocentric model is not likely to be true.
Weight is a force.
It is the force of gravity pulling on a mass at the surface of a planet. A person's mass will not depend on that person's location. Weight is equal to mass times the acceleration due to gravity at a given location. The sidereal day is 3.9 minutes less than the solar day. Seasons are caused by the tilt of the Earth's rotation axis with respect to the ecliptic.