Editing Orbit (section)

===Kepler's laws===
Bodies following closed orbits repeat their paths with a certain time called the period. This motion is described by the empirical laws of Kepler, which can be mathematically derived from Newton's laws. These can be
formulated as follows:

# The orbit of a planet around the [[Sun]] is an ellipse, with the Sun in one of the focal points of that ellipse. [This focal point is actually the [[barycenter]] of the [[Solar System|Sun-planet system]]; for simplicity, this explanation assumes the Sun's mass is infinitely larger than that planet's.] The planet's orbit lies in a plane, called the '''[[Orbital plane (astronomy)|orbital plane]]'''. The point on the orbit closest to the attracting body is the periapsis. The point farthest from the attracting body is called the apoapsis. There are also specific terms for orbits about particular bodies; things orbiting the Sun have a [[perihelion]] and [[aphelion]], things orbiting the Earth have a [[perigee]] and [[apogee]], and things orbiting the [[Moon]] have a [[perilune]] and [[apolune]] (or [[periselene]] and [[aposelene]] respectively). An orbit around any [[star]], not just the Sun, has a [[periastron]] and an [[apastron]].
# As the planet moves in its orbit, the line from the Sun to the planet sweeps a constant area of the [[Orbital plane (astronomy)|orbital plane]] for a given period of time, regardless of which part of its orbit the planet traces during that period of time. This means that the planet moves faster near its [[perihelion]] than near its [[aphelion]], because at the smaller distance it needs to trace a greater arc to cover the same area. This law is usually stated as "equal areas in equal time."
# For a given orbit, the ratio of the cube of its [[semi-major axis]] to the square of its period is constant.