Starlight Lines Forum

Gravitational attraction holds the solar system 
together and makes possible orbiting satellites, 

both natural and artificial.  Such motions are 
governed by Kepler's three laws of planetary 
motion, all of which are direct consequences of 
Newton's laws of motion and gravitation:

1.  The law of orbits.	All planets move in 
    elliptical orbits with the Sun at one focus.
2.  The law of areas.	A line joining any planet

    to the Sun sweeps out equal areas in equal 
    times.
3.  The law of periods. The square of the period
T
    of any planet about the Sun is proportional 
    to the cube of the semimajor axis "a" of the 

    orbit.  For circular orbit wiht radius "r", 
    the semimajor axis "a" is replaced by "r" and

    the law is written as T^2 = (4PI^2/GM)*r^3, 
    where M is hte mass of the attraction body.  

    This result is generally valid for elliptical

    planetary orbits, when the semimajor axis "a"

    is inserted in place of the circular radius
"r".