- 1. Planets move around the sun tracing ellipses, not circles (distance from the sun varies during orbit
- (average of the farthest and closest distances on the ellipses = semi-major axis - avg distance from sun)
2. the orbits sweep out equal area in equal amt of time --> travels faster when nearer to sun, slower when farther ==> Accelerating and Decelerating in their orbits.
3. The Period of the orbit is related to the semi-major axis a
(radius of the circle - distance between that planet and the sun as P2
--> more distant planets orbit the sun at slower avg speeds, obeying a precise mathematical relationship P2