ESS Kepler's Law Notes

1. Gravity & Kepler’s
Laws

2. Gravity
• An attractive force between
two objects exerted over a
distance
• Magnitude determined by the
mass of the two objects and the
distance between them

3. Law of Orbits
• All planets move in elliptical orbits, with
the sun at one focus

4. Eccentricity
• Ratio between the
major axis and the
distance between the
foci; closer to 1 more
“oval-like”, closer to 0
the more “circle-like”

5. Law of Areas
• A line that connects a planet to
the sun sweeps out equal areas
in equal times
• When a planet is closer to the
sun, the force of gravity goes
up, therefore the planet
accelerates and speeds up;
farther away planets go slower

6. Law of Periods
• The square of the period of any
planet is proportional to the
cube of the semi-major axis of
its orbit
• T2 = a3
• T = period; Earth years of one
revolution around sun
• a = AU’s from the sun