Download Kepler`s Laws - Hewlett

Kepler's Laws
Johannes Kepler came up with 3 laws to describe the motion of the Planets:
Kepler’s 1st Law: Planets move in orbits that are ellipses (flattened circles/more like an
oval).
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Orbits are slightly eccentric ellipses.
Every ellipse has two focus points, with the Sun positioned at one of the foci.
I. The orbits of the planets are ellipses, with the Sun
at one focus of the ellipse.
Eccentricity- describes the degree of roundness of an ellipse
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Eccentricity of 0.0 = Perfect Circle
Eccentricity of 1.0 = Straight Line
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Eccentricity Equation= distance between foci
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length of major axis
Kepler’s 2nd Law
The planets move such that the line between the Sun and the Planet sweeps out the same
area in the same time no matter where the planet is in the orbit.
II. The line joining the planet to the Sun sweeps out
equal areas in equal times as the planet travels
around the ellipse.
Perihelion- planets position when it is closest to the Sun
Aphelion- planets position when it is farthest from the Sun
Orbital Motion Questions
1. When would the orbital velocity of a planet be the greatest and when would it
be the least?
2. When would a planet’s kinetic energy be the greatest, and when would it be
the least?
3. When would a planet’s potential energy be the greatest and when would it be
the least?
4. During what season would Earth be traveling the fastest around the Sun?
Explain why?
Kepler’s 3rd Law
The square of the period of the orbit of a
planet is proportional to the mean
distance from the Sun cubed.
In simple terms: The farther a planet is
from the Sun, the larger its orbit and the
longer its period. Period- time it takes a
planet to complete a revolution around
the Sun
T2
R3
T=Perio
d
R=Orbit
al Radius