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Statement of Kepler's First Law
We can now state Kepler's First Law clearly:
Planets orbit the sun in ellipses with the sun at one focus.
This statement means that if a point $P$ represents the position of a planet on an ellipse, then the
distance from this point to the sun (which is at one focus) plus the distance from $P$ to this other
focus remains constant as the planet moves around the ellipse. This is a special property of
ellipses, and is illustrated clearly in . In this case $d_1 + d_2 = l_1 + l_2 = $ a constant as the
planet moves around the sun.
Figure %: Sum of distances to each focus is a constant.
As marked on the figure, the closest point that the planet comes to the sun is known as the
aphelion and the farthest point that the planet moves from the sun is called the perihelion.
Kepler's Second Law
Statement of Kepler's Second Law
Kepler's Second Law can be stated in several equivalent ways:
Figure %: A planet sweeps out equal areas in equal times.
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This note was uploaded on 02/09/2012 for the course PHY PHY2053 taught by Professor Davidjudd during the Fall '10 term at Broward College.
 Fall '10
 DavidJudd
 Physics

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