Kepler's Laws of Planetary Motion
An ellipse is a squashed circle that can be drawn by punching two thumb tacks into some paper,
looping a string around the tacks, stretching the string with a pencil, and moving the pencil
around the tacks while keeping the string taut. The figure traced out is an ellipse and the thumb
tacks are at the two foci of the ellipse. An oval shape (like an egg) is not an ellipse: an oval tapers
at one end, but an ellipse is tapered at both ends (Kepler had tried oval shapes but he found they
did not work).
There are some terms I will use frequently in the rest of this book that are used in discussing any
sort of orbit. Here is a list of definitions:
1.
Major axis
the length of the longest dimension of an ellipse.
2.
Semimajor axis
one half of the major axis and equal to the distance from the center of
the ellipse to one end of the ellipse. It is also the average distance of a planet from the
Sun at one focus.
3.
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 Fall '10
 EmilyHoward
 Astronomy, Kepler's laws of planetary motion, Celestial mechanics, Semimajor axis

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