Transcript Planetary Orbits

Planetary Orbits
The ancient Greeks (Aristotle and Plato) thought
the only perfect shapes were the circle and line.
All things fall in a line toward Earth, except things
in Heaven (the stars and planets) which must move
in a circle, with Earth at the center, in order to be
“perfect”.
By the 3rd Century B.C., astronomers could not explain
the actual movement in the sky of the planets, if they
moved in circles around the Earth. So they came up
with a system of “epicycles”, planets moving around
circles on circles, which sort of explained the motion.
As centuries passed, and instruments became more
accurate, astronomers had more and more problems
predicting exactly where the planets should be, and
they added more and more epicycles.
One of the most obvious problems was Mars. Venus,
Mercury and the Moon move in one direction across the
sky. If you see it one night, the next night it will have
moved a little bit over, the next a little more, and so on,
all in the same direction.
Mars will do that, but sometimes stops and goes
backwards (retrograde motion), and then changes again.
In the early 1600s, German mathematician Johannes
Kepler solved the problem with his Three Laws of
Planetary Motion.
Law 1: The orbit of the planets are ellipses with the Sun
at one focus.
Eccentricity describes how circular or elliptical an
orbit is. It is defined as “d ”, the distance between
the foci, divided by “l ”, the length of the major axis
(the widest part of the ellipse).
major axis
d
foci
Law Two: A line from a planet to the Sun sweeps over
equal areas in equal intervals of time.
Law Three: A planet’s orbital period squared (years) is
proportional to its average distance to the Sun (AU)
cubed.
Py2 = a AU3