Planetary Orbits - Notes

This document is provided by Flinn Scientific, Inc.
It is catalog number APT226 and publication number 7329.
Johannes Kepler (1571-1630) discovered that planetary orbits are ellipses, not circles.
The kit explores elliptical orbits and Kepler's laws of planetary motion.

Before Isaac Newton, Johannes Kepler formulated his laws of planetary motion based on observations.
Nicolaus Copernicus (1473-1543) proposed in 1543 that the Sun is the center of the solar system, with planets orbiting in circular paths.
Kepler's book, New Astronomy, published in 1609, presented observations and calculations showing planets travel in ellipses, not perfect circles.
An ellipse is an elongated circle with two foci instead of one center.
The major axis is the longest distance through the center of the ellipse, and the minor axis is the shortest distance.

Eccentricity defines how "oblong" an ellipse is.
It is the ratio of the distance between the foci (f) to the length of the major axis (a).
Equation:
$e = \frac{f}{a}$
An ellipse with zero eccentricity is a circle.
The larger the eccentricity, the more flattened the ellipse.
Elliptical orbits have eccentricities between zero and one.
Most planets have eccentricities close to zero, making it difficult to distinguish between circular and elliptical orbits in small models.
Mercury, Pluto, and Halley's Comet are exceptions with larger eccentricities and more elongated paths.