AS101 Study Guide - Final Guide: Scientific Modelling, Orbital Period, Semi-Major And Semi-Minor Axes
Document Summary
Kepler"s first law: the orbit of each planet around the. Sun is an ellipse with the sun at one focus. The diagram below shows what an ellipse is: an ellipse has two foci (plural of focus) and the sun is at one focus (nothing is at the other focus). The major axis is a line drawn through the two foci and ending at each end of the ellipse. Half this distance is called the semi-major axis. The same holds true for the minor axis, perpendicular to the major axis: the ratio of the distance between the two foci to the major axis length is called the eccentricity. A circle has an eccentricity of zero (0. 00) because the two foci are on top of each other. A straight line has an eccentricity of unity (1. 0) because the two foci are as far away from each other as they can possibly be at infinity on either side of the ellipse.
