## Posts Tagged ‘mathematical astronomy’

## Ellipse

## An ellipse is the correct name for an oval shape.

The planets do not orbit in perfect circles, as was believed before Kepler’s Laws of planetary motion, but rather in slightly oval (elliptical) orbits. Their eccentricity is low so the orbits are not far off being circles. An ellipse has two foci (plural of focus), in the Solar System, the Sun lies at one of these.

The same applies to binary stars, they have elliptical orbits too.

There are several methods of drawing an ellipse. A simple way is to make a loop of string and firmly fix two pins into a drawing board, through the paper set apart along where you want the axis of the ellipse to be. Place the point of your pencil inside the string loop and use the pencil to pull the string tight. Then try to draw a freehand circle. The string will guide the pencil, producing an ellipse.

A more accurate way is to use the trammel method. Naturally, this takes longer. If you use an accurate method of drawing the planetary orbits to scale, you will probably be disappointed – they will look like circles as their eccentricity is small.