Elliptical Orbits

Although orbiting planets appear to follow a circular path, we actually find that most objects orbit along an ellipse.

An ellipse is a squashed circle with two focus points or foci. The sum of the distances to the foci from any point on the ellipse is a constant, i.e. a + b remains the same. In the diagram, x marks the location of the two foci and also of one of the points where the star or planet being orbited will reside.

The amount of 'flattening' of an ellipse is termed the eccentricity. The more squashed the circle, the more eccentric is gets. A circle can be seen as a special case of an ellipse with zero eccentricity, whilst as an ellipse becomes more flattened the eccentricity approaches one. Thus, all ellipses have eccentricities lying between zero and one.

The orbits of the planets are ellipses, but for most the eccentricities are so small that they look circular. Pluto and Mercury, however, have more eccentric orbits that are clearly not circles.

Try using the electric orrery to see how eccentric these orbits are.

The longest axis of the ellipse is called the major axis, while the short axis is called the minor axis. Half of the major axis is termed a semi-major axis.

The average separation of a planet from the Sun is equal to the length of the semi-major axis, so the radius of a planet's orbit is given by the length of the semi-major axis.