Eccentricity of an Ellipse

An ellipse is a fundamental conic section defined by the constant sum of distances from any point on its curve to two fixed points, known as the foci. This geometric property can be physically demonstrated using a pencil, string, and two pins. By anchoring the string at both ends and maintaining it taut with a pencil, one can trace the outline of an ellipse.

The shape and extent of the ellipse are determined by its eccentricity, e, defined as the ratio of the distance between the center and a focus (c) to the semi-major axis length (a). When e = 0, the ellipse is a circle. As e increases toward 1, the ellipse elongates along its major axis.

This property underpins the elliptical nature of celestial orbits. Halley’s Comet, for instance, travels along a highly eccentric elliptical orbit with e = 0.967. The major axis of its orbit measures approximately 35.88 astronomical units (AU), a common unit in astronomy equivalent to the average distance between Earth and the Sun. This means the semi-major axis, a, is 17.95 AU. The distance from the center of the ellipse to each focus (the focal distance) is given by a × e. For Halley’s Comet, this results in

The semi-minor axis b can be computed using the relationship

Placing the ellipse in standard form, centered at the origin and aligned along the x-axis, the orbital equation becomes:

From this model, the perihelion distance (closest approach to the Sun) is a − c ≈ 0.59 AU, and the aphelion distance (farthest point from the Sun) is a + c ≈ 35.29 AU. This substantial variation illustrates Halley’s orbit’s pronounced eccentricity and the usefulness of its elliptical geometry in celestial mechanics.