# 14. Mathematics for Orbits: Ellipses, Parabolas, Hyperbolas

*Michael Fowler*

## Preliminaries: Conic Sections

Ellipses, parabolas and hyperbolas can all be generated by cutting a cone with a plane (see diagrams, from Wikimedia Commons). Taking the cone to be and substituting the in that equation from the planar equation where is the vector perpendicular to the plane from the origin to the plane, gives a quadratic equation in This translates into a quadratic equation in the plane -- take the line of intersection of the cutting plane with the plane as the axis in both, then one is related to the other by a scaling To identify the conic, diagonalized the form, and look at the coefficients of If they are the same sign, it is an ellipse, opposite, a hyperbola. The parabola is the exceptional case where one is zero, the other equates to a linear term.

It is instructive to see how an important property of the ellipse follows immediately from this construction

. The slanting plane in the figure cuts the cone in an ellipse. Two spheres inside the cone, having circles of contact with the cone , are adjusted in size so that they both just touch the plane, at points respectively.It is easy to see that such spheres exist, for example start
with a tiny sphere inside the cone near the point, and gradually inflate it,
keeping it spherical and touching the cone, until it touches the plane. Now
consider a pointon the ellipse. Draw two lines:
one fromto the pointwhere
the small sphere touches, the other up the cone, aiming for the vertex, but
stopping at the point of intersection with the circle *Both
these lines are tangents to the small sphere, and so have the same length*. (The
tangents to a sphere from a point outside it form a cone, they are all of equal
length.) Now repeat with We find that the distances to the circles measured
along the line through the vertex. So in
the obvious notation -- are therefore evidently the
foci of the ellipse.

## The Ellipse

### Squashed Circles and Gardeners

The simplest nontrivial planetary orbit is a circle: is centered at the origin and has radius An ellipse is a circle scaled (squashed) in one direction, so an ellipse centered at the origin with semimajor axis and semiminor axis has equation

in the standard notation, a circle of radius scaled by a factor in thedirection. (It's usual to orient the larger axis along)

A *circle* can also be defined as the set of points
which are the same distancefrom a given point, and
an *ellipse* can be defined as the set of points such that the *sum of
the distances from two fixed points is a constant length* (which must
obviously be greater than the distance between the two points!). This is
sometimes called the *gardener's definition*: to set the outline of an
elliptic flower bed in a lawn, a gardener would drive in two stakes, tie a
loose string between them, then pull the string tight in all different
directions to form the outline.

In the diagram, the stakes are at ,
the **red lines** are the **string**, is an arbitrary point
on the ellipse.

* *is called the semimajor
axis length , the
semiminor axis, length .

* are called the foci
(plural of focus).*

Notice first that *the string has to be of length*** **, because it must stretch along the
major axis from to then
back to , and for that configuration there's a
double length of string along and a single length
from to . But
the length is the same as ,
so the total length of string is the same as the total length

Suppose now we put at. Since and
the string has length, the length** **

We get a useful result by applying Pythagoras' theorem to the triangle

(We shall use this shortly.)

Evidently, for a circle,

### Eccentricity

The *eccentricity*of
the ellipse is defined by

Eccentric just means off center, this is *how far the
focus is off the center of the ellipse*, as a fraction of the semimajor
axis. The eccentricity of a circle is zero. The eccentricity of a long thin
ellipse is just below one.

* *and on the diagram are called the *foci*
of the ellipse (plural of *focus*) because if a point source of light is
placed at , and the ellipse is a mirror, it will
reflect -- and therefore *focus* -- all the light to .

### Equivalence of the Two Definitions

We need to verify, of course, that this gardener's definition of the ellipse is equivalent to the squashed circle definition. From the diagram, the total string length

and squaring both sides of

then rearranging to have the residual square root by itself on the left-hand side, then squaring again,

,

from which, using we find

### Ellipse in Polar Coordinates

In fact, in analyzing planetary motion, it is more natural
to *take the origin of coordinates at the center of the Sun *rather than
the center of the elliptical orbit.

It is also more convenient to take coordinates instead of coordinates, because the strength of the gravitational force depends only on . Therefore, the relevant equation describing a planetary orbit is the equation with the origin at one focus, here we follow the standard usage and choose the origin at

For an ellipse of semi major axis and eccentricity the equation is:

This is also often written

where is the *semi-latus
rectum*, the perpendicular distance from a focus to the curve** **(so**) **, see the diagram below: but notice
again that *this equation has as its origin***!
(For .) **

(It's easy to prove using Pythagoras' theorem, .)

*The directrix*: writing the
equation for the ellipse can also be written as

,

where (the origin being the focus).

The line is called the *directrix*.

For any point on the ellipse, its distance from the focus is times its distance from the directrix.

### Deriving the Polar Equation from the Cartesian Equation

Note first that (following standard practice) the coordinates and used here have different origins!

Writingin the Cartesian equation,

that is, with slight rearrangement,

This is a quadratic equation forand can be solved in the usual fashion, but looking at the coefficients, it's evidently a little easier to solve the corresponding quadratic for

The solution is:

from which

where we drop the other root because it gives negative for example for This establishes the equivalence of the two equations.

## The Parabola

The parabola can be defines as the limiting curve of an ellipse as one focus (in the case we're examining, that would be ) going to infinity. The eccentricity evidently goes to one, , since the center of the ellipse has gone to infinity as well. The semi-latus rectum is still defined as the perpendicular distance from the focus to the curve, the equation is

Note that this describes a parabola opening to the *left*.
Taking the equation of this parabola is

All parabolas look the same, apart from scaling (maybe just in one direction).

The line perpendicular to the axis and the same distance
from the curve along the axis as the focus is, but outside the curve, is the
parabola's *directrix*. That is,

Each point on the curve is the same distance from the focus as it is from the directrix. This can be deduced from the limit of the ellipse property that the sum of distances to the two foci is constant. Let's call the other focus. Then So from the diagram,

*Exercises*: 1. Prove by finding the slope, etc., that
any ray of light emitted by a point lamp at the focus will be reflected by a
parabolic mirror to go out parallel to the axis.

2. From the diagram above, show that the equality easily gives the equation for the parabola, both in and in coordinates.

## The Hyperbola

### Cartesian Coordinates

The hyperbola has eccentricity In Cartesian coordinates, it has equation

and has two branches, both going to infinity approaching asymptotes The curve intersects the axis at , the foci are at , for any point on the curve,

the sign being opposite for the two branches.

*The semi*-*latus rectum*, as for the earlier
conics, is the perpendicular distance from a focus to the curve, and is Each focus has an associated directrix,
the distance of a point on the curve from the directrix multiplied by the
eccentricity gives its distance from the focus.

### Polar Coordinates

The equation with respect to a focus can be found by substituting in the Cartesian equation and solving the quadratic for

Notice that *has a limited
range*: the equation for the right-hand curve with respect to its own focus has

so

The equation for this curve is

in the range

This equation comes up with various signs! The left hand
curve, with respect to the left hand focus, would have a positive sign With origin at (on
the *left*) the equation of the *right*-hand curve is

finally with the origin at the left-hand curve is

These last two describe *repulsive*
inverse square scattering (Rutherford).

### Note: A Useful Result for Rutherford Scattering

If we define the hyperbola by

then *the perpendicular distance from a focus to an
asymptote*** ***is just*

This equation is the same (including scale) as

with

*Proof*: The triangle is
similar to triangle , so ,
and since the square of the hypotenuse is , the distance

I find this a surprising result because in analyzing Rutherford scattering (and other scattering) the impact parameter, the distance of the ingoing particle path from a parallel line through the scattering center, is denoted by Surely this can't be a coincidence? But I can't find anywhere that this was the original motivation for the notation.