# 5. The Ellipse

## Why study ellipses?

Orbiting satellites (including the earth and the moon) trace out elliptical paths.

Many buildings and bridges use the ellipse as a pleasing (and strong) shape.

### On this page...

Horizontal major axis

Vertical major axis

Centre other than origin

One property of ellipses is that a sound (or any radiation) beginning in one focus of the ellipse will be reflected so it can be heard clearly at the other focus. You can see this working in the following animation.

**Ellipses with Horizontal Major Axis**

The equation for an ellipse with a horizontal major axis is given by:

`x^2/a^2+y^2/b^2=1`

where `a` is the length from the center of the ellipse to the end the major axis, and `b` is the length from the center to the end of the minor axis.

The **foci** (plural of 'focus') of the ellipse (with horizontal major axis)

`x^2/a^2+y^2/b^2=1`

are
at (-*c*,0) and (*c*,0), where *c* is given
by:

`c=sqrt(a^2-b^2`

The **vertices** of an ellipse are at (-*a*, 0) and
(*a*, 0).

### Ellipse as a locus

The **ellipse** is defined as the locus of a point
`(x,y)` which moves so that the sum of its distances
from two fixed points (called **foci***,* or **focuses**) is
constant.

We can produce an ellipse by pinning the ends of a piece of string and keeping a pencil tightly within the boundary of the string, as follows.

We start with these 2 foci:

We pin the ends of the string to the foci and begin to draw, holding the string tight:

Our complete ellipse is formed:

### Example 1 - Ellipse with Horizontal Major Axis

### Need Graph Paper?

Find the coordinates of the vertices and foci of

`x^2/100+y^2/64=1`

Sketch the curve.

## Ellipse with Vertical Major Axis

Our first example above had a horizontal major axis.

If
the major axis is **vertical**, then the formula
becomes:

`x^2/b^2+y^2/a^2=1`

We always choose our *a* and *b* such that *a*
> *b*. The major axis is always associated with
*a*.

### Example 2 - Ellipse with Vertical Major Axis

Find the coordinates of the vertices and foci of

`25x^2+y^2=25`

Sketch the curve.

### Example 3

Find the equation of the ellipse which has a minor axis of length 8 and a vertex at (0,-5).

## Eccentricity

The **eccentricity** of an ellipse is a measure of how elongated it is. If the eccentricity approaches value 0, the curve becomes more circular, and if it approaches 1, the ellipse becomes more elongated.

We can calculate the eccentricity using the formula:

`text(eccentricity)=c/a`

### Real Example

The Sun

The Earth revolves around the sun in an elliptical orbit, where the sun is at one of the foci. (This was discovered by Keppler in 1610).

The semi-major axis is approximately
149,597,871 km long and it is known that the ratio* `c/a` * is equal to `1/60`.

(i) What are the greatest and least distances the Earth is from the sun?

(ii) How far from the sun is the other focus?

(The "semi-major axis" means half of the major axis length. In our example, it is (close to) the "average" distance of the sun from the earth, and is also known as one A.U., or "astronomical unit".)

## Ellipses with Centre Other Than the Origin

Like the other conics, we can move the ellipse so that its axes
are not on the *x*-axis and *y*-axis. We do this for convenience when solving certain problems.

For the horizontal major axis case, if we move the
intersection of the major and minor axes to the point (*h*,
*k*), we have:

`((x-h)^2)/a^2+((y-k)^2)/b^2=1`

The ellipse is as follows:

### Example 4

Sketch the ellipse with equation

`((x-1)^2)/25+((y+2)^2)/9=1`

### Conic section: Ellipse

How can we obtain an ellipse from slicing a cone?

We start with a **double cone** (2 right circular cones placed apex to apex):

When we slice one of the cones at an angle to the sides of the cone, we get an **ellipse**, as seen in the view from the top (at right).

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