An ellipse usually looks like a squashed circle:
"F" is a focus, "G" is a focus,
and together they are called foci.
The total distance from F to P to G is always the same
In other words, when we go from point "F" to any point on the ellipse and then go on to point "G", we always travel the same distance.
You Can Draw It Yourself
Put two pins in a board, put a loop of string around them, and insert a pencil into the loop. Keep the string stretched so it forms a triangle, and draw a curve ... you will draw an ellipse.
It works because the string naturally forces the same distance from pin-to-pencil-to-other-pin.
A Circle is an Ellipse
In fact a Circle is an Ellipse, where both foci are at the same point (the center).
In other words, a circle is a "special case" of an ellipse. Ellipses Rule!
An ellipse is the set of all points on a plane whose distance from two fixed points F and G add up to a constant.
Major and Minor Axes
The Major Axis is the longest diameter. It goes from one side of the ellipse, through the center, to the other side, at the widest part of the ellipse. And the Minor Axis is the shortest diameter (at the narrowest part of the ellipse).
The Semi-major Axis is half of the Major Axis, and the Semi-minor Axis is half of the Minor Axis.
Area is easy, perimeter is not!
The area of an ellipse is:
π × a × b
where a is the length of the Semi-major Axis, and b is the length of the Semi-minor Axis.
Be careful: a and b are from the center outwards (not all the way across).
(Note: for a circle, a and b are equal to the radius, and you get π × r × r = πr2, which is right!)
Rather strangely, the perimeter of an ellipse is very difficult to calculate, so I created a special page for the subject: read Perimeter of an Ellipse for more details.
But a simple approximation that is within about 5% of the true value (so long as a is not more than 3 times longer than b) is as follows:
Remember, this is only a rough approximation! (That is why the "equals sign" is squiggly.)
A tangent is a line that just touches a curve at one point, without cutting across it. Here is a tangent to an ellipse:
Here is a cool thing: the tangent line has equal angles with the two lines going to each focus! Try bringing the two focus points together (so the ellipse is a circle) ... what do you notice?
Section of a Cone
By placing an ellipse on an x-y graph (with its major axis on the x-axis and minor axis on the y-axis), the equation of the curve is:
x2a2 + y2b2 = 1
(similar to the equation of the hyperbola: x2/a2 − y2/b2 = 1, except for a "+" instead of a "−")