Constructing an EllipseOn this web page you can explore the geometric construction of an
ellipse. In Lesson 9.2 of Discovering Advanced Algebra: An
Investigative Approach, you learned the locus definition of an
ellipse.
Definition of an Ellipse
An ellipse is a locus of points P in a plane,
the sum of whose distances, d_{1} and d_{2},
from two fixed points, F_{1} and F_{2},
is always a constant, d. That is, d_{1} + d_{2}
= d, or F_{1}P + F_{2}P
= d. The two fixed points, F_{1} and F_{2},
are called foci.

The sketches on this page use this definition to construct an
ellipse. The construction is the same as the one explained in the
Exploration Constructing the Conic Sections in Chapter 8.
Sketch
This sketch shows two circles centered at F_{1} and F_{2}
and the points where they intersect. The radius of circle F_{1}
is congruent to segment AC, and the radius of circle F_{2}
is congruent to segment CB. Drag point C to trace the
intersections of the circles as the radii change. Drag points F_{1}
and F_{2} to reposition the foci, and drag point B
to change the sum of the lengths of the radii. Click X to erase the traces.
Investigate
 Slowly drag point C back and forth along segment AB.
What shape is created by the trace of the intersections of the circles?
 What is true about the sum of the distance from an intersection
point to F_{1} and the distance from the same
intersection point to F_{2}? In what ways is this sum
represented by the sketch?
 Why is the trace of the intersections an ellipse?
 Try moving F_{1}, F_{2}, and/or B,
erase the traces, then slowly drag C back and forth. Do you
still get an ellipse? In what ways has it changed?
Sketch
This sketch shows the locus of the intersections from the previous
sketch. Drag points F_{1} and F_{2} to
reposition the foci, and drag point B to change the sum of the
lengths of the radii. (Note: For technical reasons, there may be
small gaps at the ends of the ellipse.)
Investigate
 Experiment by dragging the foci farther apart and closer
together. How does the ellipse change?
 How far apart can the foci be before you no longer have an
ellipse?
 What happens when both foci are at the same point?
 Experiment by dragging point B. How does the ellipse
change? Based on the locus definition of an ellipse, what is changing?
 How can you make the ellipse taller than it is wide?
Go to the parabola
page or the hyperbola
page to explore the geometric
constructions of other conic sections. 