1. It’s gonna get eccentric…
Ellipse Activity
It’s gonna get eccentric…

2. The Big Idea
As you know, the planets do not revolve around the Sun in perfect circles.
There are a number of reasons for that but an important one is gravity.
They’re all under the influence of the Sun’s gravity, but they also exert gravitational forces on one another.

3. Ellipses
Even though orbits are never perfect circles, not every orbit is the same.
As you might guess, astrophysicists use a variety of characteristics of ellipses to quantify them:
Eccentricity
Major Axis
Semi-Major Axis

4. Key Ellipse Vocabulary
Eccentricity is the deviation of an ellipse from a perfect circle, equal to the distance between the foci divided by major axis.
Major axis is the “long distance” from the ends of an ellipse.
Semi-major axis is half the major axis.
Zero
Eccentricity
High Eccentricity

5. Your Activity
You’re going to be drawing ellipses, and to more precisely draw ellipses, you’re going to use thumbtacks to simulate the foci.
Foci is the plural of focus.
The foci of an ellipse are the two points where the sum of distances of lines drawn to each of them are always the same.
The Sun is at one of Earth’s ellipse’s foci.
In plain English, it’s the points used to make the shape of the ellipse.
How will you make your ellipses? f f

6. Ellipses
The activity will walk you through the process of using thumbtacks and a length of string to make the ellipses.
You’ll be drawing ellipses on a separate sheet of paper.
The tacks serve as the foci of the ellipse.
You’ll use the same piece of string to do all ellipses.

7. That’s it! Just one last thing to keep in mind:
These are not going to be exaggerated ellipses.
To the untrained eye, they may even appear to be circles.
Let the formula for the eccentricity of your ellipse convince you…
…and speaking of which, eccentricity is measured with this equation, where e is eccentricity, d is the distance between the foci (tacks), and L is the longest diameter of the ellipse: