Draw an Ellipse with String and Pins

This classic activity physically demonstrates the geometry behind Kepler's First Law. It's suitable for students of all ages and requires only simple materials.

Materials

A piece of cardboard or corkboard (at least 30 cm × 20 cm)
Two thumbtacks or push pins
A loop of string (about 30–40 cm total length)
A pencil
A ruler

Instructions

Place the cardboard on a flat surface.
Push the two pins into the board about 8–10 cm apart. These are the foci of the ellipse. In a real orbit, the Sun would be at one of these pins.
Loop the string around both pins. The string should have some slack—it needs to be longer than the distance between the pins.
Place the pencil inside the loop and pull it outward until the string is taut.
Keeping the string taut, drag the pencil around the pins. The pencil will trace an ellipse.

Vary the Eccentricity

Pins close together (relative to string length): The ellipse is nearly circular (low eccentricity).
Pins far apart: The ellipse is elongated (high eccentricity).
Pins at the same point: You draw a perfect circle (eccentricity = 0).

Key Takeaway

For any point on the ellipse, the sum of the distances to the two foci is constant (it equals the length of your string). This is the mathematical definition of an ellipse. In an orbit, the Sun sits at one focus and the other focus is an empty point in space.