Kepler's second law of planetary motion, specifically, the law of equal areas, is demonstrated by your high schoolers. On the provided graph paper, they mark out the designated path of Earth at two different times of the year and then compute the areas swept for comparison. Note that this activity is a sampler from a larger unit, so the first few pages of the PDF will not be of use. The rest of the unit can be purchased through the publisher's website, but this activity can easily stand alone as part of your solar system or planetary motion unit.