Kepler's Laws — Draw a Planet's Ellipse with Two Pins and a String

Between 1609 and 1619 Johannes Kepler used Tycho Brahe's precise data to prove planets move in ELLIPSES, not perfect circles, with the Sun at one focus. You will draw that exact shape.

Push two drawing pins into a board a few centimetres apart -- these are the two 'foci'. Tie a loop of string and drop it over both pins. Put a pencil inside the loop and pull it taut.

Keeping the string taut, run the pencil all the way around the two pins. It traces a perfect ellipse. Move the pins closer together and the ellipse gets rounder (closer to a circle); move them apart and it stretches. One pin marks where the Sun sits -- notice it is NOT at the centre. That off-centre Sun is Kepler's first law.

What your ellipse teaches. (1) First law: each orbit is an ellipse with the Sun at ONE focus (the other is empty). (2) Second law: a planet sweeps equal AREAS in equal times, so it races when near the Sun and dawdles when far. (3) Third law: the square of the period equals the cube of the average distance, tying every planet's speed to its distance in one clean rule. (4) Kepler found the WHAT; Newton later found the WHY, deriving all three from a single law of gravity. The same laws steer every spacecraft and, today, reveal planets around other stars -- astronomers watch a distant world obey Kepler's second law and know its orbit before they ever see it.