Credit: Natalie Fisher
• Now move the chalk around the bottle, like a compass, while keeping the rope tensed. This way, a circle forms on the floor with the bottle in the centre.
• Next, remove the bottle and put the Sun’ s model in its place. Put the model of the Earth into the orbit. This completes the circular orbit. We now have an approximation of Earth’ s orbit, but not an exact model: the orbit should be elliptical!
• To construct an ellipse, we need two bottles filled with water, and a piece of chalk.
Credit: Natalie Fisher
• Place the rope around both bottles and the piece of chalk, and tense it again. This time, the rope is shaped in a triangular form.
• If the chalk now goes around both bottles with the rope tensed, the result is an ellipse. In this model, the position of one of the bottles would represent the Sun and the piece of chalk would be the planet.
• Now replace one bottle and the chalk with the models of the Sun and the Earth, and remove the other bottle. We have a realistic( although probably exaggerated) model of Earth’ s elliptical orbit around the Sun!
With the help of this method, very different ellipses can be constructed. How does the shape of the ellipse change if we diminish the distance between the bottles?
• How does the shape of the ellipse change if we increase the distance between the bottles?
• In reality, the two focal points( bottles) are very close to each other, making Earth’ s orbit almost circular. Think about it: we don’ t even notice the varying distance in temperature!
Mathematically, the weights or nails in our construction mark the so-called‘ foci’ of an ellipse. The larger their distance, the more elongated the ellipse becomes. If a planet revolves on a very