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