The Bridge V Learning Edition 1 ; our solar system | Page 33

Materials
•
•
•
•
•
Plastic planets (Box)
Plastic Sun (Box)
Yellow rope (Box)
Two full bottles of water
Chalk
Learning Objectives
Learn about the elliptical orbits of planets.
Background Science
Planets do not orbit the Sun in perfect circles, but in ellipses. The definition of a circle is that
every point on the circle has the same distance to the centre. For an ellipse, the definition is that
every point on the ellipse has the same combined distance to both focal points. In a planet’s
orbit, the Sun acts as one of the focal points. The other (imaginary) focal point is very close to the
Sun (compared to the large scales in question), making the ellipse almost a circle.
Credit: Natalie Fisher
Why are planetary orbits elliptical? There are three possible shapes of an object’s path (apart
from a straight line, which isn’t realistic since there are always gravitational forces around): a
parabola, a circle and an ellipse. In the case of a parabola, a planet would fly in from outer
space; its orbit would be bent by the Sun, and it would fly off again to infinity. Of course, the Solar
System would run quickly out of planets if the orbits were shaped like this. That leaves circular
and elliptical orbits. Circular orbits are simply too perfectly round to occur in nature. It would be
infinitely coincidental if a planet were to fly in a perfect circle. Hence, planets have elliptical orbits.
Full description
•
In order to graphically represent a circular orbit, knot two ropes together and place
them around a water-filled bottle and a piece of chalk. Make sure the distance
between the chalk and the bottle is such that the rope is tensed.