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Circular and elliptical orbits
Planets have orbits that are close to circular. However, comets have elliptical orbits.
To demonstrate the different orbits on the gravity well, begin by placing a heavy ball on the sheet to represent a central mass like the Sun. Simulate the circular orbit of a planet by rolling a marble so that it forms a circular orbit. To simulate the elliptical orbit of a comet, roll a marble so it narrowly misses the central mass. This will cause it to have an elongated elliptical orbit.
Related resources:
Comets – article
To catch a comet – the Rosetta Mission – article
IMAGE: NASA Robert Simmon
