Why can't I find Halley's Comet at the Battle of Hastings?


    Unfortunately, predicting comet positions on past trips around the sun is one area of astronomy where simulation programs like Starry Night are not very effective. As a comet goes around sun, its orbit is altered by the gravitational influence of the sun and planets in ways that can't be predicted by Starry Night. The gas and dust released by the comet as it gets closer to the Sun also causes the orbit of the comet you are interested in to change. For example, the orbital period of Halley's comet has varied from 76 years to 79 years over the last thousand years. The bottom line is that the orbital elements for a comet in Starry Night are valid only for its most recent trip around the sun and can't be used to predict its appearance in the more distant past.