Encke's Comet

This object was seen in 1786, and at some subsequent returns, before it was discovered by Encke in 1819 to be a periodic comet. It has since been observed at numerous returns. It revolves round the sun in about three and one-third years, having the shortest period of any comet known. It has exhibited very prominently the feature, shown by many comets, of a contraction of the head or coma on approaching the sun. Encke's comet long manifested the most remarkable feature of a shortening of its period by about two and a half hours in each revolution. The suggestion was made that this was due to resistance by a rare medium generally diffused in the solar system, which had the effect of reducing its aphelion distance and diminishing its orbit. This, however, is improbable, as other comets show no similar effect. The cause remains undiscovered, and a further extraordinary fact is that the shortening of period is found since 1868 to have been only half of its previous amount.