The shrinking of the ice fraction of comet nucleus is considered as the principal factor of the variation of nongravitational effects for
2P/Encke. Equations for determination of parameters of Marsden formula for nongravitational effects are developed. The least-square differential correction solution based on all available observations from 1786 to 2010 is obtained.
A posteriori RMS residual is 32.7 times greater than RMS residual a priori, but this is the first solution consistent with observations in all apparitions of
2P/Encke. The error in the representation of time of perihelion passagein 1786 is 1.3 days versus 58 days in JPL theory. It may be concluded that present model is one and a half orders better than that of JPL, but one and a half orders worse than «ideal» theory.