To consider a sessile drop on an ideal solid surface in equilibrium with a vapor phase, the classic Young equation was given. The derivation of the Young equation was based on both the mechanics and the energy knowledge. According to the constant volume of the liquid in the wetting process of the liquid on a smooth and homogeneous solid surface and the low energy law, Young equation was ob-tained through the mathematic method in this paper. The previous work indicated that the contact angle θ was a function of time, but the coefficient can be obtained only through experiments. It was assumed that the liquid was steady Newtonian flow. Then the relationships between the dynamic contact angles and the wetting time were found in terms of the equilibrium of the spreading force and the restoring force. An immediate theoretical justification for the dependence of contact angles and the time was given. It was assumed that the effect of the gravity on wetting was negligible in the investigation. Under what conditions was the gravity negligible? The criterion of the gravity on the wetting process of the liquid was proposed when contact angles were greater than 90°. If the criterion, I, was much smaller than 1, the effect of the gravity on the wetting process could be ignored. If the criterion, I, was equal to or larger than 1, the effect of the gravity on the wetting process could not be ignored. On mercury-mica systems, the gravity may be considered only when the equilibrium contact radius reached 1.5 mm. To consider a sessile drop on an ideal solid surface in equilibrium with a vapor phase, the classic Young equation was given. The derivation of the Young equation was based on both mechanics and the energy knowledge. According to the constant volume of the liquid in the wet work process of the liquid on a smooth and homogeneous solid surface and the low energy law, Young equation was obtained through the mathematic method in this paper. The previous work indicated that the contact angle θ was a function of time, but the It was assumed that the liquid was steady newtonian flow. Then the relationships between the dynamic contact angles and the wetting time were found in terms of the equilibrium of the spreading force and the restoring force. An immediate theoretical justification for the dependence of contact angles and the time was given. It was assumed that the effect of the gravity on wetting was negligible in the investigatio the criterion of the gravity on the wetting process of the liquid was proposed when the contact angles were greater than 90 °. If the criterion, I, was much smaller than 1, the effect of the gravity on the wetting process could be ignored. If the criterion, I, was equal to or larger than 1, the effect of the gravity on the wetting process could not be ignored. On mercury-mica systems, the gravity may be considered only when the equilibrium contact radius reached 1.5 mm.