Based on the assumption of separation of the charges in matter of cosmic bodies the possibility of obtaining the magnetic moment by these bodies is proved. The magnitude of the magnetic field appears proportional to the angular velocity of the body’s rotation and to the radius of convective layer. The periods of change of polarity of magnetic field of the Earth and the Sun are calculated by means of the size the convective layer and the convection speed. The solar activity appears the consequence of periodic transformation of the thermal energy into the electromagnetic form of energy.