  We derive expressions for the tidal field exerted by a spherically symmetric
galaxy having an extended mass distribution, and use our analysis to calculate
tidal perturbations and heating of stars in a globular cluster or a satellite
galaxy orbiting in the external potential. We consider a Hernquist model and
several power-law models for the primary. We integrate the tidal force over
accurate orbits in the given potential by using the impulse approximation and
by introducing adiabatic corrections that allow for the time variation of the
force. We compare these results with the conventional straight path
approximation. We also check the results with direct N-body simulations and
find very good agreement. Heating on highly eccentric orbits dominates as the
adiabatic corrections strongly damp the energy change on low eccentricity
orbits. The results are illustrated for the example of globular cluster NGC
6712. For the orbital eccentricities higher than 0.9 the future lifetime of NGC
6712 is less than 10^{10} yr. Our analysis can be used to study tidal
interactions of star clusters and galaxies in a semi-analytical manner.
