  We present a simple, yet accurate approximation for calculating the cosmic
microwave background anisotropy power spectrum in adiabatic models. It consists
of solving for the evolution of a two-fluid model until the epoch of
recombination and then integrating over the sources to obtain the CMB
anisotropy power spectrum. The approximation is useful both for a physical
understanding of CMB anisotropies, as well as for a quantitative analysis of
cosmological models. Comparison with exact calculations shows that the accuracy
is typically better than 20 percent over a large range of angles and
cosmological models, including those with curvature and cosmological constant.
Using this approximation we investigate the dependence of the CMB anisotropies
on the cosmological parameters. We identify six dimensionless parameters that
uniquely determine the anisotropy power spectrum within our approximation. CMB
experiments on different angular scales could in principle provide information
on all these parameters. In particular, mapping of the Doppler peaks would
allow an independent determination of baryon mass density, matter mass density
and Hubble constant.
