Measurements of the absorption spectrum of Si, made with high resolution, near the main absorption edge, at various temperatures between 4.2\ifmmode^\circ\else\textdegree\fi{}K and 415\ifmmode^\circ\else\textdegree\fi{}K, have revealed fine structure in the absorption on the long-wavelength side of this edge. This structure has been analyzed and can be interpreted in terms of indirect transitions involving, in general, phonons with energies corresponding to temperatures of 212\ifmmode^\circ\else\textdegree\fi{}K, 670\ifmmode^\circ\else\textdegree\fi{}K, 1050\ifmmode^\circ\else\textdegree\fi{}K, and 1420\ifmmode^\circ\else\textdegree\fi{}K. The form of the absorption associated with each type of phonon indicates that, as well as the formation of free electron-hole pairs taking place, excitons with a binding energy \ensuremath{\sim}0.01 ev are produced in the absorption process. The temperature dependence of the indirect energy band gap has been found and using this along with data on the intrinsic carrier density indicates an increase with temperature of the combined density-of-states effective mass of the electrons and holes. A smoothing out of the basic features of the curves is observed and shown to be consistent with relaxation broadening. A discussion of the significance of the energies of the phonons taking part in the indirect transitions to the lattice vibrational spectrum of Si is given.