  Analysis of f-mode frequencies has provided a measure of the radius of the
Sun which is lower, by a few hundredths per cent, than the photospheric radius
determined by direct optical measurement. Part of this difference can be
understood by recognizing that it is primarily the variation of density well
beneath the photosphere of the star that determines the structure of these
essentially adiabatic oscillation modes, not some aspect of radiative
intensity. In this paper we attempt to shed further light on the matter, by
considering a differently defined, and dynamically more robust, seismic radius,
namely one determined from p-mode frequencies. This radius is calibrated by the
distance from the centre of the Sun to the position in the subphotospheric
layers where the first derivative of the density scale height changes
essentially discontinuously. We find that that radius is more- or-less
consistent with what is suggested by the f modes. In addition, the
interpretation of the radius inferred from p modes leads us to understand more
deeply the role of the total mass constraint in the structure inversions. This
enables us to reinterpret the sound-speed inversion, suggesting that the
positions of the photosphere and the adiabatically stratified layers in the
convective envelope differ nonhomologously from those of the standard solar
model.
