The authors have applied to the Sun a method for calibrating, in absolute flux units, Ca II H profiles of late-type stars. After comparing, in the region 3948 - 3882 Å, an LTE synthetic spectrum with the data of the solar flux atlas by Kurucz et al. (1984), they have defined the wavelength ranges where observations agree with computations, based on specific radiative equilibrium models and collisional broadening parameters. By fitting in these regions the spectrum of the moon observed at ESO with the corresponding synthetic spectrum, the authors derived a calibration factor that enables them to calibrate, in absolute flux units, the whole observed range.