La Silla atmosphere transmission function T(λ) for AM = 1 as evaluated from full-Moon spectra taken with HARPS just before the eclipse (Table 2). The spectrum is normalized at 6866 Å. To verify this experimental T(λ) spectrum, a transmission fit based on the model of Hayes & Latham (1975) for the Rayleigh and aerosol components normalized at 6866 Å is displayed (blue dashed line). We also add an ozone transmission spectrum to the model (light-blue solid line) that correctly fits the observed wide Chappuis band. The best least-squares fit for Rayleigh and ozone over La Silla that night is obtained for an airmass of AM = 0.77 for ozone and Rayleigh, and an optical depth of 0.181 at 5320 Å for the aerosols, between 4000 and 6866 Å. The fit for ozone is made only where O₂ and H₂O absorption bands are absent. The O₂γ and B absorption bands are clearly seen, around 6300 and 6900 Å respectively, the other deep bands are due to H₂O. The small gap in HARPS data is visible between 5300 and 5340 Å (Table 1).