  We analyze and fit visible spectra of a red horizon at sunrise. The shape of
the spectra consist of a blue continuum followed by a red bump. The reddest
spectra are well fitted by the product of a spectrum of extinguished sunlight
(Rayleigh extinction + ozone absorption) and 1/lambda^4. The former is
essentially the radiation field in the outer atmosphere, at the scattering
volume location; the latter corresponds to Rayleigh scattering by the gas.
Moving to higher altitudes, a second component, corresponding to the spectrum
of a blue sky, must be added. The spectra we have obtained are similar to
spectra of red nebulae, suggesting there may be other explanations than an
emission process to the red color of some nebulae.
