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The characterization of the polarimetric properties of the planet Earth isimportant for the interpretation of expected observations and the planning offuture instruments. We present a multi-wavelengths and multi-phase set ofbenchmark values for the polarization signal of the integrated light from theplanet Earth derived from new polarimetric observations of the earthshineback-scattered from the Moon's dark side. Using a new, specially designed widefield polarimeter we measured the fractional polarization of the earthshine inthe B, V, R and I filters for Earth phase angles alpha between 30{\deg} and110{\deg}. The phase dependence of the earthshine polarization is fitted by afunction p x sin(alpha)^2. To determine the polarization of the planet Earth wecorrect our earthshine measurements by a polarization efficiency function forthe lunar surface derived from measurements of lunar samples from theliterature. The polarization of the earthshine decreases towards longerwavelengths and is about a factor 1.3 lower for the higher albedo highlands.For mare regions the measured maximum polarization is about 13 % at quadraturein the B band. The resulting fractional polarizations for Earth are 24.6 % forthe B band, 19.1 % for the V band, 13.5 % for the R band, and 8.3 % for the Iband. Together with literature values for the spectral reflectivity of Earth weobtain a contrast between the polarized flux of the Earth and the (total) fluxof the Sun with an uncertainty of less than 20 % and we find that the bestphase to detect an Earth twin is around an Earth phase alpha=65{\deg}. Thepolarimetric models of Earth-like planets from Stam (2008) are in qualitativeagreement with our results but there are also significant differences whichmight guide more detailed computations.

Measurement of the earthshine polarization in the B, V, R, and I band as function of phase