  Polarization during a lunar eclipse is a forgotten mystery. Coyne & Pellicori
(1970) reported the detection of significant polarization during the lunar
eclipse on 1968 April 13. Multiple scattering during the first transmission
through the Earth's atmosphere was suggested as a possible cause of the
polarization, but no conclusive determination was made. No further
investigations on polarization during a lunar eclipse are known. We revisit
this mystery with an interest in possible application to extrasolar planets, if
planetary transmitted light is indeed polarized, it may be possible to
investigate an exoplanet atmosphere using "transit polarimetry." Here we report
results of the first spectropolarimetry for the Moon during a lunar eclipse on
2015 April 4. We observed polarization degrees of 2-3% at wavelengths of
500-600 nm, in addition, an enhanced feature was detected at the O$_2$ A band
near 760 nm. The observed time variation and wavelength dependence are
consistent with an explanation of polarization caused by double scattering
during the first transmission through the Earth's atmosphere, accompanied by
latitudinal atmospheric inhomogeneity. Transit polarimetry for exoplanets may
be useful to detect O$_2$ gas and to probe the latitudinal atmospheric
inhomogeneity, and it is thus worthy of serious consideration.
