  We have discovered that Europa, Ganymede and Callisto are bright around 1.5
{\mu}m even when not directly lit by sunlight, based on observations from the
Hubble Space Telescope and the Subaru Telescope. The observations were
conducted with non-sidereal tracking on Jupiter outside of the field of view to
reduce the stray light subtraction uncertainty due to the close proximity of
Jupiter. Their eclipsed luminosity was $10^{-6}$-$10^{-7}$ of their uneclipsed
brightness, which is low enough that this phenomenon has been undiscovered
until now. In addition, Europa in eclipse was <1/10 of the others at 1.5
{\mu}m, a potential clue to the origin of the source of luminosity. Likewise,
Ganymede observations were attempted at 3.6 {\mu}m by the Spitzer Space
Telescope but it was not detected, suggesting a significant wavelength
dependence. The reason why they are luminous even when in the Jovian shadow is
still unknown, but forward-scattered sunlight by haze in the Jovian upper
atmosphere is proposed as the most plausible candidate. If this is the case,
observations of these Galilean satellites while eclipsed by the Jovian shadow
provide us a new technique to investigate Jovian atmospheric composition, and
investigating the transmission spectrum of Jupiter by this method is important
for investigating the atmosphere of extrasolar giant planets by transit
spectroscopy.
