  The atmospheres of exoplanets are commonly studied by observing the transit
of the planet passing in front of its parent star. The obscuration of part of
the stellar disk during a transit will reveal aspects of its surface structure
resulting from general centre-to-limb variations (CLVs). These become apparent
when forming the ratio between the stellar light in and out of transit. These
phenomena can be seen particularly clearly during the progress of a penumbral
lunar eclipse, where the Earth transits the solar disk and masks different
regions of the solar disk as the eclipse progresses. When inferring the
properties of the planetary atmosphere, it is essential that this effect
originating at the star is properly accounted for. Using the data observed from
the 2014-April-15 lunar eclipse with the ESPaDOnS spectrograph mounted on the
Canada France Hawaii Telescope (CFHT), we have obtained for the first time a
time sequence of the penumbral spectra. These penumbral spectra enable us to
study the centre-to-limb variations of solar Fraunhofer lines when the Earth is
transiting Sun. The Na i and Ca ii absorption features reported from previous
lunar eclipse observations are demonstrated to be CLV features, which dominate
the corresponding line profiles and mask possible planetary signal. Detecting
atmospheric species in exoplanets via transit spectroscopy must account for the
CLV effect.
