  For a comprehensive understanding of planetary formation and evolution, we
need to investigate the environment in which planets form: circumstellar disks.
Here we present high-contrast imaging observations of V4046 Sagittarii, a
20-Myr-old close binary known to host a circumbinary disk. We have discovered
the presence of rotating shadows in the disk, caused by mutual occultations of
the central binary. Shadow-like features are often observed in
disks\cite{garufi,marino15}, but those found thus far have not been due to
eclipsing phenomena. We have used the phase difference due to light travel time
to measure the flaring of the disk and the geometrical distance of the system.
We calculate a distance that is in very good agreement with the value obtained
from the Gaia mission's Data Release 2 (DR2), and flaring angles of $\alpha =
6.2 \pm 0.6 $ deg and $\alpha = 8.5 \pm 1.0 $ deg for the inner and outer disk
rings, respectively. Our technique opens up a path to explore other binary
systems, providing an independent estimate of distance and the flaring angle, a
crucial parameter for disk modelling.
