Lunar occultation of the diffuse radio sky: LOFAR measurements between
  35 and 80 MHz
  We present radio observations of the Moon between $35$ and $80$ MHz to
demonstrate a novel technique of interferometrically measuring large-scale
diffuse emission extending far beyond the primary beam (global signal) for the
first time. In particular, we show that (i) the Moon appears as a negative-flux
source at frequencies $35<\nu<80$ MHz since it is `colder' than the diffuse
Galactic background it occults, (ii) using the (negative) flux of the lunar
disc, we can reconstruct the spectrum of the diffuse Galactic emission with the
lunar thermal emission as a reference, and (iii) that reflected RFI
(radio-frequency interference) is concentrated at the center of the lunar disc
due to specular nature of reflection, and can be independently measured. Our
RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design
for an Earth-isolation of better than $80$ dB to achieve an RFI temperature
$<1$ mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than
$20$ mK for Earth-based Cosmic Dawn experiments, (iii) man-made
satellite-reflected RFI temperature exceeds $20$ mK if the aggregate cross
section of visible satellites exceeds $80$ m$^2$ at $800$ km height, or $5$
m$^2$ at $400$ km height. Currently, our diffuse background spectrum is limited
by sidelobe confusion on short baselines (10-15% level). Further refinement of
our technique may yield constraints on the redshifted global $21$-cm signal
from Cosmic Dawn ($40>z>12$) and the Epoch of Reionization ($12>z>5$).
