  One of the last unexplored windows to the cosmos, the Dark Ages and Cosmic
Dawn, can be opened using a simple low frequency radio telescope from the
stable, quiet lunar farside to measure the Global 21-cm spectrum. This frontier
remains an enormous gap in our knowledge of the Universe. Standard models of
physics and cosmology are untested during this critical epoch. The messenger of
information about this period is the 1420 MHz (21-cm) radiation from the
hyperfine transition of neutral hydrogen, Doppler-shifted to low radio
astronomy frequencies by the expansion of the Universe. The Global 21-cm
spectrum uniquely probes the cosmological model during the Dark Ages plus the
evolving astrophysics during Cosmic Dawn, yielding constraints on the first
stars, on accreting black holes, and on exotic physics such as dark
matter-baryon interactions. A single low frequency radio telescope can measure
the Global spectrum between ~10-110 MHz because of the ubiquity of neutral
hydrogen. Precise characterizations of the telescope and its surroundings are
required to detect this weak, isotropic emission of hydrogen amidst the bright
foreground Galactic radiation. We describe how two antennas will permit
observations over the full frequency band: a pair of orthogonal wire antennas
and a 0.3-m$^3$ patch antenna. A four-channel correlation spectropolarimeter
forms the core of the detector electronics. Technology challenges include
advanced calibration techniques to disentangle covariances between a bright
foreground and a weak 21-cm signal, using techniques similar to those for the
CMB, thermal management for temperature swings of >250C, and efficient power to
allow operations through a two-week lunar night. This simple telescope sets the
stage for a lunar farside interferometric array to measure the Dark Ages power
spectrum.
