  We present the serendipitous discovery of a `Giant Arc on the Sky' at $z \sim
0.8$. The Giant Arc (GA) spans $\sim 1$ Gpc (proper size, present epoch), and
appears to be almost symmetrical on the sky. It was discovered via intervening
MgII absorbers in the spectra of background quasars, using the catalogues of
Zhu \& M\'enard. The use of MgII absorbers represents a new approach to the
investigation of large-scale structures (LSSs) at redshifts $0.45 \lesssim z
\lesssim 2.25$. We present the observational properties of the GA, and we
assess it statistically using methods based on: (i) single-linkage hierarchical
clustering ($\sim 4.5\sigma$); (ii) the Cuzick-Edwards test ($\sim 3.0\sigma$);
and (iii) power spectrum analysis ($\sim 4.8\sigma$). Each of these methods has
distinctive attributes and powers, and we advise considering the evidence from
the ensemble. We discuss our approaches to mitigating any {\it post-hoc}
aspects of analysing significance after discovery. The overdensity of the GA is
$\delta \rho / \rho \sim 1.3 \pm 0.3$. The GA is the newest and one of the
largest of a steadily accumulating set of very large LSSs that may (cautiously)
challenge the Cosmological Principle, upon which the `standard model' of
cosmology is founded. Conceivably, the GA is the precursor of a structure like
the Sloan Great Wall (but the GA is about twice the size), seen when the
Universe was about half its present age.
