  Dozens of planets are now discovered with large orbital obliquity, and have
become the proof for the dynamical evolution of planetary orbits. In the
current samples, there is an apparent clustering of planets around $90^\circ$,
and also an absence of planets around $180^\circ$ although the latter is
expected by some theories. Statistical extrapolation using Hierarchical
Bayesian Analysis have recently refuted the significant clustering around
$90^\circ$ and suggested that the distribution may actually be broader. In this
work, the symmetric TESS transit light curve of KELT-19Ab is analyzed using
gravity darkening to measure its true obliquity. Its large sky projected
obliquity $\lambda = -179.7^{\circ+3.7^\circ}_{\,\,-3.8^\circ}$ makes KELT-19Ab
the only currently known planet with obliquity potentially close to
$180^\circ$. We apply spectroscopic constraints on $v\mathrm{sin}i$ and
$\lambda$ as well as theoretical constraints on the limb-darkening coefficients
to find that the KELT-19Ab's obliquity is $\psi =
155^{\circ+17^\circ}_{\,\,-21^\circ}$, in favor of a flipped orbit. The result
is consistent with the statistically inferred uniformity of obliquity
distribution, and also highlights the applicability of the gravity darkening
technique to symmetric light curves.
