  One of the key properties of any binary is its viewing angle (i.e.,
inclination), $\theta_{\rm obs}$. In binary neutron star (BNS) mergers it is of
special importance due to the role that it plays in the measurement of the
Hubble constant, $H_0$. The opening angle of the jet that these mergers launch,
$\theta_j$, is also of special interest. Following the detection of the first
BNS merger, GW170817, there were numerous attempts to estimate these angles
using the afterglow light curve, finding a wide range of values. Here we
provide a simple formula for the ratio $\theta_{\rm obs}/\theta_j$ based on the
afterglow light curve and show that this is the only quantity that can be
determined from the light curve alone. Namely, it is impossible to determine
each of the angles separately without additional information. Our result
explains the inconsistency of the values found by the various studies of
GW170817 that were largely driven by the different priors taken in each study.
Among the additional information that can be used to estimate $\theta_{\rm
obs}$ and $\theta_j$, the most useful is a VLBI measurement of the afterglow
image superluminal motion. An alternative is an identification of the afterglow
transition to the sub-relativistic phase. These observations are possible only
for mergers observed at small viewing angles, whose afterglow is significantly
brighter than the detector's threshold. We discuss the implications of these
results to measurements of $H_0$ using GW observations. We show that while the
viewing angle will be measured only in a small fraction of future BNS mergers,
it can significantly reduce the uncertainty in $H_0$ in each one of these
events, possibly to a level of 4-5\%. A minority of the mergers with high
precision measurements of this kind may dominate in the future the precision in
which $H_0$ will be measured using this method.
