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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 ofspecial importance due to the role that it plays in the measurement of theHubble 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 firstBNS merger, GW170817, there were numerous attempts to estimate these anglesusing the afterglow light curve, finding a wide range of values. Here weprovide a simple formula for the ratio $\theta_{\rm obs}/\theta_j$ based on theafterglow light curve and show that this is the only quantity that can bedetermined from the light curve alone. Namely, it is impossible to determineeach of the angles separately without additional information. Our resultexplains the inconsistency of the values found by the various studies ofGW170817 that were largely driven by the different priors taken in each study.Among the additional information that can be used to estimate $\theta_{\rmobs}$ and $\theta_j$, the most useful is a VLBI measurement of the afterglowimage superluminal motion. An alternative is an identification of the afterglowtransition to the sub-relativistic phase. These observations are possible onlyfor mergers observed at small viewing angles, whose afterglow is significantlybrighter than the detector's threshold. We discuss the implications of theseresults to measurements of $H_0$ using GW observations. We show that while theviewing 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 theseevents, possibly to a level of 4-5\%. A minority of the mergers with highprecision measurements of this kind may dominate in the future the precision inwhich $H_0$ will be measured using this method.

Afterglow constraints on the viewing angle of binary neutron star mergers and determination of the Hubble constant