  There is a strong degeneracy between the luminosity distance ($D_L$) and the
observer viewing angle ($\theta_{\rm obs}$; hereafter viewing angle) of the
gravitational wave (GW) source with an electromagnetic counterpart, GW170817.
Here, for the first time, we present independent constraints on $\theta_{\rm
obs} = 32.5 ^{\circ +11.7}_{-9.7}$ from broad-band photometry of the kilonova
(kN) AT2017gfo associated with GW170817. These constraints are consistent with
independent results presented in the literature using the associated gamma ray
burst GRB170817A. Combining the constraints on $\theta_{\rm obs}$ with the GW
data, we find an improvement of 24$\%$ on $H_0$. The observer angle constraints
are insensitive to other model parameters, e.g. the ejecta mass, half-opening
angle of the lanthanide-rich region and the temperature. A broad wavelength
coverage extending to the near infrared is helpful to robustly constrain
$\theta_{\rm obs}$. While the improvement on $H_0$ presented here is smaller
than the one from high angular resolution imaging of the radio counterpart of
GW170817, kN observations are significantly more feasible at the typical
distances of such events from current and future LIGO-Virgo Collaboration
observing runs ($D_L \sim 100$ Mpc). Our results are insensitive to the
assumption on the peculiar velocity of the kN host galaxy.
