  The Stark shift of the ytterbium optical clock transition due to room
temperature blackbody radiation is dominated by a static Stark effect, which
was recently measured to high accuracy [J. A. Sherman et al., Phys. Rev. Lett.
108, 153002 (2012)]. However, room temperature operation of the clock at
10^{-18} inaccuracy requires a dynamic correction to this static approximation.
This dynamic correction largely depends on a single electric dipole matrix
element for which theoretically and experimentally derived values disagree
significantly. We determine this important matrix element by two independent
methods, which yield consistent values. Along with precise radiative lifetimes
of 6s6p 3P1 and 5d6s 3D1, we report the clock's blackbody radiation shift to
0.05% precision.
