  We report a measurement of the dynamical polarizability of dysprosium atoms
in their electronic ground state at the optical wavelength of 1064 nm, which is
of particular interest for laser trapping experiments. Our method is based on
collective oscillations in an optical dipole trap, and reaches unprecedented
accuracy and precision by comparison with an alkali atom (potassium) as a
reference species. We obtain values of 184.4(2.4) a.u. and 1.7(6) a.u. for the
scalar and tensor polarizability, respectively. Our experiments have reached a
level that permits meaningful tests of current theo- retical descriptions and
provides valuable information for future experiments utilizing the intriguing
properties of heavy lanthanide atoms.
