  We measure the mass, gap, and magnetic moment of a magnon in the
ferromagnetic $F=1$ spinor Bose-Einstein condensate of $^{87}$Rb. We find an
unusually heavy magnon mass of $1.038(2)_\mathrm{stat}(8)_\mathrm{sys}$ times
the atomic mass, as determined by interfering standing and running coherent
magnon waves within the dense and trapped condensed gas. This measurement is
shifted significantly from theoretical estimates. The magnon energy gap of
$h\times 2.5(1)_\mathrm{stat}(2)_\mathrm{sys}\;\mathrm{Hz}$ and the effective
magnetic moment of $-1.04(2)_\mathrm{stat}(8)\,\mu_\textrm{bare}$ times the
atomic magnetic moment are consistent with mean-field predictions. The nonzero
energy gap arises from magnetic dipole-dipole interactions.
