  We present high resolution Na I D spectroscopy of the beta Pic disk, and the
resonantly scattered sodium emission can be traced from less than 30 AU to at
least 140 AU from the central star. This atomic gas is co-existent with the
dust particles, suggestive of a common origin or source. The disk rotates
towards us in the south-west and away from us in the north-east. The velocity
pattern of the gas finally provides direct evidence that the faint linear
feature seen in images of the star is a circumstellar disk in Keplerian
rotation. From modelling the spatial distribution of the Na I line profiles we
determine the effective dynamical mass to be 1.40 +/- 0.05 M_sun, which is
smaller than the stellar mass, 1.75 M_sun. We ascribe this difference to the
gravity opposing radiation pressure in the Na I lines. We argue that this is
consistent with the fact that Na is nearly completely ionised throughout the
disk (Na I/Na < 10^-4). The total column density of sodium gas is N(Na) = 10^15
cm^-2.
