fitting observations to models of the galactic halo. These observations including the rotational speed of stars at the solar circle and other locations, the total projected mass density (estimated by considering the motion of stars perpendicular to the galactic disk), peak-to-trough variations in the rotation curve ([*ie.*]{} the ‘flatness constraint’), and microlensing. Taken together, these constraints can be used to estimate the local halo density to lie between 4 $\times$ 10$^{-25}$ g/cm$^{-3}$ and 13 $\times$ 10$^{-25}$ g/cm$^3$ ($0.22-0.73\,$GeV/cm$^3$) [@GGTurner]. Limits on the density of MACHO microlensing objects imply that at least 80% of this is cold dark matter. The velocity of the WIMPs is expected be close to the galactic rotation velocity, $230 \pm 20$ km/sec [@Drukier].

These observations, however, only constrain the dark matter density as averaged over scales larger than a kiloparsec or so. In contrast, the solar system moves a distance of $\sim$$10^{-3}$ parsecs relative to the dark matter halo each year. If dark matter is distributed in an inhomogeneous way over milliparsec scales ([*ie.*]{} as a collection of dense clumps and voids), then the density along the path of the Earth, as seen by direct detection experiments, could be much larger or smaller than is inferred 