  We describe a technique that finds orbits through the Galaxy that are
consistent with measurements of a tidal stream, taking into account the extent
that tidal streams do not precisely delineate orbits. We show that if accurate
line-of-sight velocities are measured along a well defined stream, the
technique recovers the underlying orbit through the Galaxy and predicts the
distances and proper motions along the stream to high precision. As the error
bars on the location and velocities of the stream grow, the technique is able
to find more and more orbits that are consistent with the data and the
uncertainties in the predicted distances and proper motions increase. With
radial-velocity data along a stream ~40deg long and <0.3deg wide on the sky
accurate to ~1 km/s the precisions of the distances and tangential velocities
along the stream are 4 percent and 5 km/s, respectively. The technique can be
used to diagnose the Galactic potential: if circular-speed curve is actually
flat, both a Keplerian potential and Phi(r) proportional to r are readily
excluded. Given the correct radial density profile for the dark halo, the
halo's mass can be determined to a precision of 5 percent.
