Geometrodynamical distances to the Galaxy's hydrogen streams

We present a geometrodynamical method for determining distances to orbital streams of H-I gas in the Galaxy. The method makes use of our offset from the Galactic Centre and assumes that the gas comprising the stream nearly follows a planar orbit about the Galactic Centre. We apply this technique to the Magellanic Stream and determine the distances to all points along it; a consistency check shows that the angular momentum is approximately constant. Applying this technique to the Large Magellanic Cloud itself gives an independent distance which agrees within its accuracy of around 10 per cent. Relaxing the demand for exact conservation of energy and angular momentum at all points along the stream allows for an increase in orbital period between the lagging end and the front end led by the Magellanic Clouds. Similar methods are applicable to other long streams of high-velocity clouds, provided they also nearly follow planar orbits; these would allow otherwise unknown distances to be determined.