Hubble's variable parameter: The long and short of it

The ''constant'' H, whose customary units are (km/sec)/Megaparsecs, is a measure of the distance scale, expansion rate, and (indirectly) age of the universe. The first determinations of its value, by Hubble himself, between 1929 and 1936, were in the range 500-550 km/sec/Mpc, implying a universe only about 2 Gyr old (less than the age of the earth as understood even then). In a series of quick steps from 1952 to 1975, the best value dropped from 500 to 250 to 125 to 50-100 km/sec/Mpc. And there it has remained ever since, with a factor of two uncertainty. The continuing discrepancies among values found by different workers using different methods are an inconvenience to the entire astronomical community, because the value of H enters into our determinations of masses and luminosities of distant objects, of the fraction of the closure density of the universe that can be present in ordinary baryonic matter, and many other things we would like to know. It is not clear that the issue will be firmly resolved in the near future, despite the ever-increasing rate of publications on the subject.