Determination of the Hubble constant

Establishing accurate extragalactic distances has provided an immense chall enge to astronomers since the 1920s, The situation has improved dramaticall y as better detectors have become available, and as several new, promising techniques have been developed. For the first time in the history of this d ifficult field, relative distances to galaxies are being compared on a case -by case basis, and their quantitative agreement is being established. New instrumentation, the development of new techniques for measuring distances, and recent measurements with the Hubble Space telescope all have resulted in new distances to galaxies with precision at the +/-5-20% level. The curr ent statistical uncertainty in some methods for measuring Ho is now only a few percent; with systematic errors, the total uncertainty is approaching /-10%. Hence, the historical factor-of-two uncertainty in the value of the H-0 is now behind us.