Sir

Further to J. L. Heilbron and W. F. Bynum's Commentary “1904 and all that” (Nature 426, 761; 2003), two anniversaries connected with supernovae are worth noting. First, there was the discovery of the Crab supernova by some Chinese astrologers in 1054 (this would be 9.5¢ in the units used by Heilbron and Bynum). The second was the discovery of one of the renaissance supernovae, the SN 1604, by Johannes Kepler — who is mentioned in the Commentary, but for different reasons.

The Greeks thought of the sky as unchanging and did not expect to find new phenomena in the heavens. Even the comets were thought of as essentially terrestrial. However, the Chinese did not have these reservations, and kept a track of any changes in the night sky, which were believed to affect the fortunes of the country and the emperor. They have always been wonderful record keepers, and the Crab supernova is one of the many ‘guest stars’ they have discovered during the past 2000 years. The Crab is now known as the ‘Rosetta Stone’ of modern astrophysics, because its extraordinary features provide clues to numerous astrophysical phenomena, including supernova remnants, pulsars and high-energy emissions.

As your Commentary noted, Kepler and Galileo changed physics and astronomy in fundamental ways, but the general populace could only be convinced of the need to revise overall Greek thinking about the heavens by more direct evidence — if at all. It is said that the appearance of the two renaissance supernovae (the first one found by Tycho Brahe in 1572) in the European skies helped to generate acceptance of these new scientific ideas about the heavens.