Interpretation of the historic Yemeni reports of supernova SN 1006: early discovery in mid-April 1006 ?

Abstract: The recently published Yemeni observing report about SN 1006 from al-Yamani clearly gives AD 1006 Apr $17 \pm 2$ (mid-Rajab 396h) as first observation date. Since this is about 1.5 weeks earlier than the otherwise earliest reports (Apr 28 or 30) as discussed so far, we were motivated to investigate an early sighting in more depth. We searched for additional evidences from other areas like East Asia and Europe. We found that the date given by al-Yamani is fully consistent with other evidence, including: (a) SN 1006 "rose several times half an hour after sunset" (al-Yamani), which is correct for the location of Sana in Yemen for the time around Apr 17, but it would not be correct for late Apr or early May; (b) the date (3rd year, 3rd lunar month, 28th day wuzi, Ichidai Yoki) for an observation of a guest star in Japan is inconsistent (there is no day wuzi in that lunar month), but may be dated to Apr 16 by reading wuwu date rather than a wuzi date; (c) there is observational evidence that SN 1006 was observed in East Asia early or mid April; for the second half of April, a bad weather (early monsoon) period is not unlikely -- there is a lack of night reports; (d) the observer in St. Gallen reported to have seen SN 1006 "for three months", which must have ended at the very latest on AD 1006 Jul 10, given his northern location, so that his observations probably started in April. We conclude that the correctly reported details give quite high confidence in the fully self-consistent report of al-Yamani, so that the early discovery date should be considered seriously.