Unprecedented Daylight Display of Kreutz Sungrazers in AD 363?

Abstract

In the context of the recently proposed contact-binary model (Sekanina 2021), I investigate the circumstances of the first perihelion passage of the Kreutz sungrazers in orbits with barycentric periods near 735 yr, following the initial near-aphelion splitting of the presumed progenitor, Aristotle's comet of 372 BC. Given favorable conditions at this breakup and at episodes of secondary fragmentation in its aftermath, the fragments should have arrived at their first perihelion nearly simultaneously, reminiscent of the anticipated outcome for the two-superfragment model's perihelion return of AD 356 (Sekanina & Chodas 2004). The relevant case of a swarm of Kreutz sungrazers is examined to appraise possible scientific ramifications of the brief remark by Ammianus Marcellinus, a Roman historian, that "in broad daylight comets were seen" in late AD 363, only seven years later. The tested scenario, which does not contradict Ammianus' narrative and is consistent with the contact-binary model, involves a set of ten sungrazers visible in the daytime, all reaching perihelion over a period of 4.6 days. As part of this work, I comment on the role of the rapidly developing, brilliant post-perihelion tail; revise the apparent magnitude typical for the first and last naked-eye sightings; compare the visibility conditions in full daylight, in twilight, and at night; and, for the first time, present circumstantial evidence that favors comet X/1106 C1 as the parent to C/1843 D1 rather than to C/1882 R1 and C/1965 S1.