"It is interesting to notice that a similar peak is also present at about half the 6th century, a time when, according to several Byzanthin historians quoted by Gibbon [5], e. g. Malala, Procopius and Theophanes, many scaring comets appeared in the sky. That was also the time of the great Justinian plague, which decimated the population of the Mediterranean region, killing up to 90% of the population according to some estimates. This depopulation was certainly a major factor which facilitated the Arab expansion some three generations later. "