At the age of twenty Augustine may have seen Halley's Comet.
Astronomers now know that what today is called Halley’s Comet appeared in the sky over North Africa in the year 374. When Augustine was twenty years of age it would have been visible at Carthage from about 1st March until late April of 374 (Julian calendar). Although it would be almost impossible that Augustine never noticed the eight-week appearance of this comet, he made no specific note of this comet in his writings. Even so, this has not deterred various scholars from musing about how the appearance of this comet might relate to the decision of Augustine to become a Manichean.
In his Confessions, Augustine states that a prelude to his becoming a Manichean was his desire for other-worldly wisdom inspired in him by his reading the Hortensius by Cicero. According to calculations from internal evidence in the Confessions, Augustine read Hortensius between November 372 and November 373. He then said that he spent time studying the Scriptures, and only after doing that without satisfaction did he turn to Manichaeism. If he read Hortensius in November 373 and then read Scriptures in the course of the following four months, his conversion to Manichaeism coincided with the appearance of Halley’s Comet.
Could Augustine there have been specifically referring to Halley’s Comet? The above quotation acquires a factual veracity if Augustine had in mind a comet he saw in 374 that since the eighteenth century has been known as Halley’s Comet.