Interesting piece from Astronomy Magazine ... the incipit:

"In Lucania, it was alleged that the heavens had been on fire; at Privernum the Sun had been glowing red through the whole of a cloudless day; at the temple of Juno Sospita in Lanuvium a terrible noise was heard in the night."

Not to mention children born of "uncertain sex," or a lamb with a pig's head in the town of Frusino.
Auroral view from Ontario
Roman sky signs followed an 11-year cycle and, like modern auroral displays, were more frequent before and after the cycle's peak. Rick Stankiewicz [larger image]
In 200 B.C., authorities of the Roman Republic recorded such events with the enthusiasm of a modern tabloid. While these omens cataloged by ancient historians won't tell us much about heaven's wrath, astronomers say they form an indirect record of what the Sun was doing 2,000 years ago.

In March, Andrew Solow of the Woods Hole Oceanographic Institution in Massachusetts published the most detailed analysis of Roman sky signs so far. He built on similar work done in 1979 by Richard Stothers of NASA's Goddard Space Flight Center.

The work of Roman historian Titus Livius (Livy, in English), who lived from 59 B.C. – A.D. 17, formed the basis of both studies. Livy's Ab Urbe Condita, written in the time of the first emperors, chronicles Rome's history with the help of written records dating back hundreds of years. Many times, he reports how Romans interpreted natural events as warnings that something was amiss in the relationship between the state and its gods.
During the Republic, Romans chose two new consuls every year, which means relative dates in Livy's work can be reliably determined. Leaving aside two-headed lambs and five-footed foals, Livy's history presents a time series of heavenly phenomena.

When Stothers embarked on his project 30 years ago, he thought Livy had done the most consistent job for the 133-year period from 223 B.C. to 91 B.C. He showed these events increased and decreased with a period of about 11 years. That's the average activity cycle for the Sun, and Stothers concluded most of the heavenly portents Romans worried about were aurorae — atmospheric glows triggered when solar storms sweep past Earth.

With descriptions such as "the sky lit up during the night," "the sky appeared to be on fire," or even "a phantom navy was seen shining in the sky," one can well imagine Livy's sources are reporting aurorae.

Solow's work, which appeared in the March 30, 2005, issue of Earth and Planetary Science Letters, shows these events happen most often 4 years and 8 years into the 11-year cycle. This twin-peaked, or bimodal, distribution matches that seen in the modern aurora record.