One of the most famous depictions of a comet on coins goes back to ancient Rome, when Augustus Caesar issued a denarius with a comet on the reverse, along with the inscription, DIVVS IVLIVS.
This was a direct reference to the comet seen in the heavens immediately after the assassination of Julius Caesar in 44BC, and the sighting of a comet, or perhaps a bright meteor, in 19-17BC that was taken as the spirit of Julius Caesar returning to show divine support for Augustus.
On the obverse the comet appears as a eight-ray star with coma. The legend beside "DIVVS IVLIVS" refers to the divine nature of roman emperors. It was believed that the gods raise among themselves the roman emperors after their death. This legend was cultivated by regnant emperors as this belief gave divine support to them making their power unquestionable.
On the reverse the portrait of Augustus Caesar to the left, and his name written in legend around.