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.
But comets appeared on coins long before the rise of the Roman Empire. In the May, 1996 issue of "The Anvil," Richard Hazzard describes a silver tetradrachm of Ptolemy V. He was born in 210BC and became the ruler of Egypt in 205BC. There were immediate campaigns by Philip V of Macedon and Antiochus III of Syria to deprive Ptolemy V of his non-Egyptian lands. Territorial losses and native revolts increased, and by 199/198BC, the reign of Ptolemy V was in jeopardy. His regent, Aristomenes, needed a way to restore confidence for the young monarch.
He struck a tetradrachm as a propaganda piece, declaring Ptolemy V as a God Manifest in the legend of the coin. The center of the coin depicts the thunderbolt of Zeus flanked by two stars. These two stars represent the comets of 210 and 204BC, that appeared at the birth and accession of Ptolemy V. Contemporary writings of the period delivered a message of hope: "although many disasters have plagued the realm, King Ptolemy is a God Manifest, whose greatness was augered in the skies by Zeus for all the world to see!"
The regent saved the dynasty, as the king's subjects believed that celestial events would bring an era of prosperity. Thus, the use of actual celestial events were clearly used on this coin as propaganda.
During the middle ages, comets were known as "hairy stars," and their representation on coins often took the form of crude and irregularly shaped symbols such as combs, bars, pyramids, etc. One of the most famous examples is discussed in detail in "Astronomical Symbols on Ancient and Medieval Coins." In 1066, William the Conqueror used Halley's comet of 1066 as his omen for victory over Harold II of England. The 1066 comet was very bright, had a short tail, and its shape was depicted on the Tapestry of Bayeaux.
Sometime in the middle of the eleventh century, a new design appeared on the deniers of the French province of Champagne. One of the earliest of these deniers was issued in Sens, and depicts a large comb with curved tines, and a cross with two annulets. On later designs, the tines became straightened, and different astronomical symbols, such as stars and crescents, are found above the comb.
Many argue that the comb in merely a pun on the name of the province, as "champ" means field, and "peigne" is the french word for comb, and there is a comb in the field of the coin. But was the center of the coin called a "field" in the eleventh century? And why were the tines curved if it were actually a comb? It is more likely a representation of this spectacular appearance of Halley's comet. As written in the Anglo-Saxon Chronicles in 1066, "Then over all England there was a sign in the skies such as had never been seen before. Some said it was the star 'comet' which some called the long-haired star." Notice how the shape of the comb and the tail of the comet from the tapestry are very similar.
By the beginning of the Renaissance, celestial events were portrayed on coins and medals more as a commemorative or religious device rather than as sovereign propaganda. Several German States coins and medals are presented here as examples.
In Frankfurt, gold ducats and silver pattern ducats with a comet motif were issued with the date of 19 November 1618. The comet of 1618 was visible to the naked eye from 6 - 25 September, but was first discovered on August 25th using a lunette (spyglass).
The comet of 1680 was the first one to be discovered using a astronomical instrument (November 14th), and was visible for four months. The medal shown above was struck as a good luck piece as protection from the bad luck brought by the comet!
The great comet of 1577 permitted Tycho Brahe to demonstrate that comets were celestial and not atmospheric phenomena. A medal was issued on 14 November 1577 that depicted this celestial visitor.