Caesar's Comet (numerical designation C/-43 K1) – also known as Comet Caesar and the Great Comet of 44 BC – was perhaps the most famous comet of antiquity. The seven-day visitation was taken by Romans as a sign of the deification of the recently dead dictator, Julius Caesar (100–44 BC).
Caesar's Comet was one of only five comets known to have had a negative absolute magnitude and was possibly the brightest daylight comet in recorded history. It was not periodic and may have disintegrated.
Caesar's Comet was known to ancient writers as the Sidus Iulium ("Julian Star") or Caesaris astrum ("Star of Caesar"). The bright, daylight-visible comet appeared suddenly during the festival known as the Ludi Victoriae Caesaris – for which the 44 BC iteration was long considered to have been held in the month of September (a conclusion drawn by Sir Edmund Halley). The dating has recently been revised to a July occurrence in the same year, some four months after the assassination of Julius Caesar, as well as Caesar's own birth month. According to Suetonius, as celebrations were getting underway, "a comet shone for seven successive days, rising about the eleventh hour, and was believed to be the soul of Caesar."
The Comet became a powerful symbol in the political propaganda that launched the career of Caesar's great-nephew (and adoptive son) Augustus. The Temple of Divus Iulius (Temple of the Deified Julius) was built (42 BC) and dedicated (29 BC) by Augustus for purposes of fostering a "cult of the comet". (It was also known as the "Temple of the Comet Star".) At the back of the temple a huge image of Caesar was erected and, according to Ovid, a flaming comet was affixed to its forehead:
To make that soul a star that burns forever
Above the Forum and the gates of Rome.
In 1997, two scholars at the University of Illinois at Chicago – John T. Ramsey (a classicist) and A. Lewis Licht (a physicist) – published a book  comparing astronomical/astrological evidence from both Han China and Rome. Their analysis, based on historical eye-witness accounts, Chinese astronomical records, astrological literature from later antiquity and ice cores from Greenland glaciers, yielded a range of orbital parameters for the hypothetical object. They settled on a 0.224 A.U. orbit for the object which was apparently visible with a tail from the Chinese capital (in late May) and as a star-like object from Rome (in late July):
A few scholars, such as Robert Gurval of UCLA and Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics, leave the comet's very existence as an open question. Marsden notes in his forward to Ramsey and Licht's book, "Given the circumstance of a single reporter two decades after the event, I should be remiss if I were not to consider this [i.e., the comet's non-existence] as a serious possibility." 
The poet Virgil writes in his ninth eclogue that the star of Caesar has appeared to gladden the fields. Virgil later writes of the period following Julius Caesar’s assassination, “Never did fearsome comets so often blaze.”  Gurval interestingly points out that this passage in no way links a comet to Caesar’s divine status, but rather links comets to his death.
Then Jupiter, the Father, spoke..."Take up Caesar’s spirit from his murdered corpse, and change it into a star, so that the deified Julius may always look down from his high temple on our Capitol and forum." He had barely finished, when gentle Venus stood in the midst of the Senate, seen by no one, and took up the newly freed spirit of her Caesar from his body, and preventing it from vanishing into the air, carried it towards the glorious stars. As she carried it, she felt it glow and take fire, and loosed it from her breast: it climbed higher than the moon, and drawing behind it a fiery tail, shone as a star.
In Shakespeare’s Julius Caesar (1599), Caesar's wife remarks on the fateful morning of her husband's murder: "When beggars die there are no comets seen. The heavens themselves blaze forth the death of princes."
In Roman coinage
Tracing the coinage from 44 BC through the developing rule of Augustus reveals the changing relationship of Julius Caesar to the Sidus Iulium. Robert Gurval notes that the shifting status of Caesar’s comet in the coinage follows a definite pattern. Representations of the deified Julius Caesar as a star appeared relatively quickly, occurring within several years of his death. However, about twenty years passed before the star completed its transformation into a comet. Starting in 44 BC, a money maker named P. Sepullius Macer created coins with the front displaying Julius Caesar crowned with a wreath and a star behind his head. On the back, Venus, the patron goddess of the Julian family, holds a starred scepter. Gurval maintains that this coin was minted about the time of Caesar’s assassination and thus probably would not have originally referred to his deification. As it circulated, however, it would have brought that idea to mind because of Caesar’s new cult. Kenneth Scott’s older work The Sidus Iulium and the Apotheosis of Caesar contests this by assuming that the comet did indeed spark this series because of similarity to other coins he produced. A series of Roman aurei and denarii minted after this cult began show Mark Antony and a star, which most likely represents his position as Caesar’s priest. In later coins likely originating near the end of Octavian’s war with Sextus Pompey, the star supplants Caesar’s name and face entirely, clearly representing his divinity.
One of the clearest and earliest correlations of Caesar to a comet occurred during the Secular Games of 17 BC when money maker M. Sanquinius fashioned coins whose reverse sports a comet over the head of a wreathed man whom classicists and numismatists speculate is either a youthful Caesar, the Genius of the Secular Games, the Julian family, or Aeneas’ son Iulus. These coins strengthened the link between Julius Caesar and Augustus since Augustus associated himself with the Julians. Another set of Spanish coins displays an eight-rayed comet with the words DIVVS IVLIVS, meaning Divine Julius.
- Ramsey, John T. and A. Lewis Licht (1997), The Comet of 44 BC and Caesar's Funeral Games, Scholars Press (Series: APA American Classical Studies, No. 39.)
- Grant, Michael (1970), The Roman Forum, London: Weidenfeld & Nicolson; Photos by Werner Forman, p. 94.
- Flare-up on July 23–25, 44 BC (Rome): −4.0 (Richter model) and −9.0 (41P/Tuttle-Giacobini-Kresák model); absolute magnitude on May 26, 44 BC (China): −3.3 (Richter) and −4.4 (41P/TGK); calculated in Ramsey and Licht, Op. cit., p. 236.
- Suetonius, Divus Julius; 88
- Pliny the Elder, Naturalis Historia, 2.93-94.
- Ovid, Metamorphoses; XV, 840.
- Ramsey and Licht, Op. cit.
- Marsden, Brian G., "Forward"; In: Ramsey and Licht, Op. cit.
- Gurval, Robert (1997). "Caesar's Comet: the Politics and Poetics of an Augustan Myth.". Memoirs of the American Academy in Rome 42: 39–71.
- Georgic 1.487-488 qtd. In Ramsey and Licht, Op. cit
- Ovid, Metamorphoses; XV; 745-842.
- Scott, Kenneth (1941). "The Sidus Iulium and the Apotheosis of Caesar". Classical Philology 36 (3): 257–272.