This article is part of a series about Julius Caesar
Personal Family Works Laudatio Iuliae amitae Anticato Commentarii de Bello Civili Commentarii de Bello Gallico De analogia Poems Quotes Alea iacta est Veni, vidi, vici Ut est rerum omnium magister usus Military campaigns Mytilene Gallic Wars Arar Bibracte Vosges Axona Sabis Atuatuci Octodurus Morbihan Invasions of Britain Ambiorix's revolt Avaricum Gergovia Alesia Uxellodunum Civil War Corfinium Brundisium Ilerda Dyrrhachium Pharsalus Alexandrian war Siege Battle of the Nile Zela Ruspina Corduba Thapsus Munda Planned invasion of the Parthian Empire Assassination Ides of March coin Theatre of Pompey Curia of Pompey Last words Et tu, Brute? Caesar's Comet Legacy Cultural depictions Eponyms Life of Caesar Temple of Caesar Caesarism Julio-Claudian dynasty Title Crossing the Rubicon
v t e

Caesar's Comet (alsoSidus Iulium ("Julian Star");Caesaris astrum ("Star ofCaesar");Comet Caesar; theGreat Comet of 44 BC;numerical designationC/−43 K1) was a seven-daycometary outburst seen in July 44 BC.^[2] It was interpreted by Romans as a sign of thedeification ofrecently assassinated dictator,Julius Caesar (100–44BC).^[3]

Based on two questionable reports—one from China (May 30) and another from Rome (July 23)—an infinite number oforbit determinations can fit the observations, but aretrograde orbit is inferred based on available notes.^[4] The comet approached Earth both inbound in mid-May and outbound in early August.^[5] It came toperihelion (closest approach to the Sun) on May 25, −43 at a solar distance of about 0.22 AU (33 million km).^[1] At perihelion the comet had asolar elongation of 11 degrees and is hypothesized to have had anapparent magnitude of around −3 as the Chinese report is not consistent with daytime visibility during May.^[6] Between June 10 and July 20 the comet would have dimmed from magnitude +1 to around magnitude +5. Around July 20, −43, the comet underwent an estimated 9 magnitude outburst in apparent magnitude^[7] and had a solar elongation of 88 degrees in the morning sky. At magnitude −4 it would have been as bright asVenus.

As a result of the cometary outburst in late July, Caesar's Comet is one of only five comets known to have had a negativeabsolute magnitude (for a comet, this refers to the apparent magnitude if the comet had been observed at a distance of 1AU from both the Earth and the Sun^[8]) and may have been the brightest daylight comet inrecorded history.^{[9][verification needed]}

In the absence of accurate contemporary observations (or later observations confirming an orbit that predicts the earlier appearance), calculation of the comet's orbit is problematic and aparabolic orbit is conventionally assumed.^[1] (In the 1800s a possible match was speculated which would give it a period of about 575 years.^[10] This has not been confirmed because the later observations are similarly insufficiently accurate.)^[10] The parabolic orbital solution estimates that the comet would now be more than 800 AU (120 billion km) from the Sun.^[11] At that distance, the Sun provides less light than the full Moon provides to Earth.

Caesar's Comet was known to ancient writers as theSidus Iulium ("Julian Star") orCaesaris astrum ("Star of Julius Caesar"). The bright,daylight-visible comet appeared suddenly during the festival known as theLudi Victoriae Caesaris—for which the 44 BC iteration was long considered to have been held in the month of September (a conclusion drawn byEdmund Halley). The dating has recently been revised to a July occurrence in the same year, some four months after theassassination of Julius Caesar, as well as Caesar's own birth month. According toSuetonius, 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."^[12]

The Comet became a powerful symbol in the politicalpropaganda that launched the career of Caesar's great-nephew (and adoptive son)Augustus. TheTemple 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".^[13]) At the back of the temple a huge image of Caesar was erected and, according toOvid, 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.^[14]

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. About twenty years passed, however, before the star completed its transformation into a comet.^[15] 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 theJulian 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.^[15] Kenneth Scott's older workThe 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.^[16] A series of Roman aurei and denarii minted after this cult began showMark Antony and a star, which most likely represents his position as Caesar's priest.^[15] In later coins likely originating near the end of Octavian's war withSextus Pompey, the star supplants Caesar's name and face entirely, clearly representing his divinity.^[15]

One of the clearest and earliest correlations of Caesar to a comet occurred during theSecular 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, orAeneas' sonIulus. 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 wordsDIVVS IVLIV,^[15] meaningdīvus Iūlius, 'Divine Julius'.

The poetVirgil writes in his ninth eclogue that the star of Caesar has appeared to gladden the fields.^[17] Virgil later writes of the period following Julius Caesar's assassination, "Never did fearsome comets so often blaze."^[18] Gurval points out that this passage in no way links a comet to Caesar's divine status, but rather links comets to his death.^[15]

Calpurnius Siculus makes mention of the comet in his epilogues as a boding sign of the civil strife that would plague Rome after Caesar's death.^[19]

It isOvid, however, who makes the final assertion of the comet's role in Julius Caesar's deification. Ovid describes the deification of Caesar inMetamorphoses (8 AD):

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.^[20]

It has been argued recently that the idea of Augustus's use of the comet for his political aims largely stems from this passage.^[21]

InShakespeare'sJulius Caesar (1599),Caesar's wife remarks on the fateful morning of herhusband's murder: "When beggars die there are no comets seen. The heavens themselves blaze forth the death of princes."

In 1997, two scholars at theUniversity of Illinois at Chicago—John T. Ramsey (a classicist) and A. Lewis Licht (a physicist)—published a book^[22] comparing astronomical/astrological evidence from bothHan China and Rome. Their analysis, based on historical eyewitness accounts, Chinese astronomical records, astrological literature from later antiquity andice cores fromGreenlandglaciers, yielded a range of orbital parameters for the hypothetical object. They settled on a perihelion point of 0.22AU for the object which was apparently visible with a tail from the Chinese capitalChang'an (in late May) and as a star-like object from Rome (in late July):

• May 18, 44 BC (China)
• July 23–25, 44 BC (Rome)
• Apparent magnitude: −4.0^[23]

Robert Gurval ofUCLA andBrian G. Marsden of theHarvard–Smithsonian Center for Astrophysics leave the comet's very existence as an open question. Marsden notes in his foreword 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."^[24]

Maik Meyer andGary Kronk in 2025 published a paper indicating that theGreat Comet of 1744 has an orbital period of about 350 years and integrating it backwards found possible matches for previous apparitions in 1402 (seeGreat Comet of 1402), 1032, 676 and 336 AD. Based on that orbit the next previous apparition would be in the middle of 44 BC, around the time Caesar's Comet was visible. However they noted that the observation reports were contradicting and considered the identification with the Great Comet of 1744 as unlikely.^[25]

• List of hyperbolic comets

• Non-periodic comets
• Julius Caesar
• Augustus
• 44 BC
• Assassination of Julius Caesar
• Astronomical objects known since antiquity
• Great comets

• Webarchive template wayback links
• Articles with short description
• Short description matches Wikidata
• All pages needing factual verification
• Wikipedia articles needing factual verification from May 2019