Caesar's Comet

Lore & Background

Caesar's Comet was known to ancient writers as the Sidus Iulium or Caesaris astrum. The comet was visible in daylight and appeared suddenly during the festival known as the Ludi Victoriae Caesaris, which has been revised to a July occurrence in 44 BC, some four months after the assassination of Julius Caesar and in his 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 was built in 42 BC and dedicated in 29 BC by Augustus for purposes of fostering a 'cult of the comet'. 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.

Based on two questionable reports, one from China (May 30) and another from Rome (July 23), an infinite number of orbit determinations can fit the observations, but a retrograde orbit is inferred. The comet approached Earth both inbound in mid-May and outbound in early August. It came to perihelion on May 25, −43 at a solar distance of about 0.22 AU. At perihelion the comet had a solar elongation of 11 degrees and is hypothesized to have had an apparent magnitude of around −3. Between June 10 and July 20, the comet would have dimmed from magnitude +1 to around +5. Around July 20, −43, the comet underwent an estimated 9 magnitude outburst and had a solar elongation of 88 degrees in the morning sky, reaching magnitude −4.

Reader's Guide

Caesar's Comet holds a significant place in history as a political and religious symbol. The comet was used by Augustus to legitimize his rule and the divine status of Julius Caesar. On Roman coinage, representations of the deified Julius Caesar as a star appeared within several years of his death, but about 30 years passed before the star completed its transformation into a comet. Coins from 44 BC by P. Sepullius Macer showed Caesar crowned with a wreath and a star behind his head, while later coins from the Secular Games of 17 BC showed a comet over the head of a wreathed man, strengthening the link between Julius Caesar and Augustus.

In literature, Virgil wrote that the star of Caesar appeared to gladden the fields, and later noted that 'never did fearsome comets so often blaze' after Caesar's assassination. Ovid in Metamorphoses (8 AD) made the final assertion of the comet's role in Caesar's deification, describing how Venus carried Caesar's spirit to the stars, where it drew a fiery tail and shone as a star. Modern scholarship by John T. Ramsey and A. Lewis Licht in 1997 analyzed historical eyewitness accounts, Chinese astronomical records, astrological literature, and ice cores to yield orbital parameters, settling on a perihelion point of 0.22 AU. However, Robert Gurval and Brian G. Marsden leave the comet's very existence as an open question, with Marsden noting the possibility that it did not exist given the single reporter two decades after the event.

Did You Know?

• The Temple of Divus Iulius was also known as the 'Temple of the Comet Star'.
• A 2025 paper by Maik Meyer and Gary Kronk found possible orbital matches for Caesar's Comet with the Great Comet of 1744, though observation reports contradict.