Around the time for Julius Caesar's assassination  (RomBC 44, March 15) a new bright transient star was observed in Rome (about year count and notation read explanation here, opens in new tab). It is referred to in the contemporary literature as sidus Iulium (Julian star/heavenly body) and Octavian, Caesar's adopted son and subsequently Augustus, is said to have managed to launch the star or comet as a sign of his father's divinity, thereby increasing his own power. The time of observation is generally assumed to be the end of July RomBC 44 during the funeral games for Caesar held by Octavian, when Caesar had been dead for more than four months. However, a star appears on coinage already during Caesar's lifetime, and Nandini B. Pandey concludes in her recent paper (ref.1):

Octavian, in other words, had no need to invent the sidus Iulium during the funeral games that summer; the idea of Caesar’s divinity and its representation by means of a star were already part of Roman cultural discourse and were already circulating in numismatic form around the time of Caesar’s death.

 

So an alternative interpretation of the sources could lead to the assumption that the sidus appeared already in RomBC 45, perhaps during the games held by Caesar in July 20 - 30 in honor of Victoriae Caesaris.

A lot has been written about this celestial phenomenon, which has been considered alternately myth or reality until an almost matching comet was found in independent Chinese astronomical records (ref.2). This comet got the denomination C/-43 K1, and an inclination of about 110 degrees and a magnitude of about -4 (which means visible in moderate daylight) were calculated. However, the time of observation in China was not the end of July, but May 18 to June 16 Astr -43, which the authors were quite troubled about. C/-43 K1 has apparently never returned, either its orbital period is very long or it has disintegrated on its way.

Now we become aware of the real problem. There is such a great number of detailed Chinese astronomical records that the observations can be unambiguously fixed on the absolute astronomical time line, and there is a real "catalog of observations" (ref.3) so that you can find a suitable event for almost every single year. The Roman records, however, are scanty and diffuse (ref.4) which means that the Roman time complex is floating astronomically and can be fixed only to the historical time line (which is a subject for interpretation as discussed above). According to our investigations (read under tab "Ancient history"), Roman time appears conventionally dated 232 years too old which would mean that Caesar died in 189 and the sidus was instead sighted in 189 or 188. Therefore C/-43 K1 would have nothing at all to do with Caesar's comet, if we are right.

For the year Astr 188 (corresponding to RomBC 45 according to our hypothesis), the Chinese records mention the appearance of a large "guest star" on July 28 until mid-August. This guest star has been identified as Comet P/Swift-Tuttle (ref.5), which has a mean orbital period of ca. 133 years and an inclination of 113 degrees.  The Chinese records say that it was "as large as a vessel with a capacity of three pints", a peak brightness of 0.1 mag (almost visible in daylight) has been calculated for this apparition of the comet. Furthermore the modern authors interpret that the description "as large as a vessel" could mean a nebulous appearance without a tail, which seems to be characteristic for Comet Swift/Tuttle (see image from NASA, opens in new window). Contrary to comet Halley, comet Swift-Tuttle is not always visible with the naked eye. In fact, the 188 return was by far the brightest in historical times, the comet was not observed again until 1737. We have found no Roman report of Comet Swift-Tuttle at its astronomical date.

To sum up, the Astr 188 apparition of Comet P/Swift-Tuttle seems to be a hot candidate for Caesar's comet. It is bright and large, visible in the right celestial location over Rome and appears at the right time of the year (July, which is Caesar's birth month and named in his honor). Its appearance a few months before Caesar's death resolves the discrepancy between numismatic evidence and written (later) sources: it was probably Caesar himself, and not Octavian, who first used a star to promote the divinity of the Julian family.

 

References:
1. Nandini B. Pandey 2013. Caesar’s Comet, the Julian Star, and the Invention of Augustus, Transactions of the American Philological Association 143, p. 405 - 449. read here
2. Ramsey, J. T. and Licht, A. L. 1997. The Comet of 44 b.c. and Caesar’s Funeral Games. Atlanta: Scholars Press. read preview here
3. Williams, J. 1871. Observations of comets, from BC 611 to AD 1640. London, printed for the author by Strangways and Walden. read here, or here
4. Barrett, A. A. 1978. Observations of Comets in Greek and Roman Sources Before A.D. 410. Journal of the Royal Astronomical Society of Canada, Vol. 72, p.81. read here
5. Yau, K., Yeomans, D., Weissman, P. 1994. The past and future motion of Comet P/Swift-Tuttle. Royal Astronomical Society. Monthly Notices, vol. 266, 305-316. read here