Caesar's Comet

Caesar's Comet (also Sidus Iulium ("Julian Star"); Caesaris astrum ("Star of Caesar"); Comet Caesar; the Great Comet of 44 BC; numerical designationC/−43 K1) was a seven-day cometary outburst seen in July 44 BC. It was interpreted by Julius Caesar's contemporaries in Ancient Rome as a sign of the deification of the recently assassinated dictator (100–44 BC).

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 based on available notes. The comet approached Earth both inbound in mid-May and outbound in early August. It came to perihelion (closest approach to the Sun) on May 25, −43 at a solar distance of about 0.22 AU (33 million km). At perihelion the comet had a solar elongation of 11 degrees and is hypothesized to have had an apparent magnitude of around −3 as the Chinese report is not consistent with daytime visibility during May. 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 and had a solar elongation of 88 degrees in the morning sky. At magnitude −4 it would have been as bright as Venus.

As a result of the cometary outburst in late July, Caesar's Comet is one of only five comets known to have had a negative absolute magnitude (for a comet, that refers to the apparent magnitude if the comet had been observed at a distance of 1 AU from both the Earth and the Sun) and may have been the brightest daylight comet in recorded history.

In the absence of accurate contemporary observations (or later observations that confirm an orbit predicting the earlier appearance), calculation of the comet's orbit is problematic, and a parabolic orbit is conventionally assumed. (In the 1800s, a possible match with the Great Comet of 1680 was speculated, which would give it a period of about 575 years. That has not been confirmed because the later observations were similarly insufficiently accurate.) The parabolic orbital solution estimates that the comet would now be more than 800 AU (120 billionkm) from the Sun. At that distance, the Sun provides less light than the full Moon provides to Earth.