Caesar's Comet

This article is part of a series about Julius Caesar
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Caesar's Comet (also Sidus Iulium ("Julian Star"); Caesaris astrum ("Star of Caesar"); Comet Caesar; the Great Comet of 44 BC; numerical designation C/−43 K1) was a seven-day cometary outburst seen in July 44 BC.^[2] It was interpreted by Romans as a sign of the deification of recently assassinated dictator, Julius Caesar (100–44 BC).^[3]

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.^[4] The comet approached Earth both inbound in mid-May and outbound in early August.^[5] 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).^[1] 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.^[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 impressive 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, this refers to the apparent magnitude if the comet had been observed at a distance of 1 AU from both the Earth and the Sun^[8]) and may have been the brightest daylight comet in recorded 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 a parabolic 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.