Caesar's Comet

C/−43 K1
Discovery
Discovery dateMay 18, 44 BC (earliest mention)
Designations
  • Comet Caesar
  • Sidus lulium "Julian Star"
  • Caesaris astrum "Star of Caesar"
  • Great Comet of 44 BC
Orbital characteristics[1]
EpochMay 25, 43 BC(JD 1705496.5)
Observation arc54 days
Perihelion0.22 AU
Eccentricity~1.00 (assumed)
Inclination110°
170°
Argument of
periapsis		17°
Last perihelionMay 25, 43 BC
Physical characteristics
–4.0
(44–43 BC apparition)

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] It was perhaps the most famous comet of antiquity.

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.

  1. ^ a b c "JPL Small-Body Database Browser: C/−43 K1". Jet Propulsion Laboratory. Retrieved 2015-11-16.
  2. ^ Ramsey, John T.; Licht, A. Lewis (1997). The Comet of 44 B.C. and Caesar's Funeral Games. Atlanta, Ga: Scholars Press. ISBN 978-0-7885-0273-6. Retrieved 16 June 2024.
  3. ^ Grant, Michael; Forman, Werner (1970). The Roman Forum. London: Weidenfeld & Nicolson. p. 94. ISBN 978-0-297-00199-7. Retrieved 16 June 2024.
  4. ^ Kronk, Gary W. (2009). Cometography: A Catalog of Comets. 1933-1959. Volume 4. Cambridge University Press. p. 125. ISBN 978-0-521-58504-0. Retrieved 16 June 2024.
  5. ^ Kronk, Gary W. (2009). Cometography: A Catalog of Comets. 1933-1959. Volume 4. Cambridge University Press. p. 22. ISBN 978-0-521-58504-0. Retrieved 16 June 2024.
  6. ^ See Ramsey pp. 122–23: (Comet absolute magnitude H1 of 3.3) + 2.5 * (n of 4) * log (Sun distance of 0.220 AU) + 5 * log (Earth distance of 1.09 AU) = perihelion apparent magnitude of −3.1.
  7. ^ The Comet of 44 B.C. and Caesar's Funeral Games (John T. Ramsey, A. Lewis Licht) p. 123
  8. ^ Hughes, David W (1990). "Cometary absolute magnitudes, their significance and distribution". Asteroids: 327. Bibcode:1990acm..proc..327H.
  9. ^ Flare-up on July 23–25, 44 BC (Rome): −4.0 (Richter model) and −9.0 (41P/Tuttle-Giacobini-Kresák model); absolute magnitude on May 26, 44 BC (China): −3.3 (Richter) and −4.4 (41P/TGK); calculated in Ramsey and Licht, Op. cit., p. 236.
  10. ^ a b François Arago (1832). Tract On Comets. Translated by John Farrar. Hilliard, Gray. p. 71.
  11. ^ "Horizon Online Ephemeris System". California Institute of Technology, Jet Propulsion Laboratory. Retrieved 2015-11-16.