Electronic Telegram No. 5663 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET C/2026 A1 (MAPS) S. Nakano, Central Bureau, has computed the following updated orbital elements (cf. CBET 5658) from 187 observations spanning 2025 Dec. 19-2026 Feb. 9 (mean residual 0".6), with corresponding "original" and "future" values of 1/a being +0.007098 and +0.006394 (+/- 0.000080) AU**-1, respectively; these yield P = 1870 years. Epoch = 2026 Mar. 21.0 TT T = 2026 Apr. 4.59815 TT Peri. = 86.35761 e = 0.9999623 Node = 7.89626 2000.0 q = 0.0057149 AU Incl. = 144.49336 Z. Sekanina, La Canada Flintridge, CA, USA, writes with regard to S. Nakano's improved set of orbital elements (above), which predicts previous perihelion in AD 357.6 +/- 28 years (Nakano, personal communication via D. W. E. Green), that it appears increasingly likely that this comet is a product of tidal fragmentation (i.e., at close proximity of perihelion) of one of the comets seen in broad daylight by the Greek/Roman historian Ammianus Marcellinus (c. 330-400 AD) from Antioch on the Orontes in late 363 AD (probably November; *Res Gestae* 25:10). In his 2009 book *The Greatest Comets in History*, D. Seargent suggested, as "wild speculation," that the reference "could ... imply several sungrazing fragments close together". Sekanina adds: "This kind of event in the second half of the 4th century fitted perfectly a scenario that I envisioned in my 2021 conceptual model of the Kreutz system (cf. website URL https://arxiv.org/abs/2109.01297) for the first perihelion passage following cascading fragmentation of Aristotle's comet (assumed in the shape of a contact binary) at large heliocentric distance. In this context, comet C/2026 A1 would be the only second- generation fragment of Aristotle's comet that we are aware of to appear after the 12th century. Its unusually long orbital period in excess of 1600 years is readily understood as that of an outlying fragment of a parent sungrazer more than 20 km across. Based on Nakano's additional orbital computations (personal communication), comets C/2026 A1 and C/1963 R1 (Pereyra) do not appear to have derived from the same first-generation fragment of Aristotle's comet, if C/1963 R1's ancestry involves the comet of September 1041." For additional information, see Sekanina's paper as posted at website URL https://arxiv.org/abs/2602.17626. At the request of Sekanina, Nakano very cautiously provides the following orbital elements when forcing T = 363 Nov. 14 TT: Epoch 363 Nov. 14.0 TT, T = 363 Nov. 14.170 TT, q = 0.00552, e = 0.99997, Peri. = 79.445 deg, Node = 359.498 deg, i = 143.898 (equinox J2000.0), a = 159.1 AU, P = 2006 years. Hidetaka Sato (Tokyo, Japan) reports that CCD images obtained remotely with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, on Jan. 28.53 UT show a strongly condensed coma 25" in diameter with a fan-like tail 30" long spanning p.a. 20-70 degrees; the magnitude was 17.8 as measured within a circular aperture of radius 14".9. On Feb. 4.44, the comet showed a strongly condensed 1'.0 coma with a 50" tail toward p.a 80 deg; the magnitude was 16.2 as measured within a circular aperture of radius 33".0. Using a similar telescope at Rio Hurtado, Chile, on Feb. 16.10, Sato found the comet to be moderately condensed with an outer coma of size 1'.5; the magnitude was 15.7 as measured in a circular aperture of radius 46".8. A. Pearce reports the following total magnitudes and coma diameters of comet C/2026 A1 from CCD images obtained with the Perth Observatory 0.35-m Schmidt-Cassegrain reflector at Bickley, W. Australia: Jan. 31.63 UT, 17.2, 0'.5; Feb. 1.60, 16.9, 0'.7; 2.63, 16.7, 0'.7; 4.54, 16.1, 1'; 5.58, 16.1, 1'; 6.56, 15.8, 1'; 7.54, 15.9, 1'; 8.61, 15.3, 1'.2; 11.54, 14.7, 2'; 12.54, 14.6, 2'; 13.55, 14.5, 2'. He measured a tail that was 0'.5-0'.7 long in the range p.a. 64-73 degrees on most of the nights. Images taken by G. Rhemann and M. Jaeger with a 0.30-m f/3.6 astrograph located at the Tivoli Farm in Namibia on Feb. 9.89 and 18.79 UT show a greenish coma, as posted at the "ICQ Comet Observations" forum on Facebook. Exposures taken on Feb. 10.80 show a fainter outer coma of size 6' and a brighter inner coma of size 150" x 140" and total mag 14.1 with a thin, straight tail 200" long in p.a. 78 degrees. An image posted at the ICQ forum by T. Lovejoy from Feb. 15.43 (four stacked 120-s exposures taken with a 0.43-m f/6.8 reflector at Siding Spring, NSW) show a 3'.0 coma of total mag 14.1 with a short tail extending 1'.6 in roughly p.a. 84 degrees. M. Masek, Institute of Physics, Czech Academy of Sciences, reports that CCD images obtained with a 6.7-cm camera lens near Cerro Paranal, Chile, on Feb. 15.09 UT yield the following total magnitudes and coma diameters: V = 13.6, 4'.1; R = 15.0, 2'.3. On Feb. 9.08, Masek measured total mag 14.5 and coma diameter 3'.2 on V-band images. The rapid brightening of C/2026 A1 has been roughly at the rate of 2.5n = 25 in the standard power-law formula Mag. = 9.5 + 5 log Delta + 2.5n log r. The indicated size of the nucleus is generally not considered large enough to survive perihelion, as noted on CBET 5658. However, there seems to be a good chance that this comet can survive to as close to the sun as r = 2.5 AU or less pre-perihelion, in which case this comet may display a bright coma and tail visible to the naked eye. This is uncharted territory with this Kreutz sungrazer being visible so far from the sun pre-perihelion, but the average light curves of most long-period comets over the course of their paths in the inner solar system tend toward 2.5n = 7.5-8.5, which is why these circulars use the default 2.5n = 8.0 for most new long-period comets when predicting total magnitudes in ephemerides. The bright comet C/1965 S1 had an average value of 2.5n = 8 to 9 on both sides of perihelion. So being prudent in expecting the current sharp rise in brightness of C/2026 A1 to level off, the following ephemeris from the above orbital elements cautiously uses H = 11.0 and 2.5n = 15 for the predicted total visual magnitudes, noting that the comet could well start to fall apart by r = 0.2 AU. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2026 02 09 03 40.39 -26 32.2 1.302 1.576 85.9 38.6 14.5 2026 02 14 03 26.37 -24 22.6 1.293 1.478 79.6 41.1 14.1 2026 02 19 03 13.66 -22 04.4 1.286 1.377 73.2 43.4 13.6 2026 02 24 03 02.09 -19 40.1 1.279 1.272 66.8 45.7 13.1 2026 03 01 02 51.42 -17 11.1 1.270 1.162 60.3 47.8 12.5 2026 03 06 02 41.34 -14 38.0 1.259 1.046 53.8 49.9 11.8 2026 03 11 02 31.44 -12 00.1 1.244 0.924 47.2 52.0 11.0 2026 03 16 02 21.20 -09 15.8 1.224 0.794 40.3 54.1 9.9 2026 03 21 02 09.80 -06 21.8 1.196 0.651 33.0 56.3 8.6 2026 03 26 01 55.77 -03 11.3 1.160 0.491 24.9 58.7 6.7 2026 03 31 01 35.62 +00 30.9 1.109 0.299 15.2 60.9 3.4 2026 04 05 01 21.20 +03 57.4 1.112 0.169 6.9 45.1 -0.4 2026 04 10 14 59.80 +13 01.4 0.890 1.811 146.3 17.9 14.6 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 February 20 (CBET 5663) Daniel W. E. Green