Electronic Telegram No. 5621 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/2025 A6 (LEMMON) Further to CBET 5594, this comet has maintained its elevated level of increased intrinsic brightness. Selected visual total-magnitude and coma- diameter estimates: Aug. 28.09 UT, 10.5, 3'.1 (V. Gonano, Udine, Italy, 20x80 binoculars); Sept. 5.11, 9.9, 3'.5 (Gonano); 14.15, 9.2, 4' (Gonano); 16.12, 8.5, 4' (Gonano); 18.11, 8.0, 5' (Gonano); 20.11, 7.8, 6'.1 (Gonano); 21.46, 8.1, 5' (C. Hergenrother, Tucson, AZ, USA, 30x125 binoculars); 29.12, 7.5, 5' (Gonano); 30.79, 7.0, 8' (K. Yoshimoto, Yamaguchi, Japan, 8x42 binoculars); Oct. 5.14, 6.3, 7' (Gonano); 9.82, 5.6, 10' (Yoshimoto); 11.84, 5.3, 10' (Yoshimoto); 13.20, 5.3, 8' (Gonano). Yoshimoto also reports the following CMOS total-magnitude, coma-diameter, and tail-length measures for C/2025 A6, obtained with a 135-mm f/2 lens (+ V filter on the first two nights): Oct. 1.83 UT, 6.9, 10'.6, 9 deg in p.a. 300 deg; Oct. 5.82, 6.2*, 9'.6, > 6 deg in p.a. 306 deg; Oct. 8.83, 5.9*, 10'.6, > 6 deg in p.a. 313 deg; Oct. 10.85, 5.5*, 11'.6, > 11 deg in p.a. 318 deg. M. Jaeger and G. Rhemann posted a mosaic photo on the "ICQ Comet Observations" Facebook forum taken on Oct. 4.12 with a 135-mm f/2.8 telephoto lens that shows an ion tail 12 degrees long. The following updated orbital elements by S. Nakano (Central Bureau) are from 759 observations spanning 2024 Nov. 12-2025 Oct. 3 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being +0.008194 and +0.009094 (+/- 0.000002) AU**-1, respectively. The comet passed 2.33 AU from Jupiter on 2025 Apr. 16 and will pass 0.60 AU from the earth on 2025 Oct. 21 UT. Epoch = 2025 Nov. 21.0 TT T = 2025 Nov. 8.53842 TT Peri. = 132.97014 e = 0.9956356 Node = 108.09803 2000.0 q = 0.5298882 AU Incl. = 143.66345 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 7.5 and 2.5n = 12 for the total visual magnitudes below, which represent the magnitudes reported since August. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 10 12 11 35.57 +42 10.1 0.718 0.816 53.8 80.9 5.7 2025 10 17 13 04.24 +38 13.2 0.623 0.738 47.7 93.7 4.9 2025 10 22 14 34.13 +27 56.9 0.598 0.667 40.6 103.7 4.3 2025 10 27 15 41.37 +14 32.5 0.655 0.605 36.2 104.1 4.0 2025 11 01 16 23.57 +02 47.6 0.778 0.559 34.1 94.4 3.9 2025 11 06 16 48.07 -06 01.9 0.938 0.533 31.9 79.5 4.1 2025 11 11 17 01.43 -12 33.6 1.109 0.533 28.7 63.1 4.4 2025 11 16 17 08.15 -17 32.3 1.275 0.558 24.6 47.6 5.0 2025 11 21 17 11.17 -21 28.2 1.427 0.604 20.1 34.3 5.6 2026 01 20 17 07.83 -45 44.3 2.089 1.550 44.5 26.4 11.4 2026 01 25 17 05.38 -47 30.0 2.080 1.628 49.8 27.5 11.6 2026 01 30 17 01.98 -49 18.7 2.067 1.706 55.0 28.3 11.9 2026 02 04 16 57.39 -51 10.5 2.049 1.782 60.4 28.8 12.1 2026 02 09 16 51.31 -53 05.3 2.028 1.858 65.9 29.0 12.3 2026 02 14 16 43.34 -55 02.3 2.005 1.932 71.4 29.0 12.4 2026 02 19 16 33.02 -57 00.2 1.982 2.006 77.0 28.7 12.6 2026 02 24 16 19.80 -58 56.5 1.958 2.079 82.7 28.2 12.8 2026 03 01 16 03.09 -60 47.5 1.937 2.151 88.4 27.4 12.9 2026 03 06 15 42.35 -62 28.1 1.919 2.222 94.1 26.4 13.1 2026 03 11 15 17.25 -63 51.7 1.906 2.293 99.7 25.3 13.2 2026 03 16 14 47.97 -64 50.7 1.899 2.362 105.1 24.0 13.4 2026 03 21 14 15.51 -65 17.6 1.899 2.431 110.3 22.6 13.5 2026 03 31 13 08.95 -64 19.5 1.926 2.567 119.3 19.8 13.8 2026 04 10 12 13.36 -61 07.3 1.993 2.700 125.5 17.6 14.2 2026 04 20 11 34.86 -56 37.4 2.102 2.831 127.8 16.3 14.5 2026 04 30 11 10.86 -51 47.0 2.251 2.959 126.1 16.0 14.9 2026 05 10 10 56.94 -47 12.6 2.436 3.085 121.4 16.2 15.3 2026 05 20 10 49.70 -43 11.5 2.649 3.209 114.9 16.6 15.7 2026 05 30 10 46.92 -39 49.4 2.882 3.331 107.6 16.9 16.1 2026 06 09 10 47.12 -37 05.4 3.131 3.452 99.8 16.8 16.4 2026 06 19 10 49.41 -34 55.9 3.387 3.570 92.0 16.5 16.8 2026 06 29 10 53.15 -33 16.6 3.646 3.687 84.3 15.9 17.1 2026 07 09 10 57.90 -32 03.1 3.904 3.803 76.8 15.1 17.4 2026 07 19 11 03.38 -31 11.6 4.156 3.917 69.5 14.1 17.7 2026 07 29 11 09.35 -30 39.0 4.398 4.029 62.4 12.9 18.0 2026 08 08 11 15.62 -30 22.2 4.628 4.140 55.6 11.7 18.2 2026 08 18 11 22.09 -30 19.0 4.843 4.250 49.1 10.4 18.5 2026 08 28 11 28.60 -30 27.6 5.041 4.359 43.2 9.1 18.7 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 October 13 (CBET 5621) Daniel W. E. Green