Electronic Telegram No. 4383 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/2015 ER_61 (PANSTARRS) This comet has undergone a sudden brightening in the last two weeks. Selected visual total-magnitude and coma-diameter estimates: Feb. 28.31 UT, 11.3, 1' (M. Goiato, Aracatuba, Brazil, 0.22-m reflector); Mar. 24.84, 8.9, 5'.5 (A. Pearce, Nedlands, W. Australia, 0.1-m reflector); 25.33, 9.5, 5' (Goiato); 28.19, 8.7, 4' (J. J. Gonzalez, Asturias, Spain, 25x100 binoculars); 30.33, 8.5, 6' (Goiato, 20x100 binoculars); 31.84, 7.6, 10' (Pearce); Apr. 1.18, 8.2, 6' (Gonzalez, Leon, Spain); 2.28, 8.4, 6' (Goiato); 3.80, 8.3, 5' (M. Mattiazzo, Swan Hill, Vic., Australia, 25x100 binoculars); 5.34, 6.4, 6' (Goiato); 5.80, 6.2, 8' (Mattiazzo, 8x40 binoculars); 7.18, 6.2, 8' (Gonzalez, 10x50 binoculars); 10.34, 6.5, 12' (Goiato, 7x50 binoculars; moonlight, clouds). Mattiazzo notes that an ion tail at least 30' long was visible on Apr. 5.80 in 25x100 binoculars. The following updated orbital elements are by S. Nakano, Sumoto, Japan, from 849 observations spanning 2015 Jan. 21-2017 Apr. 15 (mean residual 0".53). Epoch = 2017 May 7.0 TT T = 2017 May 9.94428 TT Peri. = 68.19431 e = 0.9973050 Node = 235.22032 2000.0 q = 1.0421269 AU Incl. = 6.34889 Nakano calulates original = +0.001397 and future 1/a = +0.002331 (+/- 0.000001), adding that the comet passed 0.92 AU from Jupiter on 2016 Mar. 28 UT. He notes that the residuals of Apr. 2017 observations were +5", mainly in right ascension, with respect to his orbit in the ICQ's 2017 Comet Handbook (and NK 3275). The following ephemeris is computed from the above orbital elements, with the magnitude calculated as Mag. = 6.0 + 5 log Delta + 10.0 log r. 2017 TT R. A. (2000) Decl. Delta r Elong. Mag. Apr. 7 20 59.81 -13 58.4 1.203 1.178 63.8 7.1 12 21 24.88 -11 37.5 1.187 1.142 62.2 6.9 17 21 49.62 -09 07.4 1.179 1.111 60.5 6.8 22 22 13.85 -06 31.6 1.180 1.085 58.9 6.7 27 22 37.43 -03 53.9 1.188 1.065 57.3 6.6 May 2 23 00.23 -01 17.7 1.202 1.051 55.9 6.6 7 23 22.21 +01 14.0 1.223 1.043 54.7 6.6 12 23 43.31 +03 39.0 1.248 1.043 53.8 6.7 17 00 03.54 +05 55.7 1.277 1.049 53.0 6.7 22 00 22.90 +08 03.0 1.308 1.062 52.6 6.8 27 00 41.37 +10 00.3 1.341 1.081 52.4 7.0 June 1 00 58.98 +11 47.5 1.375 1.106 52.5 7.1 6 01 15.73 +13 24.7 1.409 1.136 52.9 7.3 11 01 31.64 +14 52.3 1.443 1.171 53.5 7.5 16 01 46.73 +16 10.7 1.475 1.210 54.4 7.7 21 02 01.01 +17 20.5 1.506 1.253 55.5 7.9 26 02 14.48 +18 22.3 1.535 1.299 56.9 8.1 July 1 02 27.14 +19 16.6 1.562 1.348 58.5 8.3 6 02 39.02 +20 04.1 1.586 1.398 60.4 8.5 11 02 50.10 +20 45.3 1.607 1.451 62.5 8.6 16 03 00.39 +21 20.9 1.626 1.505 64.7 8.8 21 03 09.90 +21 51.2 1.642 1.561 67.2 9.0 26 03 18.61 +22 16.7 1.655 1.617 69.9 9.2 31 03 26.50 +22 37.9 1.665 1.675 72.8 9.3 Aug. 5 03 33.57 +22 55.0 1.672 1.733 76.0 9.5 10 03 39.82 +23 08.4 1.677 1.791 79.3 9.7 15 03 45.22 +23 18.4 1.680 1.850 82.8 9.8 20 03 49.75 +23 25.2 1.680 1.909 86.6 9.9 25 03 53.40 +23 28.9 1.679 1.968 90.6 10.1 30 03 56.14 +23 29.7 1.676 2.028 94.8 10.2 Sept. 4 03 57.96 +23 27.6 1.673 2.087 99.2 10.3 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2017 CBAT 2017 April 17 (CBET 4383) Daniel W. E. Green