The comet underwent a major outburst in brightness on 1998 June 1 when it brightened from total magnitude m1=5.5 to 3.5. See Comet Observer's Forum for more details.
A CCD-image by O. Farago (available via M. Meyer's web page) showed the comet in strong twilight on May 10 UT. The comet is located near the extreme SW corner and it shows a short tail towards NE.
Jost Jahn has (via Maik Meyer) kindly supplied the following comments concerning Farago's image: "The following observation time is valid for Otto Faragos image:
1998 5 10.81635 3 42 6.78 +23 37 26.0 025
Brian Marsden has calculated with this time, but published the
wrong time in the IAUC. The wrong time in the IAUC was an error
of Otto Farago who discovered this mistake later and corrected
it. Brian Marsden obviously forgot to change the dates for the
IAUC. (But he calculated with the right date.) The time of
exposure was 0.4 seconds." This position (RA in hours, minutes
and seconds, and Decl. in degrees, arcminutes and arcseconds) is
apparently given for equinox B1950.0.
Comparison between observed and ephemeris posititions. Accurate positions have been reported since May 20 and B. G. Marsden has calculated new elements using positions until June 1 (see below). For comparison I have investigated the positions derived from images by M. Horn and O. Farago. In addition N. Biver has determined equatorial and horizontal coordinates from his visual observations. The results are given in the tables below. sRA and sDec are the estimated uncertainties in R.A. and Decl., while sAlt is the estimated uncertainty in altitude.
Equatorial coordinates
1998 U.T. Observed position (2000.0) Observer MPC 31893
R.A. sRA Decl. sDec R.A. (2000) Decl.
h m m o ' ' h m o '
May 10.8164 3 45.06 .... +23 43.2 ... Farago 3 45.07 +23 46.7
May 16.249 4 47.5 >1.0 +12 53 15 Biver 4 45.96 +13 24.1
May 17.241 4 54.65 0.14 +11 15 2 Biver 4 54.62 +11 15.0
May 18.249 5 03.1 0.3 + 9 04 3 Biver 5 02.92 + 9 04.2
May 18.352 5 03.73 0.05 + 8 50.7 0.6 Horn 5 03.75 + 8 50.9
May 19.2413 5 10.72 0.06 + 6 59.0 0.8 Biver 5 10.68 + 6 56.9
May 20.2438 5 18.0 0.18 + 4 49 2.5 Biver 5 18.15 + 4 50.5
May 20.356 5 18.93 0.05 + 4 36.5 0.5 Horn 5 18.97 + 4 36.5
May 21.3613 5 26.12 0.03 + 2 32.8 0.2 Horn 5 26.10 + 2 32.8
May 30.383 6 20.172 0.002 -13 12.98 0.02 Horn 6 20.17 -13 12.8
Horizontal coordinates
1998 U.T. Observed Observer *Calculated* ***Sun***
Alt. sAlt Alt. Azi. Alt. Azi. U.T.
o o o o o o h m
May 10.8164 .... .... Farago 2.7 304.0 -7.0 307.3 19 35.6
May 11.228 5 .... Biver 3.98 293.39 -6.6 292.1 5 28
May 11.238 1.2 0.2 Biver 0.81 294.68 -9.8 293.6 5 43
May 13.2413 2.72 0.17 Biver 2.15 290.34 -10.5 294.6 5 47.5
May 13.2428 2.02 0.17 Biver 1.69 290.51 -11.0 294.8 5 49.6
May 13.2438 1.83 0.08 Biver 1.38 290.65 -11.3 295.0 5 51.0
May 16.240 .... .... Biver 4.50 282.66 -9.7 295.0 5 45
May 16.249 1.9 0.2 Biver 1.55 283.79 -12.4 296.4 5 58
May 17.253 1.05 0.08 Biver 0.47 281.87 -13.5 297.3 6 04
May 18.253 1.13 0.08 Biver 0.66 279.45 -13.4 297.5 6 04
May 18.352 .... .... Horn 5.0 282.6 -18.0 283.2 8 27
May 20.356 .... .... Horn 7.3 279.0 -19.4 283.0 8 33
May 21.3613 .... .... Horn 7.3 276.7 -21.1 282.5 8 40.3
No corrections have been applied for parallax (which amounts to
~0'.2 in this case). Some of the observations agree well with the
ephemeris postions, while there are clear discrepancies for other
observations. That may be due to inaccurate timings and/or the
effect of refraction. It appears that the difference between the
observed and predicted position on May 10.8164 UT is due to
non-linear effects of refraction which are not accounted for in
ordinary astrometric reductions.
There is a good agreement between Biver's observed altitudes and the true altitudes derived from ephemeris positions. The systematic difference of ~-0°.5 can be accounted for by taking refraction into consideration and the site's height above sea level. His altitudes were measured relative to the sea horizon. This gives a strong indication that the comet was observed on May 11 and May 13 UT.
Observers:
Report from Håkon Dahle (Honolulu, HI, USA):
"I wish to report the following positive observation
of this
comet:
1998 May 11.23 UT, m1=0.5:, Dia.=1'.0 (H. Dahle, Kahe Pt.,
Oahu, HI, 20x80 binoculars)
The initial observation of this comet was made on May 11 05h28m UT by Nicolas Biver, using a 256-mm f/4.95 reflector, and the comet was subsequently sighted by the undersigned using binoculars. O. Guyon and the undersigned both confirmed Biver's sighting of the comet in the 256-mm reflector. The estimate given above was made on May 11 05h35m UT, when the altitude of the comet was 4° and the altitude of the sun was -7°. The magnitude estimate is based on a comparison with Alpha Tauri observed on a significantly darker sky background and is uncertain by about 0.5 magnitudes. The comet was clearly visible in the binoculars over a period of about 10 minutes, during which low clouds occasionally interfered with the observation. The coma was highly condensed (DC value 7-8), and no tail was visible in the binoculars."
Bjørn H. Granslo (Fjellhamar, Norway): "Attempts to observe the comet in twilight failed to show it on May 7.10 UT (using 20.3-cm SCT, 80x) and May 8.87 UT (observing from Tryvann, Oslo using a 70-mm refractor, 24x) to estimated limiting magnitudes of -0.5 and +0.5, respectively. The default coma diameter was 1'.0. I made further unsuccesful attemps to see the comet on May 11.87 and May 12.86 UT."
T = 1998 May 8.6168 TDT Peri = 110°.5438 )
(e = 1.0) Node = 351°.6634 ) 2000.0
q = 0.153217 AU Incl = 62°.9273 )
These elements are based on 122 accurate positions by
G. J. Garradd
(Loomberah, Australia) and other astrometrists during
1998 May 20-June 6 UT.
Ephemeris:
1998 RA (2000) Decl d r Elong. Ph. m1 SLim Opt NLim 00h TT h m o ' AU AU o o o o o Apr 1 0 01.91 -13 18.2 1.986 1.103 20.2 W 18.2 8.3 .. . 60 S .. . Apr 3 0 06.44 -12 07.1 1.940 1.059 20.1 W 19.0 8.1 .. . 58 S .. . Apr 5 0 11.14 -10 52.3 1.895 1.015 20.1 W 19.8 7.9 .. . 56 S .. . Apr 7 0 16.04 - 9 33.6 1.849 0.970 20.0 W 20.6 7.7 .. . 54 S .. . Apr 9 0 21.14 - 8 10.6 1.802 0.925 19.8 W 21.5 7.5 .. . 52 S .. . Apr 11 0 26.49 - 6 42.7 1.755 0.878 19.6 W 22.5 7.3 .. . 50 S .. . Apr 13 0 32.12 - 5 09.4 1.708 0.830 19.2 W 23.4 7.1 .. . 47 S .. . Apr 15 0 38.06 - 3 30.2 1.661 0.781 18.8 W 24.4 6.8 .. . 44 S .. . Apr 17 0 44.38 - 1 44.3 1.613 0.731 18.3 W 25.5 6.5 .. . 42 S .. . Apr 19 0 51.13 + 0 09.0 1.565 0.680 17.6 W 26.6 6.2 .. . 39 S .. . Apr 21 0 58.41 + 2 10.5 1.518 0.627 16.8 W 27.7 5.9 .. . 35 S .. . Apr 23 1 06.32 + 4 21.2 1.469 0.573 15.9 W 28.7 5.5 .. . 31 S .. . Apr 25 1 15.02 + 6 42.2 1.421 0.517 14.8 W 29.8 5.1 .. . .. . .. . Apr 27 1 24.71 + 9 14.6 1.373 0.460 13.6 W 30.9 4.7 .. . .. . .. . Apr 29 1 35.71 +11 59.3 1.324 0.400 12.1 W 31.9 4.1 .. . .. . .. . May 1 1 48.48 +14 56.6 1.275 0.339 10.6 W 33.0 3.5 .. . .. . .. . May 3 2 03.78 +18 05.3 1.223 0.277 9.0 W 34.7 2.8 .. . .. . .. . May 5 2 22.87 +21 17.3 1.168 0.217 7.8 W 39.0 1.9 .. . .. . .. . May 7 2 47.49 +24 03.5 1.104 0.169 7.6 W 52.0 0.9 .. . .. . .. . May 9 3 17.66 +25 07.9 1.030 0.154 8.6 E 77.9 0.5 .. . .. . .. . May 11 3 47.65 +23 32.2 0.961 0.184 10.4 E 100.2 0.9 .. . .. . .. . May 13 4 13.02 +20 10.6 0.909 0.239 13.0 E 108.4 1.6 .. . .. . .. . May 15 4 34.24 +16 05.4 0.875 0.300 16.4 E 108.5 2.3 .. . 11 S .. . May 17 4 52.56 +11 46.4 0.854 0.362 20.2 E 105.2 2.8 .. . 22 S .. . May 19 5 08.83 + 7 27.7 0.844 0.423 24.3 E 100.6 3.3 .. . 30 S .. . May 21 5 23.57 + 3 16.9 0.843 0.482 28.3 E 95.7 3.8 63 S 36 S 10 S May 23 5 37.14 - 0 41.8 0.849 0.539 32.1 E 90.9 4.1 78 S 41 S 4 S May 25 5 49.76 - 4 26.1 0.861 0.594 35.8 E 86.2 4.5 84 S 44 S 1 S May 27 6 01.59 - 7 54.9 0.877 0.647 39.2 E 81.8 4.8 88 S 47 S 1 N May 29 6 12.77 -11 08.1 0.897 0.700 42.4 E 77.7 5.1 90 S 49 S 2 N May 31 6 23.39 -14 06.1 0.921 0.750 45.3 E 73.9 5.4 90 S 52 S 2 N Jun 2 6 33.51 -16 49.8 0.947 0.800 48.0 E 70.4 5.7 90 S 53 S 3 N Jun 4 6 43.20 -19 20.1 0.975 0.848 50.4 E 67.2 5.9 90 S 55 S 3 N Jun 6 6 52.50 -21 38.1 1.005 0.895 52.6 E 64.2 6.2 90 S 56 S 3 N Jun 8 7 01.47 -23 44.9 1.037 0.942 54.6 E 61.5 6.4 90 S 57 S 2 N Jun 12 7 18.50 -27 29.3 1.104 1.032 58.1 E 56.7 6.8 90 S 60 S 2 N Jun 16 7 34.52 -30 40.8 1.174 1.119 60.9 E 52.5 7.2 90 S 60 S 1 N Jun 20 7 49.71 -33 25.9 1.247 1.203 63.2 E 49.0 7.6 90 S 63 S 1 N Jun 24 8 04.18 -35 50.0 1.323 1.285 65.1 E 45.9 7.9 90 S 65 S 0 S Jun 28 8 18.06 -37 57.1 1.399 1.364 66.6 E 43.2 8.2 90 S 66 S 1 S Jul 2 8 31.40 -39 50.4 1.477 1.442 67.8 E 40.8 8.5 90 S 68 S 2 S Jul 6 8 44.29 -41 32.6 1.555 1.517 68.7 E 38.6 8.8 90 S 69 S 3 S Jul 10 8 56.78 -43 05.5 1.634 1.591 69.3 E 36.7 9.1 90 S 70 S 4 S Jul 14 9 08.93 -44 30.9 1.714 1.664 69.8 E 35.0 9.3 90 S 72 S 4 S Jul 18 9 20.77 -45 49.9 1.794 1.735 70.1 E 33.4 9.6 90 S 73 S 5 S Jul 22 9 32.35 -47 03.7 1.874 1.805 70.2 E 32.0 9.8 90 S 74 S 6 S Jul 26 9 43.70 -48 13.1 1.955 1.874 70.2 E 30.7 10.0 90 S 75 S 7 S Jul 30 9 54.84 -49 18.9 2.036 1.941 70.1 E 29.5 10.2 90 S 77 S 8 S Aug 3 10 05.80 -50 21.5 2.116 2.008 69.9 E 28.3 10.4 90 S 78 S 9 S Aug 7 10 16.60 -51 21.4 2.197 2.073 69.6 E 27.3 10.6 90 S 79 S 9 S Aug 11 10 27.26 -52 19.0 2.278 2.138 69.1 E 26.3 10.8 90 S 80 S 10 S Aug 15 10 37.80 -53 14.5 2.358 2.202 68.6 E 25.4 10.9 89 S 81 S 11 S Aug 19 10 48.25 -54 08.3 2.438 2.264 68.1 E 24.5 11.1 88 S 81 S 12 S Aug 23 10 58.63 -55 00.6 2.518 2.327 67.5 E 23.7 11.3 87 S 82 S 12 S Aug 27 11 08.93 -55 51.6 2.598 2.388 66.8 E 22.9 11.4 85 S 82 S 13 STotal magnitude: m1 = 6.5 + 5 log d + 10 log r. This magnitude formula has been adjusted to better suit the available observations of this comet. Latitudes of visibility (Southern limit SLim, optimimum latitude Opt, and Northern limit NLim) refer to a dark sky and are based on a Solar depression angle of 15° (zenith distance 105 degrees) and an altitude of the comet of (at least) 10° for SLim and NLim.
The comet was very poorly placed before perihelion and this explains why it had not been detected earlier. It remained within 15° from the Sun until May 14.
More information, reports and images are available from