The Light Curve of Comet C/2002 C1 (Ikeya-Zhang) III Mark Kidger

Due to an error in the treatment of a few points in the Excel light curve file, Figure 2 in last month's TA gave a misleading result. When small effects are being discussed, just a few data points can make an important difference to the result obtained. On analysis of all data up to the end of April, the light curve pre- and post-perihelion is found to be almost identical. The main conclusions of that study though have been validated by the addition of an extra month of data and of the extensive TA data archive. Namely that:

1. There is only a very slight perihelion asymmetry amounting to no more than 0.2 magnitudes.

2. Peak brightness was attained approximately 2 days after perihelion.

3. The peak brightness of the innermost coma was reached approximately a week before perihelion.

The complete TA archive up to the end of March with the fit to the light curve derived from the sample of data used in the April TA is shown left. Although not perfect, the fit obviously gives a good approximation to the light curve.

The peak brightness of the comet was approximately magnitude 3.5 in the days just after perihelion. However, as the geocentric distance was decreasing at that time, the date of peak apparent brightness and that of the true peak brightness are not the same. To correct for this we subtract 5*log D from the apparent magnitudes to shift them to a standard geocentric distance of 1AU. This plot is shown to the right. The dispersion in the plot is around 0.8 magnitudes, but it does appear that the true peak brightness is shifted very slightly to the right of the axis (i.e. post-perihelion). The effect is 2±1days.

This shows that there was no large perihelion asymmetry in the light curve. It also implies that the simplest form of the non-gravitational terms, as expressed in Graeme's accompanying article, may be a good approximation to the true situation in C/2002 C1 (Ikeya-Zhang), although he notes that even a very small perihelion asymmetry can have important dynamical implications (an order of magnitude estimate is that a 1 day asymmetry in the non-gravitational term leads to a 3 month change in the perihelion date at the last return.

To compare the pre- and post-perihelion behaviour in more detail a longer time base is needed for the data. As the TA light curve data for April will not be available until after the magazine deadline, this must be done with the more limited data set previously used, from mainly Spanish visual observers (119 estimates, as against 325 in one month less in the TA archive). Interestingly, between April 24^th and May 5^th there appears to be no change in the comet's apparent bright-ness, despite the fact that it should be fading at an average of 0.09mags per day.

The plot (left) shows post-perihelion data as open rhombuses and pre-perihelion data as filled rhombuses. We see that the post-perihelion data appears to be signific-antly brighter. This effect though is misleading and depends entirely on the cluster of unexpectedly bright points after April 24^th. Until then the two fits were virtually identical. Interestingly, the TA data that was available at the time of writing (which ends at T+13 days) does appear to show the comet to be about 0.2 magnitudes brighter at the same heliocentric distance post-perihelion than pre. It will be interesting to see if this asymmetry remains visible in the extended data set.

Planetary Notes

Lunar occultation of Saturn on 2002 April 16th

Pierre Girard (PGirard@compuserve.com), Chicheley, near Newport Pagnell (N52.10, W0.68) writes: Following your request for observations of this infrequent event in E 1766, here are the timings I obtained of the various stages of the occultation. All timings are for disappearance at the dark lunar limb Instrument: 254mm f/4.3 Newtonian Reflector at x145

Ingress of Titan starts at 20 51 27.5 UT

Disappearance at 20 51 29 UT Note: gradual ingress lasts 1.5 second

Ingress of rings starts at 20 57 31 UT (1st contact)

Ingress of Saturn's Globe starts at 20 57 46 UT

Globe disappears at 20 48 57 UT

Rings disappear at 20 59 14 UT

Note: Total ingress time of Saturn and its rings is 103 seconds. The dark lunar limb is perceived as extremely sharp and smooth in silhouette against the image of Saturn. Photography (conventional) was carried out at the approach stage but , by a gardening accident of my own making some 23 years ago, was prevented by the branches of my Bramley appletree at the crucial moment. No photos available yet. The physical obstruction did not affect the timings. Re-appearance at the bright limb was observed through 12x65B but was not accurately timed.

Bill Worraker (Bill.Worraker@hyprotech.com) writes:

I watched the ingress stages of the occultation using a 0.35m reflector at x250, with the following timings: 1st contact: 2002 April 16, 20:59:59 (estimated accuracy +/-1 second) 2nd contact: 2002 April 16, 21:01:53 (estimated accuracy +/-1 second) I noted a lingering patch of light on the lunar limb for a few seconds prior to final disappearance. The whole phenomenon was quite striking, though the sky was rather hazy and the seeing rather poor. For the record, my observing coordinates are 01 deg 15' west, 51 deg 36' north.

David Moore (davidmoore0@yahoo.com) writes:

Don't know if this of sufficient quality as reports go, but I managed to glimpse the disappearance in 99.999% cloudy skies from the window of the Astronomy Shop in Dublin (51.3850N, 6.2137W, 30m)! Used a Celestron 8-inch SCT at 80x magnification. A cloud gap miraculously appeared over the Moon at the instant of disappearance! Had been cloudy all evening and had been at window watching constantly from 30 minutes before disappearance (which was predicted for 20:56:47 UT from Dublin) so could get scope tracking on Moon. By time I acquired Moon, centred Saturn, and focussed it was just about disappearing back into cloud. Time was 20:56UT but could see most, if not all of Saturn. Before I could check if 'chunk' missing from Saturn's rings, it clouded in! Very few clear patches while Saturn behind Moon. Close to reappearance (predicted for 21:22:48 UT) it looked very bleak. Total cloud cover and thick too. Then a tiny break in the clouds appeared and at 21:22:46 UT I could see Saturn's rings had just reappeared but not the globe, I don't think (a very thin sheet of cloud still over the Moon made observation less than perfect but very good view nonetheless). Less than 10 seconds later it was totally cloudy again and I saw nothing of the Moon for an hour after that despite a constant vigil (just in case I could get a picture of Saturn near the Moon). I can't believe how lucky I was with the weather. Had I checked the sky for 10 seconds every 5 minutes I would have written the event off. But watching constantly for an hour or so paid huge dividends! Pity next night-time Saturn occultation visible from these parts is not for 22 years, and next bright planet to be occulted at night is in 20 years time (Mars). I'd sort of got used to seeing them over the past few months! I know Colm J. Cannon, space correspondent for Astronomy & Space magazine, saw and photographed the reappearance from Dunboyne, Co. Meath (53.3970N, 6.4685W, 70m) 17.0km away in semi-clear skies but I do not think he timed the event.

Melvyn Taylor (melvyn.taylor@breathemail.net) writes:

As a minor contribution to TA's planetary pages the following describes a first for this observer. Though not thought of great use the details are:

ILOC station code SUIW5, Lat. N53 40 34.0 Long W 01 31 03.5 (Wakefield, West Yorkshire, England). Under a very transparent sky a unique first using 220mm f/7.32 dob refl x 107 as Titan is seen to take an estimated 1s to fade then disappear at the earthshine lunar edge the time 20h 46m 56.1s UT. Stopwatch chronometer in conjunction with MSF analogue clock.

With naked eye - a first time observation - Saturn was slowly fading over 20s to 25s and its last light timed at 20h 53m 46.6; with dob. last sight of ring disappearing at 20h 54m 09s. Reappearance also seen in dob., and with Saturn becoming obvious to naked eye at 21h 40m against brightness of the 4 day old Moon.