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THE ASTRONOMER Electronic Circular No 1746      2002 Feb 25 16.31UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK             GMH at AST.STAR.RL.AC.UK
WORLD WIDE WEB                         http://www.theastronomer.org
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COMET C/2002 C1 (IKEYA-ZHANG)
Mark Kidger comments on the possibility link with earlier comets:
The following comments of mine on C/2002 C1 (Ikeya-Zhang) appeared
on Benny Peizer's "Peizergram" on February 22. Since sending it, I
have received a note from Federico Fernandez-Pardavila pointing
out that Nakano has suggested an alternative identification with
C/1661 C1. I have thus taken the liberty of revising the text.
There is considerable interest in Comet C/2002 C1 (Ikeya-Zhang)
which could potentially turn out to be the first confirmed return
of a comet with a period superior to 155 years (the current record
holder is Comet Herschel-Rigollet, last seen in 1939). Nakano has
suggested that it may be identical to C/1532 R1 but, more recently,
has suggested that C/1661 C1 offers an even better solution with
linkage possible giving residuals of just one arcsecond.
Although C/1532 R1 was observed from September 2 to December 30,
its orbit is rather uncertain. The orbit used in the IAU/CBAT/MPC
"Catalogue of Cometary Orbits" is that of Olbers, calculated in
1787. For C/1661 C1 the orbit used is the one calculated in 1785 by
Pierre Mechain. These orbits are compared below to that of Comet
Ikeya-Zhang.
As we can see, the similarity is quite impressive. Brian Marsden
estimates an orbital period of 400-500 years for Ikeya-Zhang based
on an arc of 11 days, which would be consistent with a previous
apparitio in 1532. C/1661 C1 was less well observed. It was
discovered on 1661 Feb. 3 in the dawn sky, just after passing
perihelion, with a tail already 6deg long. The comet faded rapidly
and was last seen on March 28th. Its longitude of perihelion is
very close to the corresponding value for Ikeya-Zhang, although
other parameters are not quite so close.
        C/2002 C1         C/1532 R1           C/1661 C1
T     2002 Mar. 18.9388  1532 Oct. 18.832  1661 Jan. 27.381
q     0.507200           0.51922           0.442722
Peri  34.5777            24.53             33.45
Node  93.4156            93.81             86.562
i     28.1110            32.59             33.015
e     0.991207            1.0               1.0 
The orbit of C/1532 R1 however is not completely determined,
despite the long visibility of the comet and a 1785 solution by
Mechain gave a rather different solution, with an inclination of 42
degrees, and an Ascending Node of 126 degrees, much less similar to
the orbit of Ikeya-Zhang.
C/1532 R1 was evidently an exceptional object. David Hughes's 1987
catalogue of cometary absolute magnitudes from 568 - 1978 assigns
it an absolute magnitude of +1.8, one of just 12 comets that has an
absolute magnitude of +2 or brighter, putting it into the "giant
comet" class, almost 100 times intrinsically brighter than the
average long-period comet. Current light curve fits to C/2002 C1
(Ikeya-Zhang) suggest an absolute magnitude from +6.5-7.5, average,
or slightly fainter than average. This is an evident inconsistency.
While the date culled from pre-telescopic light curves is very
uncertain, 5 magnitudes is a big difference.
In contrast, C/1661 C1 is a more normal object. David Hughes lists
its absolute magnitude as +4.6, much closer to that of Ikeya-Zhang,
particularly as the observations suggest that the comet became
diffuse and faded out rapidly.
Guy M Hurst