From: vandeweg@cs.utwente.nl (Rob Vandeweg)
Subject: Development P/Swift-Tuttle
Keywords: Brightness, coma diameter, length tails
Reply-To: vandeweg@cs.utwente.nl
Organization: University of Twente, Dept. of Computer Science
Date: Fri, 16 Oct 1992 16:26:12 GMT


             PHYSICAL DEVELOPMENT 1992t P/Swift-Tuttle

Assuming that the physical behaviour of comet P/Swift-Tuttle in 1737, 
1862 and now in 1992 is usual for this comet, we have to expect a 
rather steep increase in brightness. This may be one of the reasons 
why this comet was missed on many occasions in the past. Based on the 
assumption of the steep increase in brightness I have computed the 
physical development of the comet to be expected:

yr   mn dy      RA-2000  decl-2000 mvis coma paI   lI   paII  lII
                h  m      o   '    magn  '   o     o    o     o

1992  9 27.00   11 49.18  59  3.5   9.0   7  346   0.4  343   0.2
1992 10  2.00   12 22.52  59 19.3   8.6   7  351   0.5  346   0.3
1992 10  7.00   13  0.68  59  8.2   8.1   8  356   0.8  351   0.4
1992 10 12.00   13 43.15  58 15.7   7.7   9    3   1.1  358   0.5
1992 10 17.00   14 28.38  56 25.5   7.3  10   10   1.5    4   0.6
1992 10 22.00   15 13.94  53 24.2   6.9  10   17   2.2   10   0.7
1992 10 27.00   15 57.25  49  5.6   6.6  11   23   2.9   15   0.8
1992 11  1.00   16 36.43  43 35.0   6.1  12   29   4.0   21   0.9
1992 11  6.00   17 10.69  37  8.4   5.8  12   34   5.1   24   1.0
1992 11 11.00   17 40.08  30  9.8   5.6  12   37   6.2   27   1.2
1992 11 16.00   18  5.15  23  4.6   5.4  12   40   7.6   29   1.4
1992 11 21.00   18 26.56  16 14.3   5.3  12   43   8.6   31   1.6
1992 11 26.00   18 44.98   9 52.7   5.1  11   46   9.8   33   1.9
1992 12  1.00   19  0.99   4  6.3   5.0  11   48  10.2   34   2.0
1992 12  6.00   19 15.06  -1  4.1   5.0  11   50  10.8   35   2.1
1992 12 11.00   19 27.59  -5 41.0   5.0  10   52  10.5   36   2.3
1992 12 16.00   19 38.91  -9 48.3   4.9  10   55  10.3   37   2.5
1992 12 21.00   19 49.32 -13 30.0   5.0   9   58   9.3   38   2.4
1992 12 26.00   19 59.04 -16 50.0   5.0   9   61   8.1   37   2.1
1992 12 31.00   20  8.26 -19 51.9   5.1   9   66   6.7   36   1.8

Explanation to this table:

RA   = Right Ascension.
decl = declination.
mvis = visual brightness in magnitudes.
coma = coma diameter in arcminutes.
paI  = position angle plasma tail in degrees.
       (0 = North, 90 = East, 180 = South, 270 = West)
lI   = length plasma tail in degrees. 
paII = position angle dust tail in degrees.
       (0 = North, 90 = East, 180 = South, 270 = West)
lII  = length dust tail in degrees.

Values for coma diameter and tail lengths are true for perfect 
observational circumstances (comet high in the sky, a very 
transparant sky, no moon, no twilight, etc.). Under moderate 
circumstances very probably less than the half of the given values 
are recorded. However remember that the behaviour of comets can be 
rather unpredictable.
The comet is rather diffuse at this moment, so brightness and coma 
diameter are easily underestimated. Use a magnification of ~two times 
the objective diameter in centimeters! 

May be the brightness for the last three dates is somewhat to 
optimistic. The reason is, that I assumed a perihelion asymmetry 
(seasonal effect) to explain observations in 1737.

Note that in 1862 variations in brightness of the central 
condensation and in position angles of jets near the nucleus were 
observed, having a periodicity of 65 hours.

Reports on brightness, coma diameter, and especially on the length of 
both tails ( and appearance of the dust tail) are very welcome to me, 
in order to be able to improve given expectations and to determine 
water and dust production rates for this comet. Also reports on 
variations of details are welcome.
                           Computations were made on October 2, 1992.

                                        Rob L.W. van de Weg
                                        email: vandeweg@cs.utwente.nl