From: wgw@vax.oxford.ac.uk
Newsgroups: sci.astro
Subject: P/Swift-Tuttle's orbit - II
Date: 17 Nov 92 14:29:53 GMT
Organization: Oxford University VAX 6620

P/Swift-Tuttle's orbit --- part II

  The previous integrations of P/Swift-Tuttle's orbit were based on the
osculating elements given on IAUC 5636 (dated October 15).  These elements
satisfied the existing 1992 observations and those of October 1862 very well.
Now, as noted by Brian Marsden (A.J. vol 78, 654 ; 1973),  a number of the 
October 1862 observations appear to contain large systematic declination 
residuals.  Integrating the orbit based on these elements resulted in a 
perihelion passage in 2126 of July 6.5 and not July 11 as given on IAUC 5636
(the July 6.5 date was independently confirmed by Nick James).  In order to 
fit the published perihelion times for 1737 and 1862, using the IAUC 5636 
elements, I found it neccessary to include a nongravitational term of 
A2 = -0.035.  Use of this term brought the perihelion time forward to 
2126 June 29.

  The orbit has now been re-solved by Don Yeomans.  His current solution for
the orbital elements (MPC 21081) uses 237 observations from 1737 to 1992 and
explicitly omits the October 1862 measurements.  Integrating this orbit 
forward results in a perihelion passage of 2126 July 12.22  --  on this
basis the closest that P/Swift-Tuttle will get to the Earth is 0.156 au  on
2126 August 5/6 when it will have an elongation from the sun of 95 degrees
and so should present a good observing prospect to anyone around in 2126!
(it will be around magnitude zero if it's light curve remains similar to 
that of the current apparition).

  From these new orbital elements I find the times of perihelion passage
during the telescopic era to be,
                                         1610 Feb.  6.31 
                                         1737 June 16.07 
                                         1862 Aug. 23.50
                                         1992 Dec. 12.33 
                                         2126 July 12.22

  For the (unobserved) 1610 apparition the comet would have been at its
brightest around the middle of Jan 1610  with a magnitude of around 5.3.
At this time it would have been only about 30 degrees from the sun in the
sky.

  Integrating the orbit backwards in time yields favourable naked-eye
apparitions in  August 195 BC,  August 188  and  August 442.  Hasegawa
(Vistas in Astronomy, vol 24, 59 ; 1980)  lists comets being seen in all
three years.  For the great winter comet of 442  enough information is
given for its orbit to have been determined  -  it's recorded motion
does not appear to be compatible with it being P/S-T, even when we take
into account the likely error in predicted perihelion passage.  Of the 
two comets recorded in 188 the second one was seen in Corona Borealis
on 28/29 July 188 and may be compatible with it being P/S-T - especially
if there has been an error of one lunar month in converting the date of
observation into the western calendar (in which case it would fit the
current gravitational integration almost exactly).


     Graeme Waddington                           wgw @ vax.ox.ac.uk
Newsgroups: sci.astro