[Kstars-devel] Pluto

[Pablo de Vicente]

El Jueves, 9 de Junio de 2005 05:52, Jason Harris escribió:
> Hello,
>
> You may recall that our model of Pluto's orbit fails rather spectacularly
> if you wander more than a century or so from the present epoch.  One idea I
> have is to rewrite KSPluto as a derivative of KSAsteroid, and rely on the
> asteroid-like orbital elements of Pluto published here:
> http://ssd.jpl.nasa.gov/elem_planets.html
>
> After doing some transformations (Mean Longitude -> Mean Anomaly; Longitude
> of Perihelion -> Argument of Perihelion), I added this line to my
> asteroids.dat: 0 Pluto2            51545 39.48168677 0.24880766  17.14175
> 113.76329 110.30347  14.86205  1.00 0.15 JPL 0
>
> Pluto2 is always within 30 arcmin of Pluto, and if you add Trails to both
> and set a timestep of 1 week, you can see that they track each other very
> well in ecliptic longitude.  However, for some reason Pluto2's retrograde
> loops are significantly "flatter" than Pluto's.
>
> Now the question is, which one is correct?  For 8 June 2005, I am getting
> the following coords:
>
> Pluto:  ( 17h 32m 14s,  -14d 28' 46" )
> Pluto2: (  17h 32m 14s, -14d 58' 18" )
>
> I tried looking up Pluto's coordinates for today (suprisingly difficult to
> find!).  Here are some sites and their coords:
>
> http://www.fourmilab.ch/yoursky/ : ( 17h 32m 15s,  -14d 58' 36" )
> http://aa.usno.navy.mil/cgi-bin/aa_micaform2?calc=8&ZZZ=END :
> ( 17h 32m 42s ,  -14d 58' 52" )

Hi Jason,

I would use the coordinates provided by JPL in his program Horizons
http://ssd.jpl.nasa.gov/cgi-bin/eph. I would say it is the most authorative 
source.

2005-Jun-09 00:00 17 32 16.52 -14 58 30.7

But these coordinates may change up to 1 arcsec depending on the location on 
the Earth.

I have written a class which computes the planets coordinates from JPL data, 
but I think that it is too heavy for KStars. You need a 6 Mb file per 20 
years span. It relies on some GPL C code from ephemeris.com that I have 
slightly modified to make it work in C++. 

>
> The second one is the US Naval Observatory's web interface for the
> Astronomical Almanac, so it's pretty authoritative.  So, it looks like
> there may be something wrong with our current Pluto model, even for modern
> dates.
>
> I would like to go ahead with my plan to make KSPluto a derivative of
> KSAsteroid, and use the JPL orbital elements.  Note that the "Pluto2"
> coordinates reported above are not quite correct.  This may be due to the
> fact that I have not taken into account the fact that the elements change
> with time ("Pluto2" used the elements from J2000).  The JPL site presents
> rates of change for each value, and I will include these in the new KSPluto
> class.

Probably the last is true and the elements change with time and therefore it 
is the best option to include this variation.

Pablo.
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