Magnitude of Pluto

[salvomic]

hi,
I'm definitely wrong, but this code
print('Visual magnitude %.2f' % planetary_magnitude(apparent))
works here for the planets (Mercury-Neptune), but not with Pluto.
Can it be done? Where am I wrong?

Thank you!

[brandon-rhodes]

Correct — the planetary magnitude code is based on this paper:

https://arxiv.org/pdf/1808.01973

— which does not include a mathematical model for Pluto. If we ever run across a recent paper giving a magnitude model for Pluto, I would be happy to toss the formula into Skyfield, though!

[brandon-rhodes]

I checked an asteroid or two, and it turns out that the HORIZONS system prints out the exact magnitude formula that it uses for computing magnitude:

https://github.com/skyfielders/python-skyfield/blob/master/authorities/chiron-position#L89

So that should be added to Skyfield. But it prints a different message for Pluto:

 'APmag   S-brt' =
   Pluto's approximate apparent visual magnitude and surface brightness.
The computed value includes Charon. Values are available only for solar phase
angles in the range generally visible from Earth. If that occurs, "n.a." is
output. This is to avoid error due to extrapolation of the model beyond its
valid data-based limit.

So they are doing something fancier for Pluto. I wonder if HORIZONS has a bibliography that might mention where the formula comes from.