; Any line starting with ; is a comment.  Any line starting with
; # is a directive to say "show the following explanatory text
; if and only if the quantity in question is turned on."  Except
; that #! means "show it only if the quantity is _off_."
; Keep explanations in order of ephemeris entries,  _not_ in
; order of the quantity numbers.
;
; Not all ephemeris quantities have comments.  Much work to do
; all of this...
;
# Type 0  (observables)

#! 21

   RA/decs are astrometric J2000 coordinates (corrected for light
time lag,  but in the inertial J2000 frame,  i.e.,  not rotated
for precession or nutation,  and differential light deflection is
not included.)   Display can be turned on/off in the 'advanced
options' for ephemerides.  When turning them on,  you can select
from various format choices (base-60,  decimal, different
precisions,  etc.)
;
; The following line means "show if either quantity 22 (distance
; to object) or 23 (distance to sun) is turned on."  Which,  for
; historical reasons,  corresponds to those bits _not_ being set.
#! 22,23  delta and (solar) r

   Distances are in kilometers for distances under one million km
(about 0.00668 AU),  and in AUs for larger distances.
# 14

   The "SM" (Sun-Moon) flags consist of one or two letters,  extended
from a scheme used on JPL's Horizons system.  Letters used are :

      * Daytime (sun above horizon)
      C Civil twilight (sun below horizon,  but by less than 6 degrees)
      N Nautical twilight (-12 < sun's altitude < -6 degrees)
      A Astronomical twilight (-18 < sun's altitude < -12 degrees)
      m Moon is in the sky,  less than 50% illuminated
      M Moon is in the sky,  more than 50% illuminated
      l Object transiting in front of moon
      L Object occulted by moon
      a Below altitude/horizon limits set for the observing site
      d Outside declination limits for your site/mount
      e Outside elongation limits for your site/mount
      h Outside hour angle limits for your site/mount
      g Some galactic confusion
      G Lots of galactic confusion
      B Object is below the horizon
# 29

   Sky brightness is in magnitudes per square arcsecond,  computed
using the Krisciunas & Schaefer model,

http://articles.adsabs.harvard.edu/full/1990PASP..102..212S
http://articles.adsabs.harvard.edu/full/1991PASP..103.1033K

   This provides a good approximation to background brightness due
to moonlight,  twilight,  and airglow.
# 11

   The phase angle bisector gives the ecliptic lat/lon of the point
midway between the vector from observer to target and the vector
from sun to target.  This can be useful in analyzing light curves.
# 34

   The 'galactic confusion' is computed from star counts per unit area
from Gaia-DR2.  Values range from zero (effectively no confusion) to
100 (densest parts of the Milky Way,  Magellanic Clouds,  or globular
clusters).  See https://projectpluto.com/gal_conf.htm for details.
# 30

   'PsAng' is applied mostly to comets : it is the position angle at which
an observer would see an ionized gas tail,  if that tail pointed exactly
away from the sun (i.e.,  if the gas was found exactly along a ray going
out the comet opposite the sun).  See the related 'PsAMV' value.
# 31

   'PsAMV' is applied mostly to comets : it is the position angle at which
an observer would see a dust tail,  if that tail pointed back along the
object's orbit.  Heavier dust particles will essentially do this;  they
will come off the comet and go into a slightly higher orbit,  and
gradually fall behind the comet.  Actual mid-size particles will be found
somewhere between this angle and the one specified by 'PsAng',  the
theoretical angle at which an ionized gas trail would be seen.
# 32

   'PlAng' is the angle between the observer and the target's heliocentric
orbital plane, as measured from the target.  If it is positive,  you are
looking at the object from above the ecliptic.  This quantity can be
useful with comets.

   JPL's Horizons service calculates this quantity using the barycentric
orbital plane.  This usually results in small differences,  but they can
be larger if the perihelion distance is low.
#  5

   Apparent motions are given in arcminutes/hour,  or (equivalently)
arcseconds/minute.
#  3

   Alt/az values are in degrees.  North=0, East=90.
#  4,19

   Radial and 'space' velocities are in km/s,  relative to the observer.
Negative radial velocities mean the object is approaching the observer.
# 16

   Uncertainties are one-sigma values shown in arcseconds (unless they
are very large, in which case they are shown in arcminutes or degrees).
In theory,  the object should be within this distance of the nominal
RA/dec 67% of the time,  within twice that distance 95% of the time,
and within three times that distance 99.7% of the time.  In practice,
astrometric errors are not actually Gaussian,  which means these
uncertainties are sometimes over- or underestimated.  So you may have
to look farther,  or not nearly as far,  as the uncertainties say.

   The object will probably be offset from nominal somewhere along the
indicated position angle,  especially if the orbit is well determined.
# 38

   Three-letter constellation abbreviations specify the constellation
containing that RA/dec,  as defined by the IAU's 1930 delineation.

# Type 1   (state vectors)
# Type 2   (positions)
# Type 3   (MPCORB output)
# Type 4   (eight-line orbital elements)
# Type 5   (close approach tables)
# Type 6   (fake astrometry)
# Type 7   (currently unused)
