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    my $long = (defined $tel ? $tel->long : 0.0 );
    my $lat = (defined $tel ? $tel->lat : 0.0 );

    ($ra_app, $dec_app, my $diam) = Astro::PAL::palRdplan($self->_mjd_tt, $PLANET{$self->planet},
                                                          $long, $lat );

    # Store the diameter
    $self->diam( $diam );

    # Convert to angle objects
    $ra_app = new Astro::Coords::Angle::Hour($ra_app, units => 'rad', range => '2PI');
    $dec_app = new Astro::Coords::Angle($dec_app, units => 'rad');

    # store in cache
    $self->_cache_write( "RA_APP" => $ra_app, "DEC_APP" => $dec_app );
  }

  return ($ra_app, $dec_app);
}

=item B<rv>

Radial velocity of the planet relative to the Earth geocentre.

=cut

sub rv {
  croak "Not yet implemented planetary radial velocities";
}

=item B<vdefn>

Velocity definition. Always 'RADIO'.

=cut

sub vdefn {
  return 'RADIO';
}

=item B<vframe>

Velocity reference frame. Always 'GEO'.

=cut

sub vframe {
  return 'GEO';
}

=item B<apply_offset>

Overrided method to warn if C<Astro::Coords::apply_offset> is
called on this subclass.

=cut

sub apply_offset {
  my $self = shift;
  warn "apply_offset: applying offset to planet position for a specific time.\n";
  return $self->SUPER::apply_offset(@_);
}

=back

=begin __PRIVATE_METHODS__

=over 4

=item B<_default_horizon>

Internal helper method for C<rise_time> and C<set_time>. Returns the
default horizon. For the sun returns Astro::Coords::SUN_RISE_SET.  For
the Moon returns:

  -(  0.5666 deg + moon radius + moon's horizontal parallax )

       34 arcmin    15-17 arcmin    55-61 arcmin           =  4 - 12 arcmin

[see the USNO pages at: http://aa.usno.navy.mil/faq/docs/RST_defs.html]

For all other planets returns 0.

Note that the moon calculation requires that the date stored in the object
is close to the date for which the rise/set time is required.

The USNO web page is quite confusing on the definition for the moon since
in one place it implies that the moonrise occurs when the centre of the moon
is above the horizon by 5-10 arcminutes (the above calculation) but on the
moon data page comparing moonrise with tables for a specific day indicates a
moonrise of -48 arcminutes.

=cut

sub _default_horizon {
  my $self = shift;
  my $name = lc($self->name);

  if ($name eq 'sun') {
    return &Astro::Coords::SUN_RISE_SET;
  } elsif ($name eq 'moon') {
    return (-0.8 * Astro::PAL::DD2R);
    # See http://aa.usno.navy.mil/faq/docs/RST_defs.html
    my $refterm = 0.5666 * Astro::PAL::DD2R; # atmospheric refraction

    # Get the moon radius
    $self->_apparent();
    my $radius = $self->diam() / 2;

    # parallax - assume 57 arcminutes for now
    my $parallax = (57 * 60) * Astro::PAL::DAS2R;

    print "Refraction: $refterm  Radius: $radius  Parallax: $parallax\n";

    return ( -1 * ( $refterm + $radius - $parallax ) );
  } else {
    return 0;
  }
}

=item B<_sidereal_period>

Returns the length of the source's "day" in seconds.

=cut

sub _sidereal_period {
  my $self = shift;
  my $name = lc($self->name);

  if ($name eq 'sun') {
    return 24 * 3600;
  } elsif ($name eq 'moon') {
    return 24 * 3600 * (1 + 1 / 29.53059);
  }
  else {
    $self->SUPER::_sidereal_period();
  }
}

=back

=end __PRIVATE_METHODS__

=head1 NOTES

Usually called via C<Astro::Coords>.

=head1 REQUIREMENTS

C<Astro::PAL> is used for all internal astrometric calculations.

=head1 AUTHOR

Tim Jenness E<lt>t.jenness@cpan.orgE<gt>

=head1 COPYRIGHT

Copyright (C) 2001-2005 Particle Physics and Astronomy Research Council.
All Rights Reserved.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; either version 3 of the License, or (at your option) any later
version.

This program is distributed in the hope that it will be useful,but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc., 59 Temple
Place,Suite 330, Boston, MA  02111-1307, USA

=cut

1;