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}

=item B<apparent>

Return the apparent RA and Dec (as two C<Astro::Coords::Angle> objects) for the current
coordinates and time.

  ($ra,$dec) = $c->apparent();

Apparent RA/Dec is obtained by linear interpolation from the reference
positions. If the requested time lies outside the reference times
the position will be extrapolated.

=cut

sub apparent {
  my $self = shift;

  my $mjd1 = $self->mjd1;
  my $mjd2 = $self->mjd2;

  my ($ra_app, $dec_app);

  if ($mjd1 == $mjd2) {
    # special case when times are identical

    $ra_app = $self->{ra1};
    $dec_app = $self->{dec1};
  }
  else {
    # else linear interpolation

    my $mjd = $self->datetime->mjd;
    my $ra1  = $self->ra1->radians;
    my $ra2  = $self->ra2->radians;
    my $dec1 = $self->dec1->radians;
    my $dec2 = $self->dec2->radians;

    $ra_app = $ra1  + ( $ra2  - $ra1  ) * ( $mjd - $mjd1 ) / ( $mjd2 - $mjd1 );
    $dec_app = $dec1 + ( $dec2 - $dec1 ) * ( $mjd - $mjd1 ) / ( $mjd2 - $mjd1 );
  }

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

  $self->_cache_write( "RA_APP" => $ra_app, "DEC_APP" => $dec_app );

  return ($ra_app, $dec_app);
}

=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 interpolated position for a specific time.\n";
  return $self->SUPER::apply_offset(@_);
}

=back

=head1 NOTES

Usually called via C<Astro::Coords>. This is the coordinate style
used by SCUBA for non-sidereal sources instead of using orbital elements.

Apparent RA/Decs suitable for use in this class can be obtained
from http://ssd.jpl.nasa.gov/.

=head1 SEE ALSO

L<Astro::Coords::Elements>

=head1 REQUIREMENTS

Does not use any external PAL routines.

=head1 AUTHOR

Tim Jenness E<lt>tjenness@cpan.orgE<gt>

=head1 COPYRIGHT

Copyright (C) 2012 Science and Technology Facilities Council.
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;