Provided by: libdatetime-event-sunrise-perl_0.0506-2_all bug

NAME

       DateTime::Event::Sunrise - Perl DateTime extension for computing the sunrise/sunset on a
       given day

SYNOPSIS

         use DateTime;
         use DateTime::Event::Sunrise;

         # generating DateTime objects from a DateTime::Event::Sunrise object
         my $sun_Kyiv = DateTime::Event::Sunrise->new(longitude => +30.85,  # 30°51'E
                                                      latitude  => +50.45); # 50°27'N
         for (12, 13, 14) {
           my $dt_yapc_eu = DateTime->new(year      => 2013,
                                          month     =>    8,
                                          day       =>   $_,
                                          time_zone => 'Europe/Kiev');
           say "In Kyiv (50°27'N, 30°51'E) on ", $dt_yapc_eu->ymd, " sunrise occurs at ", $sun_Kyiv->sunrise_datetime($dt_yapc_eu)->hms,
                                                                " and sunset occurs at ", $sun_Kyiv->sunset_datetime ($dt_yapc_eu)->hms;
         }

         # generating DateTime objects from DateTime::Set objects
         my $sunrise_Austin = DateTime::Event::Sunrise->sunrise(longitude => -94.73,  # 97°44'W
                                                                latitude  => +30.3);  # 30°18'N
         my $sunset_Austin  = DateTime::Event::Sunrise->sunset (longitude => -94.73,
                                                                latitude  => +30.3);
         my $dt_yapc_na_rise = DateTime->new(year      => 2013,
                                             month     =>    6,
                                             day       =>    3,
                                             time_zone => 'America/Chicago');
         my $dt_yapc_na_set = $dt_yapc_na_rise->clone;
         say "In Austin (30°18'N, 97°44'W), sunrises and sunsets are";
         for (1..3) {
           $dt_yapc_na_rise = $sunrise_Austin->next($dt_yapc_na_rise);
           $dt_yapc_na_set  = $sunset_Austin ->next($dt_yapc_na_set);
           say $dt_yapc_na_rise, ' ', $dt_yapc_na_set;
         }

         # If you deal with a polar location
         my $sun_in_Halley = DateTime::Event::Sunrise->new(
                                        longitude => -26.65, # 26°39'W
                                        latitude  => -75.58, # 75°35'S
                                        precise   => 1,
                                        );
         my $Alex_arrival = DateTime->new(year  => 2006, # approximate date, not necessarily the exact one
                                          month =>    1,
                                          day   =>   15,
                                          time_zone => 'Antarctica/Rothera');
         say $Alex_arrival->strftime("Alex Gough (a Perl programmer) arrived at Halley Base on %Y-%m-%d.");
         if ($sun_in_Halley->is_polar_day($Alex_arrival)) {
           say "It would be days, maybe weeks, before the sun would set.";
         }
         elsif ($sun_in_Halley->is_polar_night($Alex_arrival)) {
           say "It would be days, maybe weeks, before the sun would rise.";
         }
         else {
           my $sunset = $sun_in_Halley->sunset_datetime($Alex_arrival);
           say $sunset->strftime("And he saw his first antarctic sunset at %H:%M:%S.");
         }

DESCRIPTION

       This module will computes the time of sunrise and sunset for a given date and a given
       location. The computation uses Paul Schlyter's algorithm.

       Actually, the module creates a DateTime::Event::Sunrise object or a DateTime::Set object,
       which are used to generate the sunrise or the sunset times for a given location and for
       any date.

METHODS

   new
       This is the DateTime::Event::Sunrise constructor. It takes keyword parameters, which are:

       longitude
           This is the longitude of the location where the sunrises and sunsets are observed.  It
           is given as decimal degrees: no minutes, no seconds, but tenths and hundredths of
           degrees.  Another break with the normal usage is that Eastern longitude are positive,
           Western longitudes are negative.

           Default value is 0, that is Greenwich or any location on the eponymous meridian.

       latitude
           This is the latitude of the location where the sunrises and sunsets are observed.  As
           for the longitude, it is given as decimal degrees. Northern latitudes are positive
           numbers, Southern latitudes are negative numbers.

           Default value is 0, that is any location on the equator.

       altitude
           This is the height of the Sun at sunrise or sunset. In astronomical context, the
           altitude or height is the angle between the Sun and the local horizon. It is expressed
           as degrees, usually with a negative number, since the Sun is below the horizon.

           Default value is -0.833, that is when the sun's upper limb touches the horizon, while
           taking in account the light refraction.

           Positive altitude are allowed, in case the location is near a mountain range behind
           which the sun rises or sets.

       precise
           Boolean to control which algorithm is used. A false value gives a simple algorithm,
           but which can lead to inaccurate sunrise times and sunset times. A true value gives a
           more elaborate algorithm, with a loop to refine the sunrise and sunset times and
           obtain a better precision.

           Default value is 0, to choose the simple algorithm.

           This parameter replaces the "iteration" deprecated parameter.

       upper_limb
           Boolean to choose between checking the Sun's upper limb or its center.  A true value
           selects the upper limb, a false value selects the center.

           This parameter is significant only when the altitude does not already deal with the
           sun radius.  When the altitude takes into account the sun radius, this parameter
           should be false.

           Default value is 0, since the upper limb correction is already taken in account with
           the default -0.833 altitude.

       silent
           Boolean to control the output of some warning messages.  With polar locations and
           dates near the winter solstice or the summer solstice, it may happen that the sun
           never rises above the horizon or never sets below.  If this parameter is set to false,
           the module will send warnings for these conditions. If this parameter is set to true,
           the module will not pollute your STDERR stream.

           Default value is 0, for backward compatibility.

       trace
           This parameter should  either be a false value or  a filehandle opened for output.  In
           the  latter case,  a few messages  are printed  to the filehandle, which  allows the
           programmer to  see step by step  how the sunrise and the sunset are computed.

           Used for  analysis and debugging purposes.  You need to read  the text
           doc/astronomical-notes.pod in  the sister  module Astro::Sunrise to understand what
           the traced values represent.

           Default value is 0, which does not produce trace messages.

   sunrise, sunset
       Although they come from the DateTime::Event::Sunrise module, these methods are
       "DateTime::Set" constructors. They use the same parameters as the "new" constructor, but
       they give objects from a different class.

   sunrise_datetime, sunset_datetime
       These two methods apply to "DateTime::Event::Sunrise" objects (that is, created with
       "new", not "sunrise" or "sunset"). They receive one parameter in addition to $self, a
       "DateTime" object. They return another "DateTime" object, for the same day, but with the
       time of the sunrise or sunset, respectively.

   sunrise_sunset_span
       This method applies to "DateTime::Event::Sunrise" objects. It accepts a "DateTime" object
       as the second parameter. It returns a "DateTime::Span" object, beginning at sunrise and
       ending at sunset.

   is_polar_night, is_polar_day, is_day_and_night
       These methods apply to "DateTime::Event::Sunrise" objects. They accept a "DateTime" object
       as the second parameter. They return a boolean indicating the following condutions:

       •   is_polar_night is true when the sun stays under the horizon. Or rather under the
           altitude parameter used when the "DateTime::Event::Sunrise" object was created.

       •   is_polar_day is true when the sun stays above the horizon, resulting in a "Midnight
           sun". Or rather when it stays above the altitude parameter used when the
           "DateTime::Event::Sunrise" object was created.

       •   is_day_and_night is true when neither is_polar_day, nor is_polar_night are true.

   next current previous contains as_list iterator
       See DateTime::Set.

EXTENDED EXAMPLES

         my $dt = DateTime->new( year   => 2000,
                                 month  => 6,
                                 day    => 20,
                         );

         my $sunrise = DateTime::Event::Sunrise ->sunrise (
                               longitude =>'-118',
                               latitude  =>'33',
                               altitude  => '-0.833',
                               precise   => '1'
                         );

         my $sunset = DateTime::Event::Sunrise ->sunset (
                               longitude =>'-118',
                               latitude  =>'33',
                               altitude  => '-0.833',
                               precise   => '1'
                         );

         my $tmp_rise = $sunrise->next( $dt );

         my $dt2 = DateTime->new( year   => 2000,
                                  month  => 12,
                                  day    => 31,
                          );

         # iterator
         my $dt_span = DateTime::Span->new( start =>$dt, end=>$dt2 );
         my $set = $sunrise->intersection($dt_span);
         my $iter = $set->iterator;
         while ( my $dt = $iter->next ) {
           print ' ',$dt->datetime;
         }

         # is it day or night?
         my $day_set = DateTime::SpanSet->from_sets(
           start_set => $sunrise, end_set => $sunset );
         print $day_set->contains( $dt ) ? 'day' : 'night';

         my $dt = DateTime->new( year   => 2000,
                          month  => 6,
                          day    => 20,
                          time_zone => 'America/Los_Angeles',
                           );

         my $sunrise = DateTime::Event::Sunrise ->new(
                              longitude =>'-118' ,
                              latitude  => '33',
                              altitude  => '-0.833',
                              precise   => '1'

         );

         my $tmp = $sunrise->sunrise_sunset_span($dt);
         print "Sunrise is:" , $tmp->start->datetime , "\n";
         print "Sunset is:" , $tmp->end->datetime;

NOTES

   Longitude Signs
       Remember, contrary to the usual convention,

       EASTERN longitudes are POSITIVE,

       WESTERN longitudes are NEGATIVE.

       On the other hand, the latitude signs follow the usual convention:

       Northen latitudes are positive,

       Southern latitudes are negative.

   Sun Height
       There are a number of sun heights to choose from. The default is -0.833 because this is
       what most countries use. Feel free to specify it if you need to. Here is the list of
       values to specify the sun height with:

       •   0 degrees

           Center of Sun's disk touches a mathematical horizon

       •   -0.25 degrees

           Sun's upper limb touches a mathematical horizon

       •   -0.583 degrees

           Center of Sun's disk touches the horizon; atmospheric refraction accounted for

       •   -0.833 degrees

           Sun's supper limb touches the horizon; atmospheric refraction accounted for

       •   -6 degrees

           Civil twilight (one can no longer read outside without artificial illumination)

       •   -12 degrees

           Nautical twilight (navigation using a sea horizon no longer possible)

       •   -15 degrees

           Amateur astronomical twilight (the sky is dark enough for most astronomical
           observations)

       •   -18 degrees

           Astronomical twilight (the sky is completely dark)

   Notes on the Precise Algorithm
       The original method only gives an approximate value of the Sun's rise/set times.  The
       error rarely exceeds one or two minutes, but at high latitudes, when the Midnight Sun soon
       will start or just has ended, the errors may be much larger. If you want higher accuracy,
       you must then select the precise variant of the algorithm. This feature is new as of
       version 0.7. Here is what I (module creator) have tried to accomplish with this.

       a)  Compute sunrise or sunset as always, with one exception: to convert LHA from degrees
           to hours, divide by 15.04107 instead of 15.0 (this accounts for the difference between
           the solar day and the sidereal day.

       b)  Re-do the computation but compute the Sun's RA and Decl, and also GMST0, for the
           moment of sunrise or sunset last computed.

       c)  Iterate b) until the computed sunrise or sunset no longer changes significantly.
           Usually 2 iterations are enough, in rare cases 3 or 4 iterations may be needed.

       However, I (second module maintainer) have checked with a few external sources, to obtain
       test data. And actually, using the value 15.0 gives results closer to what Stellarium  and
       the NOAA solar calculator give.  So I will use value 15.0, unless I find a bug in the
       precise algorithm as presently implemented.

   Notes on polar locations
       If the location is beyond either polar circle, and if the date is near either solstice,
       there can be  midnight sun  or polar night.  In this case, there  is neither  sunrise nor
       sunset,  and the  module "carp"s that the sun never  rises or never sets. Then, it
       returns the time at which the sun is at its highest or lowest point.

       When computing twilights instead of  sunrises / sunsets, the limit for polar locations
       extends a little beyond the polar circle. For example, for  nautical twilights  (12
       degrees  below the  horizon), the  limits where midnight sun might happen is  12 degrees
       southward of the Arctic Circle  and 12  degrees northward  of the  Antarctic Circle,  that
       is, about 54° latitude instead of 66°33′.

DEPENDENCIES

       This module requires:

       •   DateTime

       •   DateTime::Set

       •   DateTime::Span

       •   Params::Validate

       •   Set::Infinite

       •   POSIX

       •   Math::Trig

BUGS AND CAVEATS

       Using a latitude of 90 degrees (North Pole or South Pole) gives curious results.  I guess
       that it is linked with a ambiguous value resulting from a 0/0 computation.

       Using a longitude of 177 degrees, or any longitude near the 180 meridian, may also give
       curious results, especially with the precise algorithm.

       The precise algorithm should be thoroughly analysed, to understand why the value 15.04107
       advised by Paul Schlyter does not give the expected results.

       The precise algorithm is not tested with polar locations. At least, it is tested with a
       near-polar location,  Fairbanks, at the time when the night is at its shortest, that is,
       in June.

AUTHORS

       Original author: Ron Hill <rkhill@firstlight.net>

       Co-maintainer: Jean Forget <JFORGET@cpan.org>

SPECIAL THANKS

       Robert Creager [Astro-Sunrise@LogicalChaos.org]
           for providing help with converting Paul's C code to perl.

       Flávio S. Glock [fglock@pucrs.br]
           for providing the the interface to the DateTime::Set module.

       Eric Jensen
           for  positive and  interesting advices  about the  new version  of the module

CREDITS

       Paul Schlyter, Stockholm, Sweden
           for his excellent web page on the subject.

       Rich Bowen (rbowen@rbowen.com)
           for suggestions.

       People at <https://geocoder.opencagedata.com/>
           for noticing an endless loop condition in Astro::Sunrise and for fixing it.

COPYRIGHT and LICENSE

   Perl Module
       This program is distributed under the same terms as Perl 5.16.3: GNU Public License
       version 1 or later and Perl Artistic License

       You can find the text of the licenses in the LICENSE file or at
       <https://dev.perl.org/licenses/artistic.html> and
       <https://www.gnu.org/licenses/gpl-1.0.html>.

       Here is the summary of GPL:

       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 1, 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., <https://www.fsf.org/>.

   Original C program
       Here is the copyright information provided by Paul Schlyter for the original C program:

       Written as DAYLEN.C, 1989-08-16

       Modified to SUNRISET.C, 1992-12-01

       (c) Paul Schlyter, 1989, 1992

       Released to the public domain by Paul Schlyter, December 1992

       Permission is hereby granted, free of charge, to any person obtaining a copy of this
       software and associated documentation files (the "Software"), to deal in the Software
       without restriction, including without limitation the rights to use, copy, modify, merge,
       publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
       to whom the Software is furnished to do so, subject to the following conditions:

       The above copyright notice and this permission notice shall be included in all copies or
       substantial portions of the Software.

       THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
       INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
       PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY CLAIM, DAMAGES
       OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT
       OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

SEE ALSO

       perl(1).

       DateTime Web page at <http://datetime.perl.org/>

       DateTime::Set

       DateTime::SpanSet

       Astro::Sunrise

       DateTime::Event::Jewish::Sunrise

       Astro::Coords

       Astro::PAL

       Paul Schlyter's homepage at <https://stjarnhimlen.se/english.html>

       The NOAA solar calculator at <https://www.esrl.noaa.gov/gmd/grad/solcalc/>