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

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.