sunrise

  ($sunrise, $sunset) = sunrise( { year    => $year,      month      => $month,
                                   day     => $day,
                                   lon     => $longitude, lat        => $latitude,
                                   tz      => $tz_offset, isdst      => $is_dst,
                                   alt     => $altitude,  upper_limb => $upper_limb,
                                   precise => $precise,   polar      => $action,
                                   trace   => $fh } );
  ($sunrise, $sunset) = sunrise(YYYY,MM,DD,longitude,latitude,Time Zone,DST);
  ($sunrise, $sunset) = sunrise(YYYY,MM,DD,longitude,latitude,Time Zone,DST,ALT);
  ($sunrise, $sunset) = sunrise(YYYY,MM,DD,longitude,latitude,Time Zone,DST,ALT,precise);

Returns the sunrise and sunset times, in HH:MM format.

The first form uses a hash reference to pass arguments by name. The other forms are kept for backward compatibility. The arguments are:

year, month, day

The three elements of the date for which you want to compute the sunrise and sunset. Months are numbered 1 to 12, in the usual way, not 0 to 11 as in C and in Perl's localtime.

Mandatory, can be positional (#1, #2 and #3).

lon, lat

The longitude and latitude of the place for which you want to compute the sunrise and sunset. They are given in decimal degrees. For example:

    lon => -3.6,  #  3° 36' W
    lat => 37.17, # 37° 10' N
 Eastern longitude is entered as a positive number
 Western longitude is entered as a negative number
 Northern latitude is entered as a positive number
 Southern latitude is entered as a negative number
    

Mandatory, can be positional (#4 and #5).

tz

Time Zone is the offset from GMT

Mandatory, can be positional (#6).

isdst

1 if daylight saving time is in effect, 0 if not.

Mandatory, can be positional (#7).

alt

Altitude of the sun, in decimal degrees. Usually a negative number, because the sun should be under the mathematical horizon. But if there is a high mountain range sunward (that is, southward if you live in the Northern hemisphere), you may need to enter a positive altitude.

This parameter is optional. Its default value is -0.833. It can be positional (#8).

upper_limb

If this parameter set to a true value (usually 1), the algorithm computes the sun apparent radius and takes it into account when computing the sun altitude. This parameter is useful only when the "alt" parameter is set to 0 or -0.583 degrees. When using -0.25 or -0.833 degrees, the sun radius is already taken into account. When computing twilights (-6 to -18), the sun radius is irrelevant.

Since the default value for the "alt" parameter is -0.833, the default value for "upper_limb" is 0.

This parameter is optional and it can be specified only by keyword.

polar

When dealing with a polar location, there may be dates where there is a polar night (sun never rises) or a polar day. The default behaviour of the module is to emit a warning in these cases ("Sun never rises!!" or "Sun never sets!!"). But some programmers may find this inconvenient. An alternate behaviour is to return special values reflecting the situation.

So, if the "polar" parameter is set to 'warn', the module emits a warning. If the "polar" parameter is set to 'retval', the module emits no warning, but it returns either 'day' or 'night'.

Example:

  # Loosely based on Alex Gough's activities: scientist and Perl programmer, who spent a year
  # in Halley Base in 2006. Let us suppose he arrived there on 15th January 2006.
  my ($sunrise, $sunset) = sunrise( { year => 2006, month => 1, day => 15,
                                      lon => -26.65, lat => -75.58, # Halley Base: 75°35'S 26°39'W
                                      tz  => 3, polar => 'retval' } );
  if ($sunrise eq 'day') {
    say "Alex Gough saw the midnight sun the first day he arrived at Halley Base";
  }
  elsif ($sunrise eq 'night') {
    say "It would be days, maybe weeks, before the sun would rise.";
  }
  else {
    say "Alex saw his first antarctic sunset at $sunset";
  }
    

This parameter is optional and it can be specified only by keyword.

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 to understand what the traced values represent.

This parameter is optional and it can be specified only by keyword.

precise

Choice between a precise algorithm and a simpler algorithm. The default value is 0, that is, the simpler algorithm. Any true value switches to 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 use the precise algorithm. This feature is new as of version 0.7. Here is what I 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.

This parameter is optional. It can be positional (#9).

For Example

 ($sunrise, $sunset) = sunrise( 2001, 3, 10, 17.384, 98.625, -5, 0 );
 ($sunrise, $sunset) = sunrise( 2002, 10, 14, -105.181, 41.324, -7, 1, -18);
 ($sunrise, $sunset) = sunrise( 2002, 10, 14, -105.181, 41.324, -7, 1, -18, 1);

sun_rise, sun_set

  $sun_rise = sun_rise( { lon => $longitude, lat => $latitude,
                          alt => $altitude, upper_limb => $bool,
                          offset  => $day_offset,
                          precise => $bool_precise, polar => $action } );
  $sun_set  = sun_set ( { lon => $longitude, lat => $latitude,
                          alt => $altitude, upper_limb => $bool,
                          offset  => $day_offset,
                          precise => $bool_precise, polar => $action } );
  $sun_rise = sun_rise( $longitude, $latitude );
  $sun_rise = sun_rise( $longitude, $latitude, $alt );
  $sun_rise = sun_rise( $longitude, $latitude, $alt, $day_offset );

Returns the sun rise time (resp. the sun set time) for the given location and for today's date (as given by DateTime), plus or minus some offset in days. The first form use all parameters and transmit them by name. The second form uses today's date (from DateTime) and the default altitude. The third form adds specifying a custom altitude. The fourth form allows for specifying an integer day offset from today, either positive or negative.

The parameters are the same as the parameters for "sunrise". There is an additional parameter, "offset", which allows using a date other than today: +1 for to-morrow, -7 for one week ago, etc.

The arguments are:

lon, lat

The longitude and latitude of the place for which you want to compute the sunrise and sunset. They are given in decimal degrees. For example:

    lon => -3.6,  #  3° 36' W
    lat => 37.17, # 37° 10' N
 Eastern longitude is entered as a positive number
 Western longitude is entered as a negative number
 Northern latitude is entered as a positive number
 Southern latitude is entered as a negative number
    

Mandatory, can be positional (#1 and #2).

alt

Altitude of the sun, in decimal degrees. Usually a negative number, because the sun should be under the mathematical horizon. But if there is a high mountain range sunward (that is, southward if you live in the Northern hemisphere), you may need to enter a positive altitude.

This parameter is optional. Its default value is -0.833. It can be positional (#3).

offset

By default, "sun_rise" and "sun_set" use the current day. If you need another day, you give an offset relative to the current day. For example, +7 means next week, while -365 means last year.

This parameter has nothing to do with timezones.

Optional, 0 by default, can be positional (#4).

time_zone

Time Zone is the Olson name for a timezone. By default, the functions "sun_rise" and "sun_set" will try to use the "local" timezone.

This parameter is optional and it can be specified only by keyword.

upper_limb

If this parameter set to a true value (usually 1), the algorithm computes the sun apparent radius and takes it into account when computing the sun altitude. This parameter is useful only when the "alt" parameter is set to 0 or -0.583 degrees. When using -0.25 or -0.833 degrees, the sun radius is already taken into account. When computing twilights (-6 to -18), the sun radius is irrelevant.

Since the default value for the "alt" parameter is -0.833, the default value for "upper_limb" is 0.

This parameter is optional and it can be specified only by keyword.

polar

When dealing with a polar location, there may be dates where there is a polar night (sun never rises) or a polar day. The default behaviour of the module is to emit a warning in these cases ("Sun never rises!!" or "Sun never sets!!"). But some programmers may find this inconvenient. An alternate behaviour is to return special values reflecting the situation.

So, if the "polar" parameter is set to 'warn', the module emits a warning. If the "polar" parameter is set to 'retval', the module emits no warning, but it returns either 'day' or 'night'.

This parameter is optional and it can be specified only by keyword.

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.

This parameter is optional and it can be specified only by keyword.

precise

Choice between a precise algorithm and a simpler algorithm. The default value is 0, that is, the simpler algorithm. Any true value switches to the precise algorithm.

For more documentation, see the corresponding parameter for the "sunrise" function.

This parameter is optional and it can be specified only by keyword.

For Example

 $sunrise = sun_rise( -105.181, 41.324 );
 $sunrise = sun_rise( -105.181, 41.324, -15 );
 $sunrise = sun_rise( -105.181, 41.324, -12, +3 );
 $sunrise = sun_rise( -105.181, 41.324, undef, -12);

Trigonometric functions using degrees

Since the module use trigonometry with degrees, the corresponding functions are available to the module user, free of charge. Just mention the tag ":trig" in the "use" statement. These functions are:

sind, cosd, tand

The direct functions, that is, sine, cosine and tangent functions, respectively. Each one receives one parameter, in degrees, and returns the trigonometric value.

asind, acosd, atand

The reverse functions, that is, arc-sine, arc-cosine, and arc-tangent. Each one receives one parameter, the trigonometric value, and returns the corresponding angle in degrees.

atan2d

Arc-tangent. This function receives two parameters: the numerator and the denominator of a fraction equal to the tangent. Use this function instead of "atand" when you are not sure the denominator is not zero. E.g.:

  use Astro::Sunrise qw(:trig);
  say atan2d(1, 2) # prints 26,5
  say atan2d(1, 0) # prints 90, without triggering a "division by zero" error
    

equal

Not really a trigonometrical function, but still useful at some times. This function receives two floating values and an integer value. It compares the floating numbers, and returns "1" if their most significant digits are equal. The integer value specifies how many digits are kept. E.g.

  say equal(22/7, 355/113, 3) # prints 1, because :  22/7   = 3.14285715286 rounded to 3.14
                              #                     355/113 = 3.14159292035 rounded to 3.14
  say equal(22/7, 355/113, 4) # prints 0, because :  22/7   = 3.14285715286 rounded to 3.143
                              #                     355/113 = 3.14159292035 rounded to 3.142
    

Precise Algorithm

The explanations for the precise algorithm give a pretty good reason for using an angular speed of 15.04107 instead of 15. Yet, computations with 15.04107 do not give results conforming to what the NOAA website and Stellarium give, while computations with 15 give conforming results. The implementation of the precise algorithm should be analysed and checked to find the reason why 15.04107 does not give the proper results.

Kwalitee

The CPANTS tools do not recognize the LICENSE POD paragraph. But any human reader will admit that this LICENSE paragraph exists and is valid.

Haiku-OS CPAN Tester

The built-in test t/06datetime.t fails on Haiku-OS because there is no way to extract the timezone name from the system parameters. This failure does not affect the core functions of Astro::Sunrise.

Also reported from a user working on a partially configured FreeBSD machine, see <https://github.com/jforget/Astro-Sunrise/issues/16>.

Hopefully, this will be fixed in the current version.

Perl Module

Copyright (C) 1999-2003, 2013, 2015, 2017, 2019, 2021 Ron Hill and Jean Forget, all rights reserved. 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:

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.