Original SUNRISET.C code modified for use as an Arduino library.
Computes Sun rise/set times, start/end of twilight, and the length of the day at any date and latitude.
All credit for the code goes to Paul Schlyter.
Original code on the site: http://www.stjarnhimlen.se/comp/sunriset.c
Please check out Paul Schlyter's website: https://www.stjarnhimlen.se/english.php
How to compute rise/set times and altitude above horizon, by Paul Schlyter, Stockholm, Sweden: https://www.stjarnhimlen.se/comp/riset.html