When does the fasting day become too long?

When does the fasting day become too long?

Detailed Qur'an

This is a challenging topic for many Muslims; however there may be a way to calculate this directly from the Qur’an.

وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّوا الصِّيَامَ إِلَى اللَّيْلِ

                    

… and eat and drink until the white thread (of light) becomes distinct to you from the black thread (night/darkness) of the dawn. Then strictly observe the fast until/to the night ...

Qur'an (2:187)

The ability to see the white thread against a dark night sky is likely limited by the length of the day, in very long days, the night sky may not actually become dark at any point.  For example, when there is no longer astronomical twilight (the sun does not go 18 degrees below the horizon) the sky never quite gets dark.  Muslims should calculate at what point (how many degrees below the horizon) the sky stops being dark, and this will define the maximum length of the day acceptable for fasting.

Days that are longer than this should not be fasted (as you will not have actually witnessed the start point as per 2:187) and charity/feeding of the poor should take place instead for those who can.

I have updated the fasting document with this section.  I will research the topic myself and welcome answers from others.  Please post in the comments if you have answers or ideas.