Salat al-Isha (Night)

Salat al-Isha (Night) is mentioned in the following verse in conjunction with dawn (fajr)

[24:58] O you who believe, permission must be requested by your servants and the children who have not attained puberty (before entering your rooms). This is to be done in three instances—before the Dawn Prayer, at noon when you change your clothes to rest, and after the Night Prayer. These are three private times for you. At other times, it is not wrong for you or them to mingle with one another. GOD thus clarifies the revelations for you. GOD is Omniscient, Most Wise.

(٥٨) يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لِيَسْتَـْٔذِنكُمُ ٱلَّذِينَ مَلَكَتْ أَيْمَـٰنُكُمْ وَٱلَّذِينَ لَمْ يَبْلُغُوا۟ ٱلْحُلُمَ مِنكُمْ ثَلَـٰثَ مَرَّٰتٍ مِّن قَبْلِ صَلَوٰةِ ٱلْفَجْرِ وَحِينَ تَضَعُونَ ثِيَابَكُم مِّنَ ٱلظَّهِيرَةِ وَمِنۢ بَعْدِ صَلَوٰةِ ٱلْعِشَآءِ ثَلَـٰثُ عَوْرَٰتٍ لَّكُمْ لَيْسَ عَلَيْكُمْ وَلَا عَلَيْهِمْ جُنَاحٌۢ بَعْدَهُنَّ طَوَّٰفُونَ عَلَيْكُم بَعْضُكُمْ عَلَىٰ بَعْضٍ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ لَكُمُ ٱلْـَٔايَـٰتِ وَٱللَّهُ عَلِيمٌ حَكِيمٌ

The following verse informs us how to determine when dawn starts. It specifies when "the white thread of light becomes distinguishable from the dark thread of night." A simple method of determining this is when the east can become distinguishable from the west during astronomical twilight at dawn. This is when the first light appears horizontally along the horizon. This coincides with the time when the sun is approximately 18 degrees below the horizon in the morning.

[2:187] Permitted for you is sexual intercourse with your wives during the nights of fasting. They are the keepers of your secrets, and you are the keepers of their secrets. GOD knew that you used to betray your souls, and He has redeemed you, and has pardoned you. Henceforth, you may have intercourse with them, seeking what GOD has permitted for you. You may eat and drink until the white thread of light becomes distinguishable from the dark thread of night at dawn. Then, you shall fast until sunset. Sexual intercourse is prohibited if you decide to retreat to the masjid (during the last ten days of Ramadan). These are GOD's laws; you shall not transgress them. GOD thus clarifies His revelations for the people, that they may attain salvation.

(١٨٧) أُحِلَّ لَكُمْ لَيْلَةَ ٱلصِّيَامِ ٱلرَّفَثُ إِلَىٰ نِسَآئِكُمْ هُنَّ لِبَاسٌ لَّكُمْ وَأَنتُمْ لِبَاسٌ لَّهُنَّ عَلِمَ ٱللَّهُ أَنَّكُمْ كُنتُمْ تَخْتَانُونَ أَنفُسَكُمْ فَتَابَ عَلَيْكُمْ وَعَفَا عَنكُمْ فَٱلْـَٔـٰنَ بَـٰشِرُوهُنَّ وَٱبْتَغُوا۟ مَا كَتَبَ ٱللَّهُ لَكُمْ وَكُلُوا۟ وَٱشْرَبُوا۟ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلْخَيْطُ ٱلْأَبْيَضُ مِنَ ٱلْخَيْطِ ٱلْأَسْوَدِ مِنَ ٱلْفَجْرِ ثُمَّ أَتِمُّوا۟ ٱلصِّيَامَ إِلَى ٱلَّيْلِ وَلَا تُبَـٰشِرُوهُنَّ وَأَنتُمْ عَـٰكِفُونَ فِى ٱلْمَسَـٰجِدِ تِلْكَ حُدُودُ ٱللَّهِ فَلَا تَقْرَبُوهَا كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ ءَايَـٰتِهِۦ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ

The start of the Night Contact Prayer would be just the opposite end of the day when one can still distinguish east from west in the sky. This occurs when the sun is 18 degrees below the horizon.

This is also confirmed by God's messenger, Dr. Rashad Khalifa, as specified in Appendix 15 of The Authorized English Version of the Quran, where he specifies:

"The Night Prayer can be observed after the twilight disappears from the sky (24:58)."