Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
संध्या त्रिनाडी प्रमितार्कबिंबोर्द्धोदयास्ततः ।प्राक्पश्चाद्याम्यसौम्ये चेत्पुण्यं पूर्वापरेऽहनि ॥ २६५ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 265.
Twilight is three ghatika (24-minute unit). It is measured from when the Sun's disc is half risen or half set. Say the entry falls before or after it, in the southern or northern course. Then the merit is on the day before or after. (Our text is unclear here: which case goes with which day.)
Simple meaning: Twilight lasts three ghatikas. An entry in twilight moves its holy time to the day before or after.
यादृशेनेंदुना भानोः संक्रांतिस्तादृशं फलम् ।
The Moon's state at the Sun's entry decides whether the entry is good or bad for a person.
संक्रांतेः परतो भानुर्भुक्त्वा यावद्भिरंशकैः ।
After the Sun enters a sign and covers some degrees, the next turn of its course follows.
संक्रांतिग्रहगर्क्षंवा जन्मन्युभयपार्श्वयोः ।
The star of a sankranti or an eclipse next to the birth star is not wanted for vows and marriage.