Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
षडशीतिमुखा ज्ञेया द्विस्वभावेषु राशिषु ।तुलाघटाजयोर्ज्ञेयो विषुवत्सूर्यसंक्रमः ॥ २६३ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 263.
In the dual signs, the entry is known as Shadashitimukha. The Sun's crossing into Libra and Aries is the equinox.
Simple meaning: The Sun's entry into Libra and Aries is the equinox.
याम्यायने स्थिरे त्वाद्याः पराः सौम्येन्दुमूर्तिभैः ।
Holy hours fall before or after the Sun's entry, by its course and sign. Sixteen ghatikas are holy.
संध्या त्रिनाडी प्रमितार्कबिंबोर्द्धोदयास्ततः ।
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.