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Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 1 · Basics and Panchanga
ज्योतिषमूलं पञ्चाङ्गं च
रवींद्वोर्मानयोगार्द्धं षष्ट्या संगुण्य भाजयेत् ।तयोर्भुक्त्यंतरेणाप्तं स्थित्यमर्द्धां नाडिकादिवत् ॥ १८४ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 54, verse 184.
Take half the sum of the sizes of the Sun and Moon. Multiply by 60. Divide by the difference of their motions. The result is the half-duration, in ghatikas (24-minute unit) and so on.
Simple meaning: Half the sum of the Sun's and Moon's sizes, times 60, divided by the difference in motion, gives the half-duration.
स्थिरीकृतार्द्धरात्रार्द्धौ द्वयोर्विवरलिप्तकाः ।
The gap between the fixed Moon and the midnight Moon gives the time of the point.
पातकालः स्फुटो मध्यः सोऽपि स्थित्यर्द्धवर्जितः ।
The mid-time less the half-duration is the start. Plus the half-duration is the end.
आद्यंतकालयोर्मध्ये कालो ज्ञेयोऽतिदारुणः ।
The time between the start and the end is harmful and should be avoided for every task.