Surya Siddhanta
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षष्ठ्या सम्गुण्य सूर्येन्द्वोर् भुक्त्यन्तरविभाजिते । स्याताम् स्थितिविमर्दार्धे नाडिकादिफले तयोः॥
These roots multiplied by 60 and divided by the diurnal motion of the Moon from the Sun give the Sthityardha the half duration of the Eclipse and mardárdha the half duration of the total darkness in ghaṭikás (respectively).
english translation
Chapter 4
Verse 12
Verse 14