Surya Siddhanta
स्पृशेद् यत्र ततो वृत्तम् ग्राहकार्धेन सम्लिखेत् । तेन ग्राह्यम् यदाक्रान्तम् तत् तमोग्रस्तम् आदिशेत् ॥
Meet one another, from that point describe a circle with a radius equal to half the eclipsing body : whatever of the eclipsed body is included within it, that points out as swallowed up by the darkness (tamas).
english translation
spRzed yatra tato vRttam grAhakArdhena samlikhet । tena grAhyam yadAkrAntam tat tamograstam Adizet ॥
hk transliteration by Sanscript