Surya Siddhanta
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तुल्यौ राश्यादिभिः स्याताम् अमावास्यान्तकालिकौ । सूर्येन्दू पौर्णमास्यन्ते भार्धे भागादिकौ समौ॥
The places of the Sun and the Moon found at the time of the new moon are equal (to each other) in signs, (degrees) &c. and at the instant of the full moon they are at the distance of 6 signs from each other.
english translation
Chapter 4
Verse 6
Verse 8