Agni Purana (Jyotisha Selections) · by Vyasa
Chapter 2 · War Astrology: Yogas, Nakshatras
युद्धजयार्णवः (योगाः नक्षत्राणि)
प्रतिपदादौ ककारादीन्सकारं नैर्ऋते पुनः ।राहोर्मुखे तु भङ्गः स्यादिति राहुरुदाहृतः ॥ १२ ॥
In the full Agni Purana this is chapter 123, verse 12.
Starting from Pratipada, write the letters from ka. Write sa in the south-west again. There is defeat at Rahu's mouth. This is how Rahu is described.
Simple meaning: Letters are written from ka, starting from the first moon day. Defeat comes at Rahu's mouth.
विष्टिरग्नौ पौर्णमास्यां करोलीन्द्रे तृतीयकम् ।
Vishti moves by moon day: south-east on the full moon, east on the third, south on the seventh, north on the tenth.
चतुर्दश्यां तु वायव्ये चतुर्थ्यां वरुणाश्रये ।
On the fourteenth she is north-west, on the fourth west and on the bright eighth south. Avoid the eleventh.
रौद्रश्चैव तथा श्वेतो मैत्रः सारभटस्तथा ।
This lists the first eight named parts of the day.