Agni Purana (Jyotisha Selections) · by Vyasa
Chapter 3 · Name Letters and Chakras
युद्धजयार्णवः (चक्राणि)
अग्नौ कुजे भवेत्सौम्ये स्थिते राहुर्बुधे सदा ।फणिराहुस्तु प्रहरमैशे वह्नौ च राक्षसे ॥ ९ ॥
In the full Agni Purana this is chapter 142, verse 9.
On Tuesday he is in the south-east. On Wednesday Rahu always stands in the north. Phani Rahu (serpent Rahu) stays one prahara (three-hour watch) in each corner. He goes to the north-east, the south-east, the south-west, (Our text is unclear here: Tuesday repeats Friday's south-east, and Monday is not given.)
Simple meaning: Rahu's direction changes each weekday. Serpent Rahu moves round the corners, three hours in each.
वायौ संवेष्टयित्वा च शत्रुं हन्तीशसंमुखम् ।
Serpent Rahu circles and kills the enemy. At full moon, Tithi Rahu is in the south-east.
अमावास्यां वायवे च राहुः संमुखशत्रुहा ।
At the new moon Rahu is in the north-west. The letters are split into groups by direction.
शुक्ले त्यजेत्कुजगुणान्धाद्या मान्ताश्च पूर्वतः ।
In the bright half, avoid certain things. Letters are set in the east and north. Avoid the look of the moon day.