Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 5 · Portents, Comets
ग्रहचारः केतवश्च
विष्णुचक्रोत्कृत्तशिराः पङ्गुः पीयूषपानतः ।अमृत्युतां गतस्तत्र खेटत्वे परिकल्पितः ॥ ७९ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 79.
Vishnu's discus severed Rahu's head and left him without limbs. Yet the nectar he drank made him immortal. So he was made a planet there.
Simple meaning: Vishnu's discus severed Rahu's head, but nectar made him immortal and he became a planet.
वरणधातुरर्केन्दू तुदते सर्वपर्वणि ।
Rahu troubles the Sun and Moon at new and full moons, but at times is too far to eclipse them.
षण्मासवृद्ध्या ग्रहणं शोधयेद्रविचंद्रयोः ।
Eclipses of the Sun and Moon are found six months apart. Seven gods rule the eclipse seasons in turn.
ब्रह्मेन्द्विंद्रधनाधीशवरुणाग्नियमाह्वयाः ।
A junction day ruled by Brahma brings growth to animals, crops and the twice-born.