Srimad Bhagavatam Canto 08, Chapter 09, Text 26

SB 8.9.26

siras tv amaratam nitam
 ajo graham aciklpat
yas tu parvani candrarkav
 abhidhavati vaira-dhih
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Rahu’s head, however, having been touched by the nectar, became immortal. Thus Lord Brahma accepted Rahu’s head as one of the planets. Since Rahu is an eternal enemy of the moon and the sun, he always tries to attack them on the nights of the full moon and the dark moon.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Since Rahu had become immortal, Lord Brahma accepted him as one of the grahas, or planets, like the moon and the sun. Rahu, however, being an eternal enemy of the moon and sun, attacks them periodically during the nights of the full moon and the dark moon.
Srimad Bhagavatam Canto 08, Chapter 09, Text 25
Srimad Bhagavatam Canto 08, Chapter 09, Text 27