SB 8.9.26

śiras tv amaratāṁ nītam
 ajo graham acīkḷpat
yas tu parvaṇi candrārkāv
 abhidhāvati vaira-dhīḥ
Synonyms: 
śiraḥ — the head; tu — of course; amaratām — immortality; nītam — having obtained; ajaḥ — Lord Brahmā; graham — as one of the planets; acīkḷpat — recognized; yaḥ — the same Rāhu; tu — indeed; parvaṇi — during the periods of the full moon and dark moon; candra-arkau — both the moon and the sun; abhidhāvati — chases; vaira-dhīḥ — because of animosity.
Translation: 
Rāhu’s head, however, having been touched by the nectar, became immortal. Thus Lord Brahmā accepted Rāhu’s head as one of the planets. Since Rāhu 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: 

Since Rāhu had become immortal, Lord Brahmā accepted him as one of the grahas, or planets, like the moon and the sun. Rāhu, however, being an eternal enemy of the moon and sun, attacks them periodically during the nights of the full moon and the dark moon.