siras tv amaratam nitam
ajo graham aciklpat
yas tu parvani candrarkav
sirah—the head; tu—of course; amaratam—immortality; nitam—having obtained; ajah—Lord Brahma; graham—as one of the planets; aciklpat—recognized; yah—the same Rahu; tu—indeed; parvani—during the periods of the full moon and dark moon; candra-arkau—both the moon and the sun; abhidhavati—chases; vaira-dhih—because of animosity.
Rahuís head, however, having been touched by the nectar, became immortal. Thus Lord Brahma accepted Rahuís 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.
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.
Link to this page: https://prabhupadabooks.com/sb/8/9/26