Rāhu was thought to be a decapitated demon head that floated through the universe, eating the Sun and the Moon and causing eclipses. Originally, Rāhu had been a an ordinary demon, 'drinking the nectar', when his enemy Hari came long and chopped his head off. Due to a boon of Brahmā, Rāhu's decapitated, floating head shows up on certain new-moon and full-moon days and, as mentioned, eats the luminous planets. This is the belief that Lalla relates when he disproves of the notion, using parallax.
Varāhamihira later on used the name of Rāhu when he described the lunar nodes. The nodes are the two points where the moon's path crosses the Sun-Earth plane, and thus the only two points at which an eclipse can occur. He dubbed one node Rāhu's Head, and the other, Rāhu's tail.