Although the thunderbolt revolved around and cut Vṛtrāsura's neck with great speed, his head fell of only after the time it took the sun, moon and other luminaries to complete a northern and southern journey, at a time suitable for killing him. Although the thunderbolt moved quickly and cut all around his neck--not just in one place, since it was very tough--the head, after being cut off, fell to the ground only after a year, after the planets moved through the northern and southern courses. Vārtra-hatye means “at a time suitable for slaying Vṛtrāsura.” Or the suffix ya on hatye may be derived from daṇḍādibhyo yaḥ (Pāṇini 5.1.66). It is added to the words with the meaning “deserving.” Thus the phrase then means “deserving to kill Vṛtrāsura, or for the purpose of killing him.”