vajraḥ - the thunderbolt; tu - but; tat-kandharam - his neck; āśu-vegaḥ - although very fast; kṛntan - cutting; samantāt - all around; parivartamānaḥ - revolving; nyapātayat - caused to fall; tāvat - so many; ahaḥ-gaṇena - by days; yaḥ - which; jyotiṣām - of the luminaries like the sun and moon; ayane - in moving to both sides of the equator; vārtra-hatye - at the time suitable for killing Vṛtrāsura.

Although the thunderbolt revolved around Vṛtrāsura’s neck with great speed, separating his head from his body took one complete year — 360 days, the time in which the sun, moon and other luminaries complete a northern and southern journey. Then, at the suitable time for Vṛtrāsura to be killed, his head fell to the ground.