vajras tu tat-kandharam āśu-vegaḥ
kṛntan samantāt parivartamānaḥ
nyapātayat tāvad ahar-gaṇena
yo jyotiṣām ayane vārtra-hatye
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.
Link to this page: https://prabhupadabooks.com/sb/6/12/33