 According to the second opinion, nightfall occurs exactly 72 minutes after sundown (or, equivalently, the sun falls 6–16° below the horizon).In practice, when the question is whether it is too late to do a something that must be done "by day," the practice is to follow one of the earlier opinions.When the question is whether it is too early to do something that must be done "by night," the practice is to follow one of the later opinions.