Of a star or planet: rising at sunset, or in older usage, visible at nightfall. Its appearance belongs to the hinge between day's last light and the first full dark. Where nocturnal merely names the hours, acronychal marks a specific celestial timing, an appointment kept with dusk.

From Greek akronychos "at the beginning of night," from akros "highest; extreme" + nyx "night." The word entered English through the language of observational astronomy, where naming became a way of keeping faith with recurrence.