In Sybilline Leaves (1817) this gloss is absent. But manuscript variants include:

Between the Tropics there is no Twilight. As the Sun’s last Segment dips down, and the evening Gun is fired, the Constellations appear arrayed.

No Twilight where there is no Latitude nor yet on either side within the Park & Race-course of the Sun.

Within the Tropics there is no Twilight. At the moment, the second, that the Sun sinks, the Stars appear all at one as if at the word of command announced by the evening Gun, in our W. India Islands.

Within the Tropics there is no Twilight. As the Sun sinks, the Evening Gun is fired, and the starry Heaven is at once over all, like men in ambush that have been listening for the signal—& Hark!—now.

No Twilight within the course of the Sun.