The metaphor in the final stanza has two effects. "A starlit flag unfurled" compares the land to a flag.
On one level, this metaphor conveys the power of finally seeing land after all that time at sea. One light becomes an entire starlit flag. The hope that one light gives is power.
However, there is a much deeper level to this metaphor. It likens the land to the American flag, indicating he had both seen land and discovered a nation simultaneously. At this point, Columbus "gained a world."
