Ok, they knew the earth was round so if they could find the equator they could do latitude, but how’d they do longitude?
Did you miss the part about the writer of the story?
No need to mark the equator for Latitude, the Pole star is a clear starting point.
Longitude can be based on any commonly known point agreed on by others. We just use Greenwich, England and Paris out of the common agreement of the English and Europeans.
That the world is round, as has been noted, was well established by the Greeks. The curious thing to some is that Columbus thought it was smaller, by a significant margin, than what the Greeks had measured. He was wrong, and only the unknown factor of the ‘New World’ kept him from being an unfortunate footnote in History.
In this context they are not referring to longitude as a position on the earth’s surface, but rather the east/west position of a particular star relative to a reference point in the night sky.