How did people cross the open ocean before they could measure where they were?

One of two arcs in Mission-42's Discovery atlas. Each chapter is researched against tier-1 and tier-2 sources, interrogated by the Inquiry Council, and revised in response before publication. Chapters cross-link to PANGEA for shared methods — celestial navigation, marine taxonomy, plate tectonics — and across to the Space arc where the journeys mirror each other.