Navigators still faced the problem of not knowing how far east or west they were. Latitude is how far north or south you are. You can tell that with an astrolabe. Longitude is how far east or west.
It was a problem Amerigo Vespucci tried to solve. In 1502, he wrote: “…I learned [my longitude] … by the eclipses and conjunctions of the Moon with the planets…” He was trying to find longitude by observing the Moon’s and Mars’ positions in relation to the Earth. Not only was this an overly-complicated method, it had several drawbacks—mainly it only worked during a specific astronomical event.
I’d be a clod not to link Rudyard Kipling’s poem, which I quoted in the title above—http://www.kiplingsociety.co.uk/poems_eastwest.htm
While we’re at it, here’s Bob Hope and Jane Russell in The Paleface, singing Buttons & Bows.