system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Before we saw the Earth for the first time in space, how did we know what our continents and lands looked like?
gpt
Let's say that it is 1920 (so, no GPS other satellites). You have just found a island in the Pacific and want to know where it goes on the map. There are two things you would need to figure out to do that: how far north or south is the island, and how far east or west it is. One way to do this is by sighting the stars.

The stars look like they are fixed to a sort of celestial sphere that rotates around the Earth, with its axis of rotation going through our north and south poles. By looking at the north-south positions of the stars, you can figure out how tilted the celestial sphere appears to be from your position, which tells you how far north you are. For instance, if you see the North Star, you can almost directly read your north-south position. If it's overhead, you're close to the North Pole, and if it's on the horizon, your near the equator.

Determining your latitude (east-west) [EDIT: sorry, that should be longitude] position is much more tricky, and there is a long history concerning it, but it basically boils down to clocks. As the celestial sphere spins in sky, the stars will appear to be in different positions east or west depending on how far east or west you are. If you have an accurate clock and a star chart, you can determine where the stars would be at that time if you were at a reference point on the Earth (for instance Greenwich, England). Looking at where the stars actually are in your sky at that time, you can determine how far east or west you are.

A book that's well worth a read on this subject is *Longitude*. It discussed the problems and history of determining your position while navigating.