Before inertial navigation the re was compass AND celestial navigation AND dead reckoning. Everybody figured out sun rises in the east and sets in the west, the north star. Many early civilizations had much more, google "viking navigation".
With a compass you can do dead reckoning, which means I went on this compass heading at a certain speed for a certain time, so I must be here. I believe Columbus used this. They would throw a log over the stern witha rope that had knots and count how many knots went by in X time using an hourglass. They would write down how many knots in a log book. Admittedly often large error, but sometimes pretty close.
Sextant was invented in 1757, forerunners existed before. Then latitude is easy at night, for daytime latitude and longitude, you need to know exactly what time it is. So when they got chronometer and sextant (except clouds) would tell you where you are very precisely.
All this has worked beautifully since for 200 years and is based on RE astronomy. If it just happens to work on FE, quiter a coincidence. I do not believe FE has a technical explanation for how celestial navigation works. RE does.