[CCIP]

One thing that bears mentioning is that maybe the main navigation instrument was and remains the gyrocompass - the trick is that the gyrocompass doesn't tell you your position, only how far you travelled and in what direction. Submarines at that time carried gyrocompasses.

The trick is that over time, a gyrocompass is prone to error and drift, so you need regular fixes to update your precise position and compensate for gyro errors. In familiar territory, as already mentioned, you had a variety of sources - including visual navigation and radio beacons. But out at sea, as Steve mentioned, until very very recently, there was no more accurate method of getting a precise fix than celestial navigation. Until GPS became fully operational in the 1990s, it remained the best and most accurate method. As late as the 60s, it was still the only real navigational reference for aircraft flying long distances over oceans (before gyrocompasses that were both light and accurate enough were developed, and became basis for automated inertial navigation systems). So yes, celestial navigation is no longer used - but it was in fact not until the 1990s that technology to fully replace it actually existed, although more drift-resistant, portable gyroscopes, navigational computers and better radio navigation lessened reliance on it over the years.

To this day, all precise navigation in fact continues to rely chiefly on inertial gyros, which provide much faster and smoother tracking than radio signals, which are prone to interruptions; GPS and other sources are simply used for regular automated corrections to these gyros. It's not actually all that fundamentally different from how ships operated back in the 1940s - the difference is that the source for fixes is weather-independent and the process of updating the fixes and calibrating the gyrocompasses is fully automated, removing some of the human error from the equation.

It's also worth noting that celestial navigation is still extremely accurate. Not quite to the level of GPS, but a lot closer than one might think. It's just slower, involves more work (either for a human or a computer) and weather-dependent.