Here's a URL to a good summary of navigation. The use of the sextant or its prececessors to determine latitude, essentially north-south position was comparatively easy, and had been in use for many years. But the determination of east-west position was far more difficult, depending at first on the ability to make a time-piece highly accurate for long periods of time and not affected by rocking of ship at sea.

Here's another fine article about navigation:

Of course now that we have Global Position Units of incredible accuracy, all the above is mostly obsolete.