Until quite recently with the advent of first the Sat Nav (Satellitte Navigation) system, and then the more sophisticated Global Positioning System (GPS), the Offshore Yacht navigator needed a knowledge of astro navigation in order to determine his vessels position when out of sight of land. Even today the availability and accuracy of GPS should not be taken for granted (I am ignoring the reliability of a yacht electrical systems here on the ship's GPS - as often a battery powered hand held is carried) and there thus remains a place for the traditional navigation skills as a back up.
To navigate by the sun, planets and stars all that is required are a set of books, a sextant, a deck watch (with known error rate) and the knowledge to use them.
The astro-navigator needs to be ready to grab any opportunity to obtain sights in order to calculate the ship's position. A clear view of the heavenly body and the horizon below it is required, and so cloud and poor visibility (e.g. mist and fog) may foil the navigators preparations for a series of sights.
Typically sights are taken at the following times:
morning twilight sights of planets and/or stars
an observation of the sun around two hours before local noon
an observation of the sun at local noon which provides the vessels latitude
an observation of the sun around two hours after local noon
evening twilight sights of planets and/or stars
In yacht navigating by traditional means watch keeping needs to be arranged so that the navigator can be on deck at these times.
The morning and evening sights can provide an observed position, whilst the observations of the sun after taking into account the movement of the vessel during the period can provide an observed position. The accuracy of a position from sun sights is partially determined by the accuracy of the course steered by the yacht's crew and their entry of appropriate detail in the vessel's deck log.
Astro-navigation is not difficult provided you have the patience to learn the theory and then to develop the technique of taking sights at sea. The navigator's technique and the prevailing visibility/sea conditions will determine the accuracy of each sight taken.