The kindest way of describing finding longitude using the difference between local and Greenwich is 'approximate'. Small errors in time can produce surprisingly large errors in longitude. Still, it may be better than no longitude at all.
In 24 hours the earth turns through 360°. This means that in every hour it turns through 15° or that a difference of one degree of longitude equals four minutes of time (see Figure 17.2). Before leaving port, a ship's chronometer would have been set to GMT. Each day the time difference shown on the carefully cosseted chronometer, between GMT and local noon as noted from the sun, was calculated and turned into longitude east or west of Greenwich. Together with a noon latitude this would provide a noon fix.
If you have a watch set to Greenwich time and know its error, then use this method to produce a longitude, but treat its answers with healthy scepticism.
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