Graphical methods of obtaining data for so many engineering problems are in use and have proven so satisfactory that one is impelled to wonder why an exact science like navigation does not find use for more such methods. The solution of sights of nearly every kind requires first the obtaining of the Greenwich mean time and date, and when one is out of practice or meets one of the border line cases, the Greenwich mean time puzzles the oldest hand.
It is easy enough to take the watch time and apply the C-W and CC to it to find the chronometer time but then comes the question as to whether to add 12 hours to the chronometer time or not and what the date is. But it is always easy to know the watch time and the calendar date and if we had some way of taking these and telling directly what the Greenwich date and hour angle was we would get away from all the difficulty.
In an effort to find some method of picking out the GMT after applying the confusing rules in Muir and memorizing several thumb rules, etc., the following astonishingly easy graphical method was developed.
On the accompanying chart the horizontal lines are time lines for any latitude or longitude and correspond to your watch time. The vertical lines are the longitude lines. East or West; the diagonal lines are the GMT lines. To find the GMT, note your calendar date and watch time, then find on the chart the intersection of the line representing the longitude that you are in with the line representing the watch time and read from the nearest diagonal line the hours of the GMT. If the intersection lies below the zero hour line the GMT date is the same as your calendar date; if the intersection lies above the zero hour line the date is one day preceding your calendar date.
In crossing the i8oth meridian from west longitude to east longitude add one to your calendar date and proceed as in the general case. In crossing from east longitude to west longitude subtract one from your calendar date and proceed as in the general case.
The principle on which the chart is constructed is the very simple one of drawing in the lines representing the loci of all places having the same GMT at the same time.