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...he must get his longitude, also, and this, if he chooses, he can find from the same sextant sight, by means of his chronometer. 

(A chronometer is a very fine and adjusted clock. One Harrison, by inventing a clock that would keep accurate time in all temperatures, and under all circumstances, received in 1773, a reward of $100.000 offered in the reign of Queen Anne for the best method of finding a ship longitude. The method is simplicity itself, in theory, and theories are all we are after now)

You know, of course, that the earth revolves on its axis once in each 24 hours. As the earth turns from the W. to E. and the meridians of long. run N. and S., you can see that the 360° line of longitude passes under the [sun] one after the other, in 24 hours. And that means that a new degree passes every 4 minutes.

Now, it is noon at any place on the earth when the [sun] is at the highest point in the heavens at that place. That is, it is noon at any place when the meridian, which passes through that place, is directly under the [sun]. Consequently, as it takes 4 minutes for the [sun] to pass through one degree, difference of longitude becomes merely a difference of time.

Now, the captains' chronometer is kept running with the most anxious exactitude to the time of Greenwich, England, which you remember, is longitude zero. When the captain finds the noon for his locality (by noting when the sun is at its highest point, as explained, he has but to compare his time with the Greenwich time, and figure 4 minutes to each degree. So, in the case we have before us, at the moment the captain made “8 bells” – noon – someone, stationed for that purpose at the chronometer marked the exact chronometer time. Suppose that was 4:52 PM. That is 4 hours and 52 minutes after noon, or 292 minutes after noon. Counting four minutes of time to each degree of longitude, he would be 73° of longitude to the W. of Greenwich. The captain knows,..