Longitude, in geography, an arc of the equator included between the meridian of a place and the meridian whence the degrees are counted, which is usually called the first meridian. The ancient geographers drew the first meridian through Ferro, the westernmost of the Canary islands, and they are still followed by the geographers of Germany and eastern Europe (who draw it, however, a little E. of the island). The English call the first meridian that which passes through Greenwich; the French, that of Paris; the Spaniards, that of Madrid. The inhabitants of the United States generally use Greenwich, though the longitude from Washington is also used. - An easy method of ascertaining the longitude at sea had been wanted since the improvements in navigation, and after the improvement of the quadrant by Hadley and Godfrey it was the thing most desired to make navigation perfect. When deduced from the course and the distance, as was then the custom, the mariner had but little trust in his own work; and as late as 1820 vessels at sea on speaking each other never omitted the inquiry, "What is your longitude ? " a common sympathy also causing them on "heaving in sight" to steer toward each other.
Almost every method of determining the longitude depends on the obtaining the difference of time between your first meridian and that which passes through the place where you are; the time at the latter can be easily obtained by means of altitudes of the sun or other heavenly bodies, but the great difficulty is to find the time elsewhere, the difference of time being one hour to 15° of longitude. John Werner was the first to recommend the use of lunar distances for this purpose (1514); but at that time there were neither lunar tables nor instruments for measuring the distance between the moon and a star. Gemma Frixius was the first to suggest the use of timekeepers (1530), but the art of watch-making was then in its infancy. The great importance to navigation of determining the longitude induced various governments to offer rewards for some practical method. Spain offered 1,000 crowns for its solution as early as 1598, and the states of Holland soon after 10,000 florins; but it was not till 1714 that encouragement was offered in Great Britain, when an act was passed in parliament allowing £2,000 toward making experiments, and offering a reward to the person who should discover the best means of determining the longitude at sea, proportioned to the degree of accuracy that might be attained by such discovery.
The result was the invention by John Harrison and the gradual perfection of the chronometer, which is now in general use, and to which alone we are indebted for the shortening of passages at sea, as by its use vessels can steer as direct for port as the ocean and winds will allow without fear of falling to leeward as formerly. (See Clocks and Watches, and Harrison, John.) "When the final award was made to Mr. Harrison, the acts concerning longitude were repealed, except so much as related to the publishing of the nautical almanac and other useful tables. It was also enacted that any person who should discover a method for finding the longitude by means of a timekeeper the principles of which had not previously been made public, should be entitled to a reward of £5,000, if after certain trials made by the commissioners the said method should enable a ship to keep her longitude during a voyage of six months within 60 geographical miles or a degree of a great circle, of £7,500 if within 40 geographical miles, and of £10,000 if within 30 geographical miles.
If the method be by improved astronomical tables, the author becomes entitled to £5,000 when they show the distance of the moon from the sun and stars within 15" of a degree, answering to about 7' of longitude, after allowing half a degree for errors of observation and under certain restrictions, and after comparison with astronomical observations for a period of 18 1/2 years, during which the lunar irregularities are supposed to be completed. The same rewards were likewise offered to any person who should discover any other method of determining the longitude at sea with the accuracy above mentioned. At that period timekeepers were expensive and less to be depended on than at the present day; they could easily be consulted, but prudent mariners were cautious, and the motion of the heavenly bodies was more to be trusted. The eclipses of Jupiter's satellites, although answering well on land for determining longitude, could not be observed at sea on account of the ship's motion, and the more practical method of observing lunar distances for this purpose was adopted, Dr. Maskelyne, the astronomer royal, being the first to introduce them into use. The difficulties which had attended this method when first suggested were now removed.
Prof. Mayer of Gottingen had formed lunar tables sufficiently correct to induce the commissioners of longitude to promote their practical application by the annual publication of the nautical almanac, commencing in 1767, and proper instruments for observing lunar distances had come into general use. The method of ascertaining the longitude at sea by lunar observations was followed until the great perfection with which chronometers were made caused it to be neglected. - The last discovered and most accurate of all methods of determining differences of longitude is by the use of the electro-magnetic telegraph. Captain (afterward Admiral) Charles Wilkes, of the United States navy, is entitled to the credit of having made the first attempt in that way, soon after the wires were placed between Washington and Baltimore; since that time the method of recording transits on a chronographic register by means of a galvanic circuit has been introduced, and in connection with telegraphic wires enables remote observers to record transits of the heavenly bodies simultaneously and with a degree of accuracy almost incredible.
This method is called in Europe the American, but it may with greater propriety be called the United States coast survey method, as it had its origin and was perfected in that service. The application of this method has been also extremely important as a means of testing the correctness of the various astronomical methods which have been used, or may still have to be used, for regions with which telegraphic communication is impossible or inconvenient. Prof. Asaph Hall of Washington makes the following remarks on this important subject: "The method of defining longitudes by moon culminations is so simple in theory and so easy of application, that when only an approximate value of the longitude is desired, this method will often be applied. In case, however, we wish an accurate determination of a geographical position, such as may be necessary for a station occupied in observing the transit of Venus, it is well that we should not overestimate the accuracy of this method. In many of the estimates that have been made we have an illustration of the existence of constant errors which render such estimates of accuracy wholly illusory. ... In nearly all the determinations of longitude by moon culminations, where a large number of culminations have been observed, the computed probable error of the result is only a small fraction of a second; but the telegraphic determination of the same points shows errors in the old determinations of two, three, and even four seconds of time.
Thus the longitude of San Francisco, determined from 206 moon culminations, was found to be four seconds in error. The most decisive experiment on this point is, however, the determination of longitude between Europe and America. The three determinations of longitude between Greenwich and Washington by the United States coast survey, by means of the Atlantic cable, give the difference of longitude 5 h. 8 m. 12.2 s. The following are the determinations of the same difference of longitude by moon culminations:
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showing errors - 2.9 s., - 2.2 s., - 2.6., - 0.6 s., and - 2.4 s. In his last determination, Prof. Newcomb used the observations of 1862 and 1863, when the transits both at Greenwich and Washington were recorded by the chronographic method. It seems therefore fair to conclude that a longitude determined by moon culminations may be in error two or three seconds, even if we could use an infinite number of observations."