LONGITUDE. The longitude of a place on the earth is the angle at the pole between the meridian of Greenwich and the meridian passing through the observer's place; or it is the arc of the equator intercepted between these meridians; or, since this arc is measured by the time required for the earth to turn suffi ciently to bring the second meridian into the same position held by the first, it is simply the difference of their local times, usually reckoned in hours, minutes and seconds, instead of degrees. Since it is easy for the observer to find his own local time by the methods which have been given, the problem is really this: being at any place, to find the corresponding local time at Greenwich without going there. There are three principal methods of finding longitude.
Finding Longitude by Means of Signals Simultaneously Observable at the Places be tween which the Difference of Longitude is to be I. A Lunar Eclipse. When the moon enters the shadow of the earth, the nomenon is seen at the same moment, no ter where the observer may be. By noting, therefore, his own local time at the moment, and afterward comparing it with the time at which the phenomenon was observed at wich, he will obtain his longitude from wich. 2. Ecilpses of the satellites of Jupiter maybe used in the same way, with the tage that they occur very frequently— almost every night, in fact; but the objection to them is that they are not sudden. 3. The appearance and disappearance of meteors may be and has been used to determine the difference of tude between places not more than two or three hundred miles apart, and gives very accurate results. This method is now superseded by the telegraph. 4. Artificial signals, such as flashes of powder and rockets, can be used between two stations not too far distant. The ence of longitude between the Black Sea and the Atlantic was determined by means of a chain of signal stations on the mountain tops; so also, the difference of longitude between the eastern and western extremities of the ern, boundary of Mexico. This method is now superseded by the telegraph.
Finding Longitude by Regarding the Since the moon revolves around the earth once a month, it is continually changing its place among the stars; and as the laws of its motion are now well known, and as the place which it will occupy is predicted for every hour of every Greenwich day three years in advance in the nautical almanac, it is possible to deduce the corresponding Greenwich time by any observation which will determine the place of the moon among the stars. The almanac place, however, is the place at which the moon would be seen by an observer at the centre of the earth, and consequently the actual observa tions are in most cases complicated with very disagreeable reductions for parallax before they can be made available. 1. That of Moon
Culminations.— We merely observe with a transit instrument the time when the moon's bright limb crosses the meridian of the place; and immediately after the moon we observe one or more stars with the same instrument, to give us the error of our clock. As the moon is observed on the meridian, its parallax does not affect its right ascension, and accordingly, by a simple reference to the almanac, we can ascertain the Greenwich time at which the moon had the particular right ascension deter mined by the observation. The method has been very extensively used, and would be an admirable one were it not for the effects of personal equation. 2. Lunar-Distances.— At sea it is impossible to observe the moon with a transit instrument, but we can observe its distance from the stars near its path by means of a sextant. The distance observed will not be the same that it would be if the observer were at the centre of the earth, but by a mathe matical process called °clearing a lunar* the distance as seen from the centre of the earth can be easily deduced, and compared with the distance given in the almanac. From this the longitude can be determined. 3. Occultations. Occasionally, in its passage through the sky, the moon overruns a star, or (occults* it. The star vanishes instantaneously, and, of course, at the moment of its disappearance the distance from the centre of the moon to the star is pre cisely equal to the apparent semi-diameter of the moon: we thus have a ulunar-distance( self-measured. Observations of this kind fur nish one of the most accurate methods of de termining the difference of longitude between widely separated places, the only difficulty aris ing from the fact that the edge of the moon is not smooth, but more or less mountainous, so that the distance of a star from the moon's centre is not always the same at the moment of its disappearance. 4. In the same way a solar eclipse may be employed by observing the mo ment when the moon's limb touches that of the sun. This and the preceding methods (the methods of occultation and solar eclipse) do not belong in the same class with the method of lunar eclipse, because the phenomena are not seen at the same instant at different places, but the calculation of longitude depends upon the determination of the moon's place in the sky at the given time, as seen from the earth s centre.