The earth rotates once every twenty-four hours, and if at any time a star is directly south of Greenwich it is also due south of all places on the meridian of Greenwich north of the equator, and north of all places on the same meridian south of the equator; then, as the earth rotates, the meridian of Greenwich will pass from under the star, and others to the west will take its place, and in an hours time, at 1 P.M., a certain meridian to the west of Greenwich will be under the star, and in that case all places on this meridian will be an hour west of Greenwich, and so on through all the twenty-four hours, the meridian being called so many hours, minutes, or seconds, west, as it passes under any star that length of time after the meridian of Greenwich. It is immaterial whether we reckon longitude in degrees or in time, for since there are 360 degrees or twenty-four hours into which the equator is divided, each hour corresponds to 15°. We also express the longitude of a place by its distance east of Greenwich in hours, so instead of calling a place twenty-three hours west, it is called one hour east. Suppose we wish to find the longitude of any place, all that is required to be known to an observer there is the exact time that a certain star is on the meridian of Greenwich; he then observes the time that elapses before the star comes to his meridian, and this time is the longitude required.
This, of course, only shows the principle, for in practice it is not absolutely necessary for the star to be on either meridian, provided its distance on either side is known, when, of course, the difference between the times when it actually crosses the meridian can be reckoned.
In practice a difficulty arises in finding out at a distance from Greenwich what time it is there. It is of course twelve o’clock at Greenwich when the sun crosses the meridian, and it is also twelve o’clock at all the other places when the sun crosses their meridian: but if a place is two hours west of Greenwich, the sun crosses the meridian two hours later than it does at Greenwich, and consequently their clock is two hours slower than Greenwich time, hence the term “local time,” which is different for different places east or west of Greenwich. We have taken above a star for our fixed point, but obviously the sun answers the same purpose.
It will appear from this, that if we know the difference between the local times of two places, we also know the longitude of one place from the other, which is the same. A great number of ways have been tried in order to make it known at one observing station what time it is at the other. Rockets are sent up, gunpowder fired, and all kinds of signals made at fixed times for this purpose; but these, of course, only answer for short distances, so for long ones carefully adjusted chronometers have to be carried from one station to the other to convey the correct time; unless telegraph wires are laid from one place to another, as from England to America; then it is easy to let either station know what time it is at the other. For ships at sea chronometers answer well for a short time, but they are liable to variation.
There are certain astronomical phenomena the instant of occurrence of which can be foretold—and published in the nautical almanacs—such as the eclipses of Jupiter’s moons, and the position of our own moon amongst the stars. Suppose then an eclipse of one of Jupiter’s moons is to take place at 1 P.M. Greenwich time, and it is observed at a place at 2 P.M. of the observer’s local time, i.e., two hours after the sun has passed his meridian, then manifestly the clock at Greenwich is at 1 P.M. while his is at 2 P.M., and the difference of local time is one hour, and the place is one hour, or 15°, east of Greenwich. If, however, the eclipse was observed at 12 noon, then the place must be one hour west of Greenwich. The local time being one hour slower than Greenwich shows that the sun does not south till an hour after it does at Greenwich, or, in reality, the place does not come under the sun till after the meridian at Greenwich has passed an hour before, clearly showing it to be west of Greenwich.
We shall now see how easy it is to find the longitude when the two stations are electrically connected. Suppose we wish to determine the difference of longitude of two places in England,—this can be determined with the utmost accuracy in a short time if the observers have a chronograph, of the kind just described, to record the transit of a star at these two places. The observers at each station arrange that the observer at Station A shall observe the transit of a certain star on his chronograph, and the observer at Station B shall observe the transit of the same star on his, and then with the faithful clock, beating seconds and marking them on the surface of both chronographs simultaneously, the difference of sidereal time between the transit of the same star over Station A and Station B will be an absolute distance to be measured off in as delicate a way as possible by comparison of the roller of each chronograph, and will give exactly how much time elapses between the two transits. This is the longitude required. There are various methods of utilizing the same principle, as, for instance, one chronograph only may be used, and both observers then register their transits on the same cylinder. But when we have to deal with considerable distances, such as between England and the United States, then we no longer employ this method. From Valentia we telegraph to Newfoundland in effect “Our time is so-and-so,” and then the observer at Newfoundland telegraphs to Valentia “Our time is so-and-so.”
In this way the absolute longitude of the West of Ireland and America and the different observatories of Europe has been determined with the greatest accuracy.
So it appears there are two methods, the first showing one time, say Greenwich time, at both places, and showing the difference in times of transit of stars; or secondly, having the clock at each place going to its own local time, so that a certain star transits at the same local time at each place, and finding the difference between the two clocks.
[15]. It was found, that between the passing of the spark into the gun, and the ignition of the powder and discharge of the piece, one tenth of a second elapsed.