With the advent of mechanical devices, the water clocks disappeared
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Captain Scott had the misfortune to lose his solar tables on one of his south polar trips and had he not been extremely resourceful the result might have been very serious. As he was located between the True and the Magnetic South Poles his compass was practically useless. He tells in his book that he instructed one of his subordinates to steer due west by his compass, when he wanted to go due east.
On this occasion, Scott remembered the latitude of his base and the date when the Sun first appeared; from those he could calculate the declination of the Sun on that day. He had in his notebook the declination of the Sun on one or two recent days, and with the help of these notations he plotted a curve for the future declinations of the Sun and used it for the balance of his trip.
For times when the Moon is not visible, astronomers have devised a system by which the eclipses of Jupiter's satellites can be used as time signals. They are very hard to observe precisely with portable instruments, and if neither the Moon nor Jupiter is visible, there is nothing to do but give yourself up for lost.
Altogether no system for determining "absolute" longitude is really satisfactory; and if you will insist on being stranded the best thing to take along is a radio to catch the time signals given at frequent intervals all over the world. They have practically displaced all other devices.
Time is measured by the angle at one of the poles or by the segment of arc on the equator, intersected by the meridian vertically under the Sun, referred to a standard meridian. Because of the need for this reference the poles are the only two places on the Earth at which time cannot be measured, even though all such measurements are referred to them. The reason is very simple.
Suppose a party of four people went to the Arctic Circle and set up a post exactly on the pole, around which they stood in a ring. For one the time might be noon, for the second it would be 6:00 A.M., for the third, midnight, and for the fourth 6 P.M. But exactly at the post, where all meridians converge time would not be measurable. Stand ever so slightly on one side or the other and you will be on some meridian and can then find the time; but the movement of a small distance would alter the determination (except to return home) very materially.
No teacher should accept as valid the excuse that a pupil did not know the time because he could not go to the North Pole to find it.
Even so eminent an author as Jules Verne slipped on this point, for he made his famous character Captain Nemo, on his trip underseas to the South Pole, "wait till the Sun was on the Meridian before he took his latitude observations." Latitude observations must be taken on or near the Meridian but, if Captain Nemo was at the South Pole as his observations subsequently proved, the altitude of the Sun, and so the determined latitude, would have been the same whatever the direction of the Sun, except for a small change in declination during a few hours. But that difference is so small that in all probability it would have been less than his errors of observation. If he had been really particular, he would have taken four observations nearly at right angles to one another as Commander Peary did.
We seem to insist that the luggage of explorers should consist of curious implements; but, if you should ever happen to take a garden sundial to a place near either of the poles, remember that the gnomon must be adjusted there to point almost straight up at the pole of the heavens so that it will remain parallel to the axis of the Earth, with the hour lines radial from it.
Returning now to more temperate regions, and considering the problem of time as a whole, we can easily see that just as a standard meridian is required, so it is necessary to have a date line, where the day will start and end. This has been set up at the meridian 180° away from Greenwich, the dividing line between East and West, as far as Greenwich is concerned. Originally it was intended to be a straight line, but as a matter of fact it is swung a bit to allow islands which have been in the habit of trading with one another to keep the same dates.
When midnight occurs on this line, the date changes, and the day moves westward as the Earth turns to the east. It passes over Australia, Siberia and the Asiatic continent, Europe and Africa, and when Big Ben strikes midnight in London the Australian day is already half-gone, and high noon is shining over the Pacific date line. The day moves westward, over the Atlantic, the North and South American continents, and ends where it began on the international date line in the middle of the Pacific Ocean.
The effects are most surprising. A passenger on an east-bound boat, say from Australia to San Francisco, lives the same day over twice. He goes to bed on a Saturday evening and wakes up again on Saturday morning. Conversely a passenger traveling westward loses a day out of his life. He goes to bed Saturday evening and when he wakes it is Monday. He can never again say that he has never missed a Sunday service. Philosophers may take different views of the advantages and disadvantages of living the same day over twice and of skipping a day entirely. Two friends were discussing their trips around the world. One had gone eastward and the other westward. Said the latter, "I don't think it is fair. Since we started you have had six more meals in the same time than I have!"
Many of the seemingly irreconcilable paradoxes can be solved if the international date line is taken into consideration. But light opera and novels would both lose and gain thereby. There was an old song in The Belle of New York which took for its premise the irreversability of dates. It concerned a passenger who was talking to a ticket agent about a trip to "Morrow, Ohio," and the refrain went thus:
"Can I go to Morrow and return from there today?
The train which goes to Morrow is an hour upon its way;
You can not go to Morrow and return from there today."
Some years ago a novel was written around an unusual plot. It started in the ordinary way of a love match between a poor boy and the daughter of a rich man who declared, "You shall never marry my daughter until there are two Sundays in one week." Fathers should know that no such difficulties are insurmountable. But the boy was despondent. He left home and wandered aimlessly until the Yukon gold excitement caught him and he rushed up to Alaska. There he heard church bells ringing on two consecutive days--and wondered and asked. He found that Juneau had been settled by two classes of people, the Russians from Siberia and the Americans from Washington and California.
Between America and Siberia lies the international date line. The Russians brought their Sunday eastward with them and the Americans brought their Sunday westward. Religion is a traditional thing, and though the two colonies were able to reconcile their civil days, they were unwilling to change the Sabbath worship of their different churches; hence the bells rang out for Sunday on two consecutive days--and on this occasion they rang a third time--for a wedding service.
The greatest eclipse of the Sun that had occurred in many years was visible on June 8. It lasted for over seven minutes, nearly as long as it is possible for any eclipse to last. But unfortunately it was only visible on some remote islands in the Pacific Ocean, or from the deck of a steamer. The path of totality crossed the date line from west to east, and the eclipse ended on the day before it began. In this chapter we have traced the divisions of the day from the time when only sunrise and sunset were used to determine them, up to modern times.
With the advent of mechanical devices, the water clocks disappeared, and with them went the unequal hours which were impractical for the smooth running of an ordinary clock. Sundials were relegated to gardens and bathing beaches; nocturnals and astrolabes to glass cases in museums. The effect has been a continuous effort to get greater and greater regularity, till now we can buy and carry on our wrists, hardly noticing their presence, watches that keep time well within all our ordinary requirements; and the appeal to the watchman, "What of the night?" is heard no more.
But still in observatories, the systems we have described are in use; and from them the time we keep or hear over the radio derives its ultimate source. For in the last analysis our only means of keeping time is by astronomy.
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