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Their Theory and Construction
By Albert E. Waugh
Dover Publications, Inc.Copyright © 1973 Albert E. Waugh
All rights reserved.
Fortunately we need not start by defining time, since the concept has perplexed philosophers and lexicographers and served as the basis for learned and inconclusive arguments. We shall assume, with the man in the street, that we know what time is, and that our problem is measuring it rather than defining it.
Very early in human history men must have recognized the passage of time. Its major subdivisions were marked by the sequence of day and night and by the passage of the seasons. Timekeeping at this level involved merely counting the days or the years—or, with the American Indian, the cycles of moon phases. There were no such obvious subdivisions of the day, and no one knows when men first began to count the hours, nor what they used to measure their passage. It is certain that one of the early methods involved observations of shadows. As the hours of any day passed it was apparent that the shadows changed slowly in their direction and in their length. The shadows of early morning were long, and stretched toward the west. As noon approached the shadows grew shorter and swung into the north. Then through the afternoon the shadows lengthened again and reached toward the east. The hour of the day could be estimated from either the length or the direction of the shadow.
At first the day was apparently merely divided into three parts—morning, afternoon and night—by the three phenomena of dawn, noon and sunset. Dawn and sunset were obvious, and noon was the moment when the shadows were shortest for the day. Later, men noted the change in length of shadows more carefully, and judged the time of day roughly by measuring their own shadows—stepping them off with their own feet, "heel to toe." The Venerable Bede gave a table about 700 A.D. for use in telling the time of day by this method (Table 1.1).
In interpreting this table one must understand that in Bede's day men counted the hours from dawn, so that the hour of "3" means "the end of the third hour after dawn." The time from dawn to sunset was divided into 12 equal "hours," but since the time from dawn to sunset was longer in summer than in winter, the "hours" of summer were also longer than the "hours" of winter. For many centuries these "unequal hours" or "temporary hours" were used over much of the earth. The "hours" of any one day were equal, but the "hours" of summer long. It is for this latter reason that we refer to them as "unequal hours."
We shall have occasion to speak further of these old unequal hours, but the modern reader may find it amusing to experiment with a roughly comparable table computed for modern hours and for the latitude of New York City or Chicago (Table 1.2). If you live fairly close to this latitude you may test the table by stepping off the length of your own shadow with your own feet and comparing the time estimated from the table with that shown on your watch.
Chaucer, who wrote his Canterbury Tales about 1390 or 1400 A.D., gives at least two illustrations of this method of telling the time of day. In the opening lines of his "Parson's Prologue" he says:
It was four o'clock according to my guess,
Since eleven feet, a little more or less,
My shadow at the time did fall,
Considering that I myself am six feet tall.
And near the opening of the Introduction to his "Man of Law's Tale" he tells us:
... the shadow of each tree
Had reached a length of that same quantity
As was the body which had cast the shade;
And on this basis he conclusion made:
... for that day, and in that latitude,
The time was ten o'clock....
But in many ways the direction of a shadow is a more satisfactory time-teller than its length. Boy Scouts are told that they can tell the direction from their watches. They are instructed to hold the watch face upwards and point the hour hand toward the sun. The south point will then lie, it is said, half way between the hour hand and 12 o'clock. This rule is actually very rough, but perhaps better than none at all.
We do not know when men first began to use instruments which were at all similar to modern sundials. A stone fragment in a Berlin museum is thought to be the earliest known sundial, dating from about 1500 B.C. The Bible mentions what some authorities take to have been a sundial (although the meaning is by no means certain) in the days of Ahaz, king of Judah some 700 years before Christ. About a century later the Greek philosopher and astronomer Anaximander of Miletus is said to have introduced the sundial into Greece. Herodotus, who lived in Asia Minor and Greece about 450 B.C., tells us that "It was from the Babylonians that the Greeks learned about the pole, the gnomon and the twelve parts of the day"; and sundials had become so common in Rome by 200 B.C. that the comic dramatist Plautus condemned in verse "the wretch who first ... set a sundial in the market place to chop my day to pieces." Vitruvius, a contemporary of Julius Caesar, bemoaned the fact that he could not invent new types of sundials, since the field was already exhausted. He lists a dozen or more types, giving the names of their inventors. We do not know anything about the appearance of many of these early dials, and cannot guess the degree of their accuracy.
Many medieval English churches carry what appear to be crude sundials cut or scratched directly into the stone of their walls. These appear to have been used primarily to note the times of the prayers. One of these dials, at Kirkdale in Yorkshire, carries an inscription in Old English which reads in part, "This is the day's sun-marker at every tide." This will be understood only if we realize that the Saxons divided the day not into hours, but into "tides"—from which we still get such words as "noontide" and "eventide."
Sometime and somewhere—no one knows when or where—it was discovered that the shadow cast by a slanting object might be a more accurate timekeeper than the shadow cast by a vertical one. If, in fact, the shadow-casting object was parallel to the earth's axis, the direction of its shadow at any given hour of the day was constant regardless of the season of the year. It has been suggested that this discovery occurred in the first century A.D., but be that as it may, men had now discovered the system which remained the principal basis for time-telling for nearly thirteen centuries. In fact, sundials remained in use long after the invention of the clock, since early clocks were erratic and needed frequent correction by the sundial. Our frontispiece reproduces an old print showing three gentlemen waiting to set their watches when the sun dial shows that the moment of noon has arrived, and many a New England housewife paced her morning's chores with the movement of the shadows across the kitchen floor. While men have used many other means of telling time—sandglasses, waterclocks, candles and graduated oil lamps (or else relied on the crowing of cocks and other natural phenomena), nevertheless for at least ten and perhaps twenty centuries the sundial was the major timekeeping device used by man.CHAPTER 2
Kinds of Time
American newspapers on July 21, 1969, featured accounts of man's first landing on the moon, and at various places they gave the time of Neil Armstrong's first step on the moon as:
10:56 P.M. Eastern Daylight Time on the 20th.
9:56 P.M. Eastern Standard Time on the 20th.
2:56 A.M. Greenwich Mean Time on the 21st.
3:56 A.M. British Summer Time on the 21st.
Here were four different ways of describing the same moment of time; yet they were but four of a great many possible ways which must certainly have been used by newspapers in various parts of the world as they interpreted the news for their readers. If we are to design a sundial to "tell time," we must first decide what kind of time it is to tell.
The Sun's Apparent Motion. Every schoolchild knows that the earth revolves around the sun even though it looks as though the sun were revolving around us. For our purposes it really makes no difference, since a sundial designed to tell time on an earth with the sun revolving around it would be identical in every detail with one designed for use on an earth which was revolving around the sun. In our treatment of the matter we shall ordinarily describe things as they seem rather than as they really are. We shall thus speak of the sun's "rising in the east" and "moving across the sky from east to west" until it "sets in the west" in the evening.
Differences in Longitude. A train running from New York to San Francisco appears in Albany before it reaches Chicago, and in Chicago before it reaches Denver. Similarly as the sun moves across the sky from east to west it appears first to people living on the East Coast, and later to people living farther west. When we see the sun directly south of us at midday it has already passed its high point for people to our east and they see it already falling in the west, while people to our west see it still rising higher in their eastern sky. They all see the same sun at the same moment, but they see it in different directions reflecting differences in their points of view.
The meridians are imaginary lines running along the earth's surface from the North to the South Pole, lying everywhere exactly in a north-south direction. Our own meridian is, then, nothing more than a north-south line running through the particular spot where we happen to be. At any moment of time the sun is over one of these meridians, and everyone who is located on that meridian says that it is noon. Everyone east of that meridian says it is afternoon, and everyone to the west calls it morning. If we are to keep time by the sun we must realize that all places on the same meridian (all places due north and south of each other) will have the same time, but in all other places on the earth's surface the time will be different—later in places to the east and earlier in places to the west. Places on the same meridian are said to have the same longitude, and we commonly measure longitudes by their angular distances east or west from the standard meridian which passes through Greenwich, England. One of the earth's meridians starts (like all meridians) at the North Pole, runs due south through Greenwich, continues south across the earth's Equator, and finally reaches the South Pole. This is the standard meridian, with longitude 0º. Another meridian runs from the North Pole through New York City across the Equator to the South Pole. The arc of the Equator between these two meridians measures 73º50', and since New York is west of Greenwich we say that the longitude of New York City is 73º50' west. Similarly the longitude of Tokyo, Japan, is 139º45' east of Greenwich. Since the sun makes one complete circuit of the heavens in 24 hours, passing over 360º of longitude, it obviously passes at the rate of 15º of longitude each hour, or through 1 degree each 4 minutes. If we note that the sun has covered an angle of 30º since it was last on our meridian we know that it is 2 hours past noon. The sun's angular distance from our meridian at any moment is called the sun's hour angle, and as we see from the preceding examples, if the sun's hour angle is 45º west, the time is 3 P.M., while when the hour angle is 30º east it is 10 A.M.
Local Apparent Time. If we are to tell the time by the sun's hour angle, no two points will share the same time unless they lie on the same meridian with one directly north of the other. The time is thus localized to a particular meridian, and since it is also based on the apparent motion of the sun we call it local apparent time and often symbolize it with its initials as L.A.T. This is the kind of time shown on most sundials, and until about a century ago it was the kind of time almost universally used in daily life. Yet it suffers from disadvantages which have led most people to discard it in favor of some other kind of time.
For one thing, it is inconvenient to use a system of timekeeping which is so narrowly localized. No two places have the same L.A.T. unless they lie on the same meridian. Two cities lying 100 miles apart in an east-west direction will differ by about 7 1/2 minutes in L.A.T., while two towns only 13 1/2 miles apart will differ in L.A.T. by 1 minute. There is even a difference of about 1/4 second in L.A.T. at opposite ends of a football field if it lies in an east- west direction, and precisely accurate clocks would show slightly different times in different rooms of the same house.
Local Mean Time. The second disadvantage of L.A.T. arises from the fact that when we measure days by the sun they turn out to differ among themselves in length. About Christmastime the days are about minute longer than average and in mid-September about 20 seconds shorter. These small discrepancies accumulate until in mid-February the sun reaches the meridian almost minutes later than it would if all days were equal in length, and early in November the sun reaches the meridian about minutes too early. These variations of 14 minutes one way and 16 minutes the other amount to just over hour, which would be decidedly inconvenient for scientific purposes, and today we would consider it unacceptable even for everyday affairs.
Instead, then, of reckoning time from the irregularly moving real sun, we usually reckon it from an imaginary mean sun—a fictitious heavenly body which moves in the celestial equator at a constant speed which is just equal to the average speed with which the real sun moves in the ecliptic. If the real sun and the mean sun start off together, the real sun, moving irregularly, will sometimes run ahead of the mean sun and sometimes lag behind it, but at the year's end they will finish the course together.
Time measured by the hour angle of the real or apparent sun is called apparent time, whereas if we measure the hour angle of the mean sun we find the mean time. Mean time has the advantage that it is uniform, but since the mean sun, like the real sun, is over but one meridian at a time, mean time, like apparent time, will be local. All places on a given meridian will have the same mean time, but no other places will share that time. Hence we call it local mean time, symbolized by L.M.T., as contrasted with L.A.T.
The Equation of Time. We are especially interested in the differences between L.A.T. and L.M.T., since most sundials, influenced by the real sun, show apparent time, while our clocks and watches, running regularly, show mean time. Thus as the real sun and the mean sun run their separate courses, sometimes one ahead and sometimes the other, our sundials and our watches will reflect the same differences. Sometimes our sundials will appear to be "fast" and sometimes to be "slow" when compared with an accurate watch. Neither the sundial nor the watch is "wrong." They merely record different kinds of time.
Four times each year the real sun and the fictitious mean sun are together, and on these dates the sundial and the clock agree. While the dates of agreement vary slightly from year to year as we adjust for leap years, they fall at about April 16, June 14, September 2, and December 25. If we compare our sundial with an accurate clock we will find that from the first of the year until April 16 the dial is running behind the clock, from April 16 until June 14 the dial is ahead, from June 14 until September 2 the dial is slow again and from September 2 until Christmas the dial is fast. For the final week of the year the dial is slow again.
Excerpted from Sundials by Albert E. Waugh. Copyright © 1973 Albert E. Waugh. Excerpted by permission of Dover Publications, Inc..
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