astronomy?" We know, of course, that men would profit greatly from
a study of that science, even if it could not be turned to any
immediate bread-and-butter use; for astronomy is essentially the
science of big things, and it makes men bigger to fix their minds
on problems that deal with vast distances and seemingly endless
periods of time. No one can look upon the quietly shining stars
without being impressed by the thought of how they burned--then
as now--before he himself was born, and so shall continue after
he has passed away--aye, even after his latest descendants shall
have vanished from the earth. Of all the sciences, astronomy is
at once the most beautiful poetically, and yet the one offering
the grandest and most difficult problems to the intellect. A study
of these problems has ever been a labor of love to the greatest
minds; their solution has been counted justly among man's loftiest
achievements.
And yet of all the difficult and abstruse sciences, astronomy is,
perhaps, the one that comes into the ordinary practical daily
life of the people more definitely and frequently than any other.
There exist at least three things we owe to astronomy that must
be regarded as quite indispensable, from a purely practical point
of view. In the first place, let us consider the maps in a work
on geography. How many people ever think to ask how these maps
are made? It is true that the ordinary processes of the surveyor
would enable us to draw a map showing the outlines of a part of
the earth's surface. Even the locations of towns and rivers might
be marked in this way. But one of the most important things of all
could not be added without the aid of astronomical observations.
The latitude and longitude lines, which are essential to show the
relation of the map to the rest of the earth, we owe to astronomy.
The longitude lines, particularly, as we shall see farther on, play
a most important part in the subject of time.
The second indispensable application of astronomy to ordinary
business affairs relates to the subject of navigation. How do ships
find their way across the ocean? There are no permanent marks on
the sea, as there are on the land, by which the navigator can guide
his course. Nevertheless, seamen know their path over the trackless
ocean with a certainty as unerring as would be possible on shore;
and it is all done by the help of astronomy. The navigator's
observations of the sun are astronomical observations; the tables
he uses in calculating his observations--the tables that tell him
just where he is and in what direction he must go--are astronomical
tables. Indeed, it is not too much to say that without astronomy
there could be no safe ocean navigation.
But the third application of astronomy is of still greater
importance in our daily life--the furnishing of correct time
standards for all sorts of purposes. It is to this practical use of
astronomical science that we would direct particular attention. Few
persons ever think of the complicated machinery that must be put
in motion in order to set a clock. A man forgets some evening to
wind his watch at the accustomed hour. The next morning he finds it
run down. It must be re-set. Most people simply go to the nearest
clock, or ask some friend for the time, so as to start the watch
correctly. More careful persons, perhaps, visit the jeweller's
and take the time from his "regulator." But the regulator itself
needs to be regulated. After all, it is nothing more than any other
clock, except that greater care has been taken in the mechanical
construction and arrangement of its various parts. Yet it is but
a machine built by human hands, and, like all human works, it is
necessarily imperfect. No matter how well it has been constructed,
it will not run with perfectly rigid accuracy. Every day there will
be a variation from the true time by a small amount, and in the
course of days or weeks the accumulation of these successive small
amounts will lead to a total of quite appreciable size.
Just as the ordinary citizen looks to the jeweller's regulator to
correct his watch, so the jeweller applies to the astronomer for
the correction of his regulator. Ever since the dawn of astronomy,
in the earliest ages of which we have any record, the principal
duty of the astronomer has been the furnishing of accurate time
to the people. We shall not here enter into a detailed account,
however interesting it would be, of the gradual development by
which the very perfect system at present in use has been reached;
but shall content ourselves with a description of the methods now
employed in nearly all the civilized countries of the world.
In the first place, every observatory is, of course, provided with
what is known as an astronomical clock. This instrument, from the
astronomer's point of view, is something very different from the
ordinary popular idea. To the average person an astronomical clock
is a complicated and elaborate affair, giving the date, day of the
week, phases of the moon, and other miscellaneous information. But
in reality the astronomer wants none of these things. His one and
only requirement is that the clock shall keep as near uniform time
as may be possible to a machine constructed by human hands. No
expense is spared in making the standard clock for an observatory.
Real artists in mechanical construction--men who have attained a
world-wide celebrity for delicate skill in fashioning the parts of
a clock--such are the astronomer's clock-makers.
To increase precision of motion in the train of wheels, it is
necessary that the mechanism be as simple as possible. For this
reason all complications of date, etc., are left out. We have
even abandoned the usual convenient plan of having the hour and
minute hands mounted at the same centre; for this kind of mounting
makes necessary a slightly more intricate form of wheelwork. The
astronomer's clock usually has the centres of the second hand,
minute hand, and hour hand in a straight line, and equally distant
from each other. Each hand has its own dial; all drawn, of course,
upon the same clock-face.
Even after such a clock has been made as accurately as possible,
it will, nevertheless, not give the very best performance unless
it is taken care of properly. It is necessary to mount it very
firmly indeed. It should not be fastened to an ordinary wall, but
a strong pier of masonry or brick must be built for it on a very
solid foundation. Moreover, this pier is best placed underground
in a cellar, so that the temperature of the clock can be kept
nearly uniform all the year round; for we find that clocks do
not run quite the same in hot weather as they do in cold. Makers
have, indeed, tried to guard against this effect of temperature,
by ingenious mechanical contrivances. But these are never quite
perfect in their action, and it is best not to test them too
severely by exposing the clock to sharp changes of heat and cold.
Another thing affecting the going of fine clocks, strange as it may
seem, is the variation of barometric pressure. There is a slight
but noticeable difference in their running when the barometer is
high and when it is low. To prevent this, some of our best clocks
have been enclosed in air-tight cases, so that outside barometric
changes may not be felt in the least by the clock itself.
But even after all this has been accomplished, and the astronomer
is in possession of a clock that may be called a masterpiece of
mechanical construction, he is not any better off than was the
jeweller with his regulator. After all, even the astronomical clock
needs to be set, and its error must be determined from time to
time. A final appeal must then be had to astronomical observations.
The clock must be set by the stars and sun. For this purpose the
astronomer uses an instrument called a "transit." This is simply
a telescope of moderate size, possibly five or six feet long, and
firmly attached to an axis at right angles to the tube of the
telescope.
This axis is supported horizontally in such a way that it points
as nearly as may be exactly east and west. The telescope itself
being square with the axis, always points in a north-and-south
direction. It is possible to rotate the telescope about its axis so
as to reach all parts of the sky that are directly north or south
of the observatory. In the field of view of the telescope certain
very fine threads are mounted so as to form a little cross. As the
telescope is rotated this cross traces out, as it were, a great
circle on the sky; and this great circle is called the astronomical
meridian.
Now we are in possession of certain star-tables, computed from
the combined observations of astronomers in the last 150 years.
These tables tell us the exact moment of time when any star is
on the meridian. To discover, therefore, whether our clock is
right on any given night, it is merely necessary to watch a star
with the telescope, and note the exact instant by the clock when
it reaches the little cross in the field of view. Knowing from
the astronomical tables the time when the star ought to have been
on the meridian, and having observed the clock time when it is
actually there, the difference is, of course, the error of the
clock. The result can be checked by observations of other stars,
and the slight personal errors of observation can be rendered
harmless by taking the mean from several stars. By an hour's work
on a fine night it is possible to fix the clock error quite easily
within the one-twentieth part of a second.
We have not space to enter into the interesting details of the
methods by which the astronomical transit is accurately set in
the right position, and how any slight residual error in its
setting can be eliminated from our results by certain processes
of computation. It must suffice to say that practically all time
determinations in the observatory depend substantially upon the
procedure outlined above.
The observatory clock having been once set right by observations
of the sky, its error can be re-determined every few days quite
easily. Thus even the small irregularities of its nearly perfect
mechanism can be prevented from accumulating until they might reach
a harmful magnitude. But we obtain in this way only a correct
standard of time within the observatory itself. How can this be
made available for the general public? The problem is quite simple
with the aid of the electric telegraph. We shall give a brief
account of the methods now in use in New York City, and these may
be taken as essentially representative of those employed elsewhere.
Every day, at noon precisely, an electric signal is sent out by
the United States Naval Observatory in Washington. The signal is
regulated by the standard clock of the observatory, of course
taking account of star observations made on the next preceding
fine night. This signal is received in the central New York office
of the telegraph company, where it is used to keep correct a
very fine clock, which may be called the time standard of the
telegraph company. This clock, in turn, has automatic electric
connections, by means of which it is made to send out signals
over what are called "time wires" that go all over the city.
Jewellers, and others who desire correct time, can arrange to have
a small electric sounder in their offices connected with the time
wires. Thus the ticks of the telegraph company's standard clock
are repeated automatically in the jeweller's shop, and used for
controlling the exactness of his regulator. This, in brief, is the
method by which the astronomer's careful determination of correct
time is transferred and distributed to the people at large.
Having thus outlined the manner of obtaining and distributing
correct time, we shall now consider the question of time
differences between different places on the earth. This is a matter
which many persons find most perplexing, and yet it is essentially
quite simple in principle. Travellers, of course, are well
acquainted with the fact that their watches often need to be reset
when they arrive at their destination. Yet few ever stop to ask the
cause.
Let us consider for a moment our method of measuring time. We go
by the sun. If we leave out of account some small irregularities
of the sun's motion that are of no consequence for our present
purpose, we may lay down this fundamental principle: When the sun
reaches its highest position in the sky it is twelve o'clock or
noon.
The sun, as everyone knows, rises each morning in the east, slowly
goes up higher and higher in the sky, and at last begins to descend
again toward the west. But it is clear that as the sun travels
from east to west, it must pass over the eastern one of any two
cities sooner than the western one. When it reaches its greatest
height over a western city it has, therefore, already passed its
greatest height over an eastern one. In other words, when it is
noon, or twelve o'clock, in the western city, it is already after
noon in the eastern city. This is the simple and evident cause of
time differences in different parts of the country. Of any two
places the eastern one always has later time than the western.
When we consider the matter in this way there is not the slightest
difficulty in understanding how time differences arise. They will,
of course, be greatest for places that are very far apart in an
east-and-west direction. And this brings us again to the subject of
longitude, which, as we have already said, plays an important part
in all questions relating to time; for longitude is used to measure
the distance in an east-and-west direction between different parts
of the earth.
If we consider the earth as a large ball we can imagine a series of
great circles drawn on its surface and passing directly from the
North Pole to the South Pole. Such a circle could be drawn through
any point on the earth. If we imagine a pair of them drawn through
two cities, such as New York and London, the longitude difference
of these two cities is defined as the angle at the North Pole
between the two great circles in question. The size of this angle
can be expressed in degrees. If we then wish to know the difference
in time between New York and London in hours, we need only divide
their longitude difference in degrees by the number 15. In this
simple way we can get the time difference of any two places. We
merely measure the longitude difference on a map, and then divide
by 15 to get the time difference. These time differences can
sometimes become quite large. Indeed, for two places differing 180
degrees in longitude, the time difference will evidently be no less
than twelve hours.
Most civilized nations have agreed informally to adopt some one
city as the fundamental point from which all longitudes are to
be counted. Up to the present we have considered only longitude
differences; but when we speak of the longitude of a city we mean
its longitude difference from the place chosen by common consent as
the origin for measuring longitudes. The town almost universally
used for this purpose is Greenwich, near London, England. Here
is situated the British Royal Observatory, one of the oldest and
most important institutions of its kind in the world. The great
longitude circle passing through the centre of the astronomical
transit at the Greenwich observatory is the fundamental longitude
circle of the earth. The longitude of any other town is then simply
the angle at the pole between the longitude circle through that
town and the fundamental Greenwich one here described.
Longitudes are counted both eastward and westward from Greenwich.
Thus New York is in 74 degrees west longitude, while Berlin is in
14 degrees east longitude. This has led to a rather curious state
of affairs in those parts of the earth the longitudes of which are
nearly 180 degrees east or west. There are a number of islands
in that part of the world, and if we imagine for a moment one
whose longitude is just 180 degrees, we shall have the following
remarkable result as to its time difference from Greenwich.
We have seen that of any two places the eastern always has the
later time. Now, since our imaginary island is exactly 180 degrees
from Greenwich, we can consider it as being either 180 degrees east
or 180 degrees west. But if we call it 180 degrees east, its time
will be twelve hours later than Greenwich, and if we call it 180
degrees west, its time will be twelve hours earlier than Greenwich.
Evidently there will be a difference of just twenty-four hours, or
one whole day, between these two possible ways of reckoning its
time. This circumstance has actually led to considerable confusion
in some of the islands of the Pacific Ocean. The navigators who
discovered the various islands naturally gave them the date which
they brought from Europe. And as some of these navigators sailed
eastward, around the Cape of Good Hope, and others westward, around
Cape Horn, the dates they gave to the several islands differed by
just one day.
The state of affairs at the present time has been adjusted by a
sort of informal agreement. An arbitrary line has been drawn on
the map near the 180th longitude circle, and it has been decided
that the islands on the east side of this line shall count their
longitudes west from Greenwich, and those west of the line shall
count longitude east from Greenwich. Thus Samoa is nearly 180
degrees west of Greenwich, while the Fiji Islands are nearly 180
degrees east. Yet the islands are very near each other, though the
arbitrary line passes between them. As a result, when it is Sunday
in Samoa it is Monday in the Fiji Islands. The arbitrary line
described here is sometimes called the International Date-Line.
It does not pass very near the Philippine Islands, which are
situated in about 120 degrees east longitude, and, therefore, use
a time about eight hours later than Greenwich. New York, being
about 74 degrees west of Greenwich, is about five hours earlier in
time. Consequently, as we may remark in passing, Philippine time is
about thirteen hours later than New York time. Thus, five o'clock,
Sunday morning, May 1st, in Manila, would correspond to four
o'clock, Saturday afternoon, April 30th, in New York.
There is another kind of time which we shall explain briefly--the
so-called "standard," or railroad time, which came into general
use in the United States some few years ago, and has since been
generally adopted throughout the world. It requires but a few
moments' consideration to see that the accidental situation of
the different large cities in any country will cause their local
times to differ by odd numbers of hours, minutes, and seconds.
Thus a great deal of inconvenience has been caused in the past.
For instance, a train might leave New York at a certain hour by
New York time. It would then arrive in Buffalo some hours later by
New York time. But it would leave Buffalo by Buffalo time, which
is quite different. Thus there would be a sort of jump in the
time-table at Buffalo, and it would be a jump of an odd number of
minutes.
It would be different in different cities, and very hard to
remember. Indeed, as each railway usually ran its trains by the
time used in the principal city along its line, it might happen
that three or four different railroad times would be used in a
single city where several roads met. This has all been avoided by
introducing the standard time system. According to this the whole
country is divided into a series of time zones, fifteen degrees
wide, and so arranged that the middle line of each zone falls at
a point whose longitude from Greenwich is 60, 75, 90, 105, or 120
degrees. The times at these middle lines are, therefore, earlier
than Greenwich time by an even number of hours. Thus, for instance,
the 75-degree line is just five even hours earlier than Greenwich
time. All cities simply use the time of the nearest one of these
special lines.
This does not result in doing away with time differences
altogether--that would, of course, be impossible in the nature
of things--but for the complicated odd differences in hours
and minutes, we have substituted the infinitely simpler series
of differences in even hours. The traveller from Chicago to New
York can reset his watch by putting it just one hour later on
his arrival--the minute hand is kept unchanged, and no New York
timepiece need be consulted to set the watch right on arriving.
There can be no doubt that this standard-time system must be
considered one of the most important contributions of astronomical
science to the convenience of man.
Its value has received the widest recognition, and its use has now
extended to almost all civilized countries--France is the only
nation of importance still remaining outside the time-zone system.
In the following table we give the standard time of the various
parts of the earth as compared with Greenwich, together with the
date of adopting the new time system. It will be noticed that in
certain cases even half-hours have been employed to separate the
time-zones, instead of even hours as used in the United States.
TABLE OF THE WORLD'S TIME STANDARDS
----------------+---------------------++------------------
When it is Noon | || Date of Adopting
at Greenwich | In || Standard Time
it is | || System.
----------------+---------------------++------------------
Noon | Great Britain. ||
| Belgium. || May, 1892.
| Holland. || May, 1892.
| Spain. || January, 1901.
1 P.M. | Germany. || April, 1893.
| Italy. || November, 1893.
| Denmark. || January, 1894.
| Switzerland. || June, 1894.
| Norway. || January, 1895.
| Austria (railways). ||
1.30 P.M. | Cape Colony. || 1892.
| Orange River Colony.|| 1892.
| Transvaal. || 1892.
2 P.M. | Natal. || September, 1895.
| Turkey (railways). ||
| Egypt. || October, 1900.
8 P.M. | West Australia. || February, 1895.
9 P.M. | Japan. || 1896.
9.30 P.M. | South Australia. || May, 1899.
10 P.M. | Victoria. || February, 1895.
| New South Wales. || February, 1895.
| Queensland. || February, 1895.
11 P.M. | New Zealand. ||
----------------+---------------------++------------------
In the United States and Canada it is
4 A.M. by Pacific Time when it is Noon at Greenwich.
5 A.M. " Mountain " " " " " "
6 A.M. " Central " " " " " "
7 A.M. " Eastern " " " " " "
8 A.M. " Colonial " " " " " "
MOTIONS OF THE EARTH'S POLE
Students of geology have been puzzled for many years by traces
remaining from the period when a large part of the earth was
covered with a heavy cap of ice. These shreds of evidence all seem
to point to the conclusion that the centre of the ice-covered
region was quite far away from the present position of the north
pole of the earth. If we are to regard the pole as very near the
point of greatest cold, it becomes a matter of much interest to
examine whether the pole has always occupied its present position,
or whether it has been subject to slow changes of place upon
the earth's surface. Therefore, the geologists have appealed to
astronomers to discover whether they are in possession of any
observational evidence tending to show that the pole is in motion.
Now we may say at once that astronomical research has not as yet
revealed the evidence thus expected. Astronomy has been unable to
come to the rescue of geological theory. From about the year 1750,
which saw the beginning of precise observation in the modern sense,
down to very recent times, astronomers were compelled to deny the
possibility of any appreciable motion of the pole. Observational
processes, it is true, furnished slightly divergent pole positions
from time to time. Yet these discrepancies were always so minute as
to be indistinguishable from those slight personal errors that are
ever inseparable from results obtained by the fallible human eye.
But in the last few years improved methods of observation, coupled
with extreme diligence in their application by astronomers
generally, have brought to light a certain small motion of the pole
which had never before been demonstrated in a reliable way. This
motion, it is true, is not of the character demanded by geological
theory, for the geologists had been led to expect a motion which
would be continuous in the same direction, no matter how slow might
be its annual amount; for the vast extent of geologic time would
give even the slowest of motions an opportunity to produce large
effects, provided its results could be continuously cumulative.
Given time enough, and the pole might move anywhere on the earth,
no matter how slow might be its tortoise speed.