THE TIME O’ DAY
=What Time Is It?=—This is a question everybody is always asking
everybody else.
Did you ever stop to think what a curious thing _time_ is? No one knows
when it began nor can anyone tell when it will end, yet we measure off
a little bit from that which has gone or from that which is yet to
come, so that we may know when to eat, to start to work, or to quit,
and when to go to bed or to get up.
We know that, roughly, a year is the time it takes the four seasons to
come and go, and that this is done when the Earth travels once round
the Sun; that the month is based on the time it takes the Moon to
travel once round the Earth; that the week has nothing to do with the
Sun, Earth, Moon or Stars, but is a pure invention, and, finally, that
the day is the time it takes the Earth to turn once round on its axis.
But when we want to know what time it is we mean, of course, what the
hour, the minute and, sometimes, even the second of the day is, and
these are the small measured parts of time we want to find out about.
The time it takes the Earth to make one complete turn on its axis is
divided naturally into two parts, more or less equal, depending on
where we live, and these parts are daytime and nighttime.
This general division of time, marked by alternate daylight and
darkness, may have served every need of the cave man at first, but just
as he came to have sense enough to crawl into his cave to get out of
the rain so the blazing Sun must finally have driven into his awakening
brain its use to him as a means of marking time.
As his savage mind grew less animal and more human, ideas were formed
in it either by instinct or by the first vague glimmerings of reason
and he began to think. He saw that when the light of the Sun fell on
the trees and the rocks, long, strange, black marks, which we call
shadows, were cast by them, and he must have noticed that the shadows
swung round the trees and rocks in the opposite direction to the way
the Sun was traveling.
To mark off with his eye and place a stone at a point somewhere near
the middle of the shadow cast by the rising Sun and that cast by the
setting Sun, and which would mean that the day was half done was the
next great step.
These things were, quite likely, the first feeble efforts of the human
race to measure time, and out of which the sundial came, as well as the
crude beginnings on which the science of astronomy is based.
=Solar Time, or Time by the Sun.=—To make a sundial which would give
correct Sun time was so hard a problem that men had to think about it
for a million years before they could solve it and the chances are that
then it was invented by a boy. You will find directions for making a
simple sundial in Chapter III.
Now let’s find out how we can know when a day begins and when it ends.
On first thought this would seem to be an easy thing to do and it is if
we are not particular about being exact. You will remember that a day
is measured by the time it takes the Earth to turn once round on its
axis and what we want to know, now, is how to tell when the Earth has
made one complete turn, no more and no less.
We can tell this, you may say, by a clock or a watch, but the best of
clocks and watches are always a little fast or a little slow, and time
today is measured by the fraction of a second. So we get the _apparent
time_ from the Sun, change it into _mean time_ and set our clocks and
watches by it.
There are several ways by which we can obtain Sun time, but all of them
are based on the same principle. The way that was used by people who
first began to think about these things, and it is also a good way for
you to try, is like this:
[Illustration: FIG. 162.—DIAGRAM SHOWING HOW TO FIND SOLAR NOON.]
First you must have a good _horizon_, that is, you must be able to see
the Sun rise and set without any mountains or other things in the way.
When you begin your observations for getting solar time note exactly
where the Sun rises and where it sets on the horizon. You can easily do
this by using hills, houses and trees for marking the places. Now with
your eye draw a line between these two points.
Notice also where you are standing and let your line of sight meet the
imaginary line which joins the places where the Sun rises and sets just
as near the middle as you can, and all of which is clearly shown in
Fig. 162. Now this line will run due north and south and hence it is a
_meridian line_ which you are to use to observe the Sun.
When the Sun reaches that point in the sky where it is directly over
the middle of the imaginary line, joining the places where it rises and
sets, it is exactly noon, _Sun time_.
The next day observe the Sun in the same way and when it crosses the
meridian line again the Earth will have turned round once and you will
have a part of time measured off called a Sun or _solar day_.
When the time is taken between two succeeding crossings of a meridian
by the Sun a day so measured is called by astronomers an _apparent
solar day_, and when the Sun is on the meridian it is called _apparent
noon_.
Now the word _apparent_ means to seem, that is, something which is
obtained by observation. An apparent solar day is then the length of
a day measured by the Sun, and while we might suppose that the Sun at
least would always give a day of the same length, this is not the case
for the reason that the Earth does not travel in all parts of its orbit
round the Sun at the same rate of speed, and, further, it is tilted on
its axis; together these things make the days as measured off by the
Sun unequal and hence they are called apparent solar days.
=Mean Solar Time.=—Apparent solar days which are of unequal length were
all right as long as sundials were the only timepieces, but when clocks
and watches came into use days which were equal in length were needed
and needed badly, for a clock couldn’t be made which would keep Sun
time.
So astronomers who watched the stars by night lay awake during the day
wondering how they could make the Earth travel round the Sun at the
same speed every day in the year and just as though it was not tilted.
At last they solved the problem.
And how do you think they did it? It was as easy as rolling off a
log—when you know how. They simply _imagined_ that the Earth traveled
at a uniform speed and that it stood straight, as shown in Fig. 74.
In other words they took the _mean length_, which is another way of
saying the average length of all the apparent solar days which make up
a year, and divided it up equally. Further, the men who got up this
scheme said that every day should have not only the same length, but
that it should have 24 hours, and of course you know that hours are
divided into minutes and minutes into seconds. _Mean solar time_, then,
is really _imaginary Sun time_ and this is the time used everywhere and
watches and clocks are set by it.
=Equation of Time.=—To get the exact mean time each day you have to
know just what the apparent solar time is, and then you have to know
what the difference in time is between the apparent solar time and the
mean time, that is how many minutes and seconds to add to or subtract
from the solar time of each day to get the mean time.
This difference of time is called the _equation of time_. A table
prepared by Professor Todd of the Amherst College Observatory is given
in Appendix M and can be used for all ordinary purposes.
=Standard Time.=—A meridian, as you know, is an imaginary line running
due north and South and hence we can have a meridian whenever we want
it and as many as we like.
When the Sun crosses the meridian of those who are on it, it is noon to
them, but to no one else, for the Sun has already crossed the meridians
to the east of it and has yet to cross those to the west of it. When
the Sun crosses the meridian which passes through New York City it is
noon there and when the Sun crosses the meridians which pass through
St. Louis, Denver and San Francisco, it is noon at those places, and
this is true of every other place and of every other hour of the day.
This is the reason why every place had its own, or _local time_, before
the year of 1883.
As an illustration it takes the Sun about three hours to cross the
United States from the Atlantic to the Pacific coasts.
Now as long as people traveled on foot, or by horse, they moved so
slowly that local time did not worry them, but when railroads came into
use there was all kinds of trouble for the traveler; if he was going
west his watch was faster than the local time of the towns he passed
through and if he was going east his watch was always slower than the
local time. If he wanted the right time he had to set his watch at
every town he passed through; of course he couldn’t very well do this
and he was always in a stew.
The railroad companies were just as much put to for it was next to
impossible to make a timetable to fit the local time of each town and
still keep up a running schedule; and the result was that the railroads
finally got up a system of their own which they called _railroad time_.
This was all well enough for everybody but the poor traveler, who, not
knowing the difference in time between local time and railroad time,
nearly always found he was either an hour too early or—as it usually
happened—a minute or two too late to catch his train.
As new towns sprung up and railroads multiplied, things had come to
such a pretty pass in 1883 that nobody but the astronomers knew what
the real time was, and they wouldn’t tell; then a new time scheme was
tried out, and as it is still used we must conclude it is a fairly
good one. It is called the _zone_, or _belt system of standard time_;
the time used is called standard time because the towns and cities and
railroads all use it and there is no confusion.
To understand what _standard time_ means we have to know first what a
_standard meridian_ is. A meridian, as we have said, is an imaginary
line running due north and south anywhere we want it, but while a
standard meridian is also a line running due north and south it has a
fixed position.
The first fixed or _prime meridian_, as it is called, passes through
_Greenwich_ (pronounced Gren´-ij), which is a part of London, England.
The reason this meridian was chosen by geographers to reckon distance
east and west from is because the Royal Observatory at Greenwich is one
of the oldest in the world and it was the first from which exact time
was sent out.
If a circle is divided into 360 degrees, as shown in Fig. 163, and
it is also divided into 24 parts, each part will be a space equal to
15 degrees or 1 hour. Now geographers have divided the Earth into 24
equal parts by meridians separated by 15 degrees, and each space, or
belt, between them represents 1 hour, as shown in Fig. 164. These fixed
meridians start at the first, or prime meridian, at Greenwich and all
the other meridians are measured in degrees east or west of Greenwich
as the case may be.
[Illustration: FIG. 163.—CIRCLE DIVIDED INTO 360 DEGREES AND 24 HOURS.]
Starting west from the first, or prime meridian, which passes through
Greenwich the time at the second standard meridian, which is called the
15th meridian because it is 15 degrees from the first meridian, will be
one hour behind Greenwich time.
At every standard meridian the mean solar time is used as the standard
time and every place on it and halfway to the meridian on both sides
uses it, and so local time and standard.
This makes it very convenient for the traveler, for instead of setting
his watch at each station he does not need to set it until he has
traveled 15 degrees east or west, which is about 700 miles in our
northern latitudes, and then he turns it exactly one hour ahead or one
hour back, depending on the direction he is going.
[Illustration: FIG. 164.—THE EARTH DIVIDED INTO 24 STANDARD MERIDIANS.]
There are four standard time meridians running through the United
States, as shown in the map in Fig. 165. The one running through New
York, Pennsylvania, New Jersey and Delaware is the 75th meridian,
meaning of course that it is 75 degrees west of the prime meridian
which passes through Greenwich. Time on this meridian and halfway to
the meridians on both sides of it is called _Eastern Time_. It is just
five hours slower than Greenwich time.
[Illustration: FIG. 165.—STANDARD TIME MERIDIANS IN U. S.]
The next is the 90th meridian and this one passes through Wisconsin,
Illinois, Missouri, Tennessee, Mississippi and Louisiana. Time on and
around this meridian is called _Central Time_.
The one after this is the 105th meridian and passes through Montana,
Wyoming, Colorado, New Mexico and Texas. Time on and around this
meridian is called _Mountain Time_. The last of the four meridians, the
120th, passes through the States of Washington, Oregon and California
and time on and around this one is called _Pacific Time_. It is three
hours slower than Eastern time and 8 hours slower than Greenwich time.
Although these meridians are just one hour apart the time is not
changed on them nor exactly in the middle of a belt but at some
well-known town or city between two of the meridians, as you will see
by looking at the map, Fig. 165. Fig. 166 shows the standard time at
different cities around the world north of the equator.
=Star or Sidereal Time.=—Besides all the different kinds of time
described above there is still another and a very important kind of
time, and this is obtained by the stars.
Just as Sun, or solar time is obtained by noting when the Sun crosses
a meridian, so _star_, or _sidereal time_ is obtained by observing
when a star crosses a meridian. Now there is a difference between the
length of a day when formed by the Sun crossing the meridian twice in
succession and when formed by a star crossing the meridian twice in
succession. This difference in time, between a solar day and a sidereal
day, as they are called, is nearly four minutes.
But the point is this: an astronomer can obtain the time from watching
a star cross the meridian much more accurately than he can from the
Sun, because a star is a mere point of light, and it is easier for him
to calculate the mean solar time from the _transit_ of a star than it
is for him to go to bed, and besides he would rather do it, too.
=How Time Is Distributed.=—In this country the correct standard time is
sent out by the United States Naval Observatory at Washington to all
cities east of the Rocky Mountains, by _wire telegraph_, and all over
the Atlantic ocean and seaboard by _wireless telegraph_.
[Illustration: FIG. 166.—STANDARD TIME AT DIFFERENT CITIES.]
When time is received over the wires from Washington it is distributed
by local telegraph or by _time balls_ to various jewelry stores and to
private citizens who always want to be set right.
How an astronomer gets the correct time; how it is sent out over the
wires and by wireless and how it is distributed to the common people is
a mighty interesting piece of business. Briefly it is like this:
=How Correct Time Is Obtained.=—Every observatory has, besides its big
telescopes, a _transit instrument_, a wonderfully accurate _clock_,
and a clockwork device called a _chronograph_. The transit instrument
is nothing more than a telescope with a thin piece of clear glass with
a number of lines ruled on it with a diamond, about ⅛ inch apart, and
this ruled glass is set between the eyepiece and the object glass, as
shown in Fig. 167.
[Illustration: FIG. 167.—RULED GLASS IN TRANSIT INSTRUMENT.]
This telescope, or transit instrument—so called because it is used to
observe the _transit_, or passage, of a star across a meridian—is set
on an axis so that the telescope can be pointed to any place on the
meridian but it cannot be moved east or west.
Sometime before a star is due to cross the meridian the astronomer sets
his transit instrument so that the star will pass right across the line
of sight of his telescope.
The purpose of observing the transit, or passing of a star is to see
how much his wonderfully accurate clock has lost or gained during the
past 24 hours. So his clock is right at hand.
The chronograph is another accurate clockwork which revolves a cylinder
about the size and shape of a phonograph cylinder, and around which
is wrapped a sheet of white paper. This cylinder makes one revolution
every minute. A fountain pen marks a spiral line on the paper when the
cylinder is revolving but at every second the pen is thrown out of
position and this makes a notch in the line.
After starting the chronograph the astronomer takes an electric push
button, which is connected with and controls the lever which holds
the pen of the chronograph that makes the notches, and takes up his
position with his eye at the end of the transit instrument.
The instant he sees the star on one of the lines in the telescope he
presses the button and this closes the electric circuit and makes a big
notch in the line traced on the paper of the chronometer.
From the position of the big notches which he caused to be made on the
paper as the star crossed the ruled lines, and the little notches made
regularly every second, he can dope out just how much his clock is in
error—that is, how much it is too fast or too slow.
The next thing he does is to change this absolutely correct star time
into mean solar time, when it is ready to be sent all over the United
States east of the Rocky Mountains by telegraph. The Pacific Coast
folks get their correct time from a Government observatory at Mare
Island in San Francisco Bay.
=How Time Is Sent by Telegraph.=—The wires of the Western Union
Telegraph Company run into the United States Naval Observatory at
Washington, and for a few minutes each day all of this company’s wires
are controlled by the Government.
About five minutes before 12 o’clock noon, standard time at Washington,
each day the wires all over the country east of the Rockies are
_cleared_ and all business and other messages are cut off for the time
signals.
At five minutes of 12 sharp the United States Naval Observatory
begins to send the beats of every second of the wonderfully accurate
observatory clock, which are ticked out on telegraph sounders in all
the cities and towns.
But no, not every beat, for the 29th second of each minute, the last 5
seconds of each of the first 4 minutes, and then the last 10 seconds of
the last minute are not sent. The first click of the sounder after the
10 second rest is the noon signal, and all local clocks are set by it.
In many cities the telegraph company has special wires running to
various jewelry and other stores and these subscribers get the correct
time direct by telegraph from the Naval Observatory for their sounders
are connected in the regular line circuit.
=The Time Ball.=—The Bureau of Navigation got up what is known as the
_time ball_ for the benefit of sailing masters in particular and the
townsfolk in general.
[Illustration: FIG. 168.—THE TIME BALL.]
In New York and other cities along the seacoasts and lake ports a great
ball, weighing in the neighborhood of 100 pounds, having a diameter of
over 3 feet and with a hole in its center, is slipped over a pole or
flagstaff some 20 feet in height, which is mounted atop of some high
building where it can be seen to the best advantage.
The ball is held in position at the top of the pole by an
electro-mechanical trigger, which is placed directly in the electric
telegraph circuit that runs into the Naval Observatory at Washington.
When the time signals are being sent out from the observatory and the
telegraph key is closed at exactly noon the trigger which holds the
ball in place is released by the current and the ball drops. Fig. 168
shows a time ball atop of the old Western Union Building in New York.
[Illustration: FIG. 169.—RECEIVING TIME SIGNALS BY WIRELESS.]
=How Time is Sent by Wireless.=—At Arlington, Va., just across the
Potomac River from Washington, is one of the most powerful wireless
stations in the world, having a sending range of at least 3,000 miles.
Every day at noon time signals like those sent over the wires on land
are sent out so that every navigation officer and sailing master whose
ships are fitted with wireless apparatus can get the correct time.
All over the Atlantic seaboard boys, as well as jewelers (see Fig.
169), have wireless receiving sets and they receive the correct time
every day by wireless free of charge. No license is required to
receive wireless signals, the cost of the apparatus is little and the
experience immense. Are you in on it?