III . THE MOON
The Moon is the body nearest to us in the sky. It is so near, from the point of
view of astronomy, which has to do with very long distances, that it may be said to
be almost on top of us, the distance being so small that it would be bridged over by 30 earths,
placed one on top the other. A telegram would get there is some seconds. Its distance is only 1/400
that of the Sun, and about 1/100,000,000 that of the nearest fixed star.
If the space between the Earth and the Moon
was completely full of air, and a very
loud gun was let off on the Moon, we would
see the flame in a second or two, though
it would take almost 14 days for the sound
of the firing to get to us.
The Moon's common distance, measured
from a point at the middle of the Earth, is
238,833 miles.
1 The distance through
the Moon, its 'diameter ', is 2,160 miles.
26
The Moon goes round the Earth completely
in about 27-1/3 days or 2,360,591
seconds (measuring its position by the
stars).
It was by the help of the Moon that the
attraction between all substances in space
was first made clear and measured.
Taking the number of seconds given and
making a division by them of the complete
distance of the journey round the Earth
made by the Moon in the time named, we
get a rate of 3,357 feet a second as the rate
of motion of the Moon. If, then, the Moon
is moving in a straight line and we take
the measure of its position after a second
or a given number of seconds, we are able
to say where it may be looked for. But
we make the discovery that it is not there.
The reason is that in place of going in a
straight line, it has been going in a curve ;
in other words, it has been coming down
in the direction of the Earth. The amount
by which its motion is different from a
straight line is very small. It is only
52/1000 of an inch in every second. This
number gives the motion of the Moon
in
the direction of the Earth in a second.
27
We are able to make a measure of, let
us say, a stone falling to the Earth, which
will give us the rate at which it will come
down from any point. In addition, we
are able to make tests so as to be certain
that the numbers so got are true.
The general idea formed by the mind of
Newton was, then, that the Moon is
falling
in the direction of the Earth in the same
way as the stone, and from the same cause,
that is, the attraction present between all
substances, and which is acting between
the Earth and the Moon.
This is the relation between increase in
distance and the change in attraction :
if the distance is twice as great the force is
¼ of what it was ; at three times the
distance, the force is 1/9 ; at four times the
distance the force is 1/16 , and so on. That
is, the force gets smaller not in relation to
the greater distance, but to the square of
that amount, or if the distance becomes
less the force becomes greater by the
square of the degree to which the distance
is less.
On working out this idea, Newton saw
that if a stone was taken up to the level of
the Moon and let go, it would come down
in the direction of the Earth at the same
rate as the Moon comes down out of the
straight line.
28
This discovery was made the base of a
general idea, and it is now the View of all
those having any authority on this
question, that the same law of attraction,
becoming smaller in quite the same way
with increase of distance, is true of all
bodies and all substance everywhere in
space, the force being dependent on the
mass (or amount of substance) present.
This view has in fact become more and
more deeply rooted as the outcome of the
great number of tests to which it has been
put.
What is this force of attraction, and
what is its way of acting ? The reader
may put this question, and rightly. But
there is no good and complete answer.
Even the mind of Newton, which gave
birth to the great general theory of the law
of attraction everywhere in space acting
on all substance, was not able, though he
gave much attention and work to this
question, to come upon any answer which
would take into account all the conditions.
And the theory of Einstein in his discovery
of ' relativity ' is in the same position.
The answer given by him is not one which
may be pictured by the mind of man,
though the mathematical statement on
which it is based seems not to be open
to discussion.
29
The force of attraction is not like light,
for example, in its way of acting. Light
takes a certain time to get to us from the
Sun and stars, and if the same time was
needed by the force of attraction, the
motions of the planets would be different
from what they are. So it is clearly not a
motion, but a condition of space round
a body, like magnetism -- a greater sort of
, magnetism acting not only on certain
metals, but on all substances. In using
the words " a condition of space " the
writer does not have in mind space with
nothing in it, but a condition of the material
of which space everywhere is taken to
be full, and by which light comes to us in
the form of waves. This material is
named the ether.
Now called dark energy
The Moon is a dark body, like the
Earth, giving off no light itself, and of
which we are able to see only that part on
which the light of the Sun is falling at the
time. Being a round ball, one half of it
is, like the Earth, at all times in the sunlight.
At the new Moon, the half turned
away from us is in the Sun and in its light,
and at full Moon the half turned in our
30
direction is facing the Sun. In the same
way we get the first quarter, after the
new Moon, and the last quarter after the
full Moon. The reason is naturally that
when the Moon goes round the Earth it is
sometimes between the Sun and the Earth
(new Moon) and sometimes the Earth is
in the middle, between the Sun and the
Moon (full Moon), and there are all the
other positions between these.
The Moon itself goes round the Earth
from west to east, though naturally it
takes part in what seems to be the motion
of the sky from east to west.
Measured by its position among the
stars, as we said before, the journey of the
Moon round the Earth takes about 27
days 7 hours 43 minutes. But it is not
then back in the same position in relation
to the Sun and the Earth. In that time
the Moon and Earth together have been
moving round the Sun from west to east,
and if the starting-point was the new
Moon, about 2 days and 5 hours are
needed in addition for the Moon to make p
up for this motion. S0 the time between
two new Moons is 29 days 12 hours 44
minutes and 3 seconds.
31
[Sketch of eclipse.]
PICTURE 1.
The plane of the Moon's motion round
the Earth is not the same as that of the
Earth's round the Sun, but is at a small
angle to it, of 5 degrees. If this was not
so, that is, if the two planes were the same,
the Sun's light would be cut off by the
Moon at every new Moon and that of the
Moon by the Sun at every full Moon,
though the cutting off would not at all
times be complete, because, if the Moon
came between at a time when the distance
between the Earth and the Moon was
greater than it commonly is, the shade
sent out by the Moon would not be great
enough ; and the complete body of the
Sun would not be covered by it. Round
the dark body of the Moon in front of the
Sun we would see the edge of the Sun in
the form of a ring of light.
32
As things are now, the two planes of
motion being at an angle, an ' eclipse ' as it
is named, a ' dark ' Sun or a ' dark ' Moon,
will only be seen when the Sun and the
Moon are near the line joining the points
at which the planes are cut by one another.
This line, joining the cutting points, itself
goes round in a circle, the journey taking
18 years and 11 days. So if we take note
of all the eclipses seen in one time of
18 years ll days, we get a knowledge of
all the eclipses which are to come in the
future. This way of judging the coming
of eclipses was within the knowledge of
certain nations in old times, among others
the Chaldeans, who were very great
astronomers, and who gave the 18 year
measure the name of ' Saros '. Though
this measure is not quite the true one, it is
near enough to it for us to be able to make
good use of it. For example, Herodotus,
the Greek writer of history, says that,
while a light was going on between the
Lydians and the Medes, a complete eclipse
of the Sun came on, which took the fighters
by surprise, and put an end to the war.
Now, a complete eclipse is not seen at all
33
frequently in any one place, so it was
possible for astronomers to get the day
of the light (as to which there had been
no agreement because the writers gave
any year from 626 to 583 B.C.) fixed, as
having been the 28th May, 585 B.C.
The number of eclipses which may be
seen somewhere on the Earth in this time
of 18 years is generally 70, 29 being
eclipses of the Moon and 41 of the Sun, but
the trouble is that, though the number of
eclipses is great, it is not possible for them
to be seen in the same place, because the
Sun and the Moon will be in a different
place in the sky every time there is an
eclipse. This will not have any effect on
the eclipses of the Moon, which are the
same for all parts of the Earth, because
the Earth is between the Sun and the Moon
and sends out its shade on to the Moon,
so that the eclipse will be seen in every
part of the Earth where the Moon is in the
sky. With the eclipses of the Sun it is
different. The Moon is so near, that if a
person is on a different point on the Earth
he will see the Moon in a different place
in the sky, and it may no longer be in front
of the Sun so as to make an eclipse. The
widest band of Earth or sea in which an
34
eclipse of the Sun is complete on the Earth
is commonly 180 miles, and this band does
not come in the same place every time, but
in a number of different places. That is
why, though there are such a number of
eclipses of the Sun, most of them are not
of any use, because a complete eclipse over
a band of sea without land will not make
any great addition to our knowledge,
which is got from measures taken with
great care on a fixed base.
On looking at the Moon through a telescope
for the first time, some surprise may
be experienced when one sees how clear
and hard every detail seems to be, quite
free from mist of any kind. The reason
is that there is no air, or only a very small
amount of air, on the Moon ; and it is now
certain that there is no water on the Moon
though the great dark spaces on it were,
when first looked at through the telescope,
taken for seas, and names were given to
them with that idea in mind, which they
have still kept, as for example, the
Mare
Tranquilitatis, Mare Imburium, and so on.
35
Possibly these great dark spaces are
full of something which was at one time
liquid, and which has now been made solid
by the cold. Under these conditions it
does not seem probable that any of the
living forms present on the Earth have
any existence on the Moon.
There area number of mountains on the
Moon and they are very high, as high as
those on the Earth, though the Moon is
, so much smaller. There are great round
rings seen on its face, through the telescope,
like those rings formed by the edges
of fire-mountains on the Earth. But if
ever there were fires in these great holes,
they have been cold for a very long time.
Everything on the Moon, when seen
through the telescope, gives the onlooker
the feeling that complete death has come
upon it, if it ever had anything living, and
that though its beautiful face is of very
great interest, it will not at any time have
that attraction for us which it would
have if there were living things on it, like
ourselves, with whom we might have the
hope of getting into touch some day and
learning their experiences and the story
of the Moon, so as to make our knowledge
of the secrets of existence deeper and
wider.
36
The Moon at all times keeps the same
face turned in our direction. We do not
see the other face ever, only a little bit of
it at the edge. But there seems to be no
reason at all for the idea, which some have
had, that the other face of the Moon may
have living beings on it. The same processes
by which the death of all living
things on one side may have been caused
would no doubt have the same effect on
the other side.
But there is one fact from which it
seems that there was a time when the
Moon had water and air.
Why does the Moon keep the same face
turned to the Earth ? The answer to this
question, now quite certain, is this: the
Earth, with its great mass, was at one
time, by its attraction, the cause of great
tides on the Moon. The tides have the
effect of making the turning motion
slower, acting, though much more softly,
as a brake does on a wheel. So that
the effect of the tides, keeping on and
on through millions of years, is that the
Moon, seen from the Earth, no longer has
any turning motion. But it seems probable
from this that, if there were tides,
there was water at some time on the Moon,
and no doubt air.
1
37
But this is not the end of the story.
What the Earth has done to the Moon,
the Moon is, by the same law of attraction,
doing to the Earth, only much more
slowly, because it is so much smaller, and
has less ' mass ', or substance. Those
waves of the tides which you take such
pleasure in watching coming in or going
out when seated near the sea on a beautiful
summer's day, are responsible, through
long millions of years, for making the great
land-edge into very small sand, and it is
they who, little by little, are getting their
grip on the Earth, very softly and slowly,
making its days longer. The amount
is very small, a small part of a second in
100 years. But it goes on and on without
stopping, and they have time enough
before them, time without end, to make
their work complete. For that reason
the time will come, after far-off millions
of years, when the Earth will no longer
make her turning motion, when her day
will, like the Moon's, be equal to the month,
and when an eye looking out from her
would see a Moon hanging for ever without
motion in the sky, her face seen in the red
light of a very old and feeble sun.
38
But it will not be our sky. Gone will
be the old star-groups, their places taken
by strange new ones, the new groups
among which our Sun's system has come
in its unending journey. Or man himself
will have gone, and no eye will be looking
out but the great round eyes of the two
dead balls, the Earth and the Moon,
turned upon one another for ever, while
somewhere on the Earth under great ice
masses are the bones of the last Earth man,
with eyes looking up for ever and seeming
to put the question : Why ?
39
IV . THE SUN
When, after a long process of development,
living things on the Earth took their
highest form in Man, it was a long time before he had any knowledge of the true
part played by the Sun in his existence. It was so bright, that he kept his eyes
turned away and only saw it for a short
time at sun-down and sun-up. The Moon,
which he was able to have a look at for such
a number of hours at a time, and which
underwent such strange changes, seemed to
him far more important, to have much
more to say to him and to be far more full
of secrets. He would keep sharp watch on it
night after night, while from a thin line it
became a half-circle, and again with deep
pleasure he would see its rays when, at
full moon, it sent them down like a mass
of silver from the sky. But again, with
a feeling of regret he would see it get
thinner and thinner till at last it was no
longer to be seen, so that he had a sense of
40
deep loss till some days later he saw it
come out of the Sun’s rays again as if it
had been given a new existence. These
deep feelings took a strong grip of the heart
of Man, s0 that, even when his offspring
later came to be great nations, they still
had, as an important part of their public
religion, the forms and signs by which they
made clear the full measure of their
pleasure at the coming back of the Moon.
But it was not long before Man came to
have a truer knowledge of the facts and
to see that it is through the heat of the
Sun that we are able to go on living, and
through its light that our road is made
clear before us on land and sea, and our
hearts are full of the pleasure of existence.
Seen from the Sun, the Earth would
be very small, only about 17 seconds of
angle measure.
1 To make it clear
how small this is, we may give a simple
example. Most readers have no doubt
seen Mars, which was in a good
position for observation in the night sky
in 1933. Seen under good conditions,
Mars seems greater in size to us than the
Earth would seem to a person looking at
it from the Sun.
41
It is clear from this that, because the
Sun seems to be much greater in size, the
true size of the two bodies is necessarily
different by the same amount. That is to
say, the size of the Sun in angle measure, is
about 32 minutes, as against the 17 seconds
of the Earth, and so the distance through
the middle of it is 106 times greater than the
distance through the middle of the Earth ;
in other words it is about 866,000 miles.
So, if the Sun was a hollow ball, the Earth
and the Moon would, if placed inside it,
have quite enough room to go round one
another in the same way as they do now.
The distance of the Sun from us is about
92,870,000 miles. How it was measured
is an interesting story. The attempt was
made by the Greeks, but they did not get
the desired outcome, and it is clear to us
now that it was not possible to get the
distance in the way it was attempted at
that time. The Greeks, and others in
later times, even for some time after the
invention of the telescope, had no idea
how very great the distance was.
42
At last the English astronomer Halley
came upon the idea of using the planet
Venus for this purpose. At certain times
Venus comes between the Earth and the
Sun, and seems like a black mark on the
Sun. Its journeys over the Sun’s face are
far from frequent, but when one takes
place, another commonly comes again
eight years later, after which there are no
more for over 100 years. To make the
desired measure, the journey of Venus
across the Sun is watched by one person
high up in the north, and by another
person low down in the south. It will,
to the man in the north, seem to be going
over a part of the Sun more to the south,
and to the man in the south, it will seem
farther north, because of their different
positions. But how are they to take the
measure of the amount? There are no
markings for that purpose on the Sun,
which is a ball of fire. So a way out was
necessary. A simple and very good idea
for this purpose was to take the measure,
not of a position on the Sun, but of the
time needed for the journey across its
face. From this it was seen how wide
the Sun was from east to west at the two
different points, and this in turn gave the
north and south positions. Then by a
simple process of working out the relation
of angles (the seeming changes in the
position of Venus caused by the distance
between the two watchers) the true
distance of the Sun was got.
43
As the outcome of this process of
measuring and the process by which it was
worked out, a distance of 95,000,000 miles
was got at first.
Through later measurings, however, by
this and by other processes, this number
has been put right, and the distance is
now taken to be about 93,000,000, as has
been said before.
This distance is a very important thing.
Though it seems so great, and though we
are not quite certain even as to some thou-
sands of miles, one way or the other,
it is the base by which the distances of the
stars are in their turn measured.
A fact which may be of interest to the
reader is that it is not very probable that
most of those who are living at the time
when these lines are printed (1934) will
still be in the land of the living when
Venus again makes a journey across the
face of the Sun. The day of that event
will be the 7th June, 2004 and a short time
after it, that is, a little less than eight
years later, on June 5th, 2012, the same
thing will take place again.
44
The last time it took place was on the
6th December, 1882, and before that on
8th December, 1874.
There is no need to give details here, as
is frequently done in works of Astronomy,
about the number of times the Sun is
greater in size than the Earth. Any one
is able to get this worked out for himself,
with the knowledge that the measure of
the Earth through the middle is about
7,912 miles, and that of the Sun 867,000
miles. Because the Sun and the Earth
have the form of a ball, it is simple to see
how much more substance is in the one
than in the other.
But though the Sun gives us existence,
it is not for that reason more important
than man. Though all our Earth and ourselves,
our feelings and mind came from the
Sun, if current opinion is right, the Sun is
itself without any living things, without
any reasoning beings, and without that
knowledge which is man’s, by which even
the Sun, from which man’s existence has
come, is measured and judged.
That is not to say, however, that man is
the greatest being in existence up to the
present time. We are not able to say
what other eyes of reasoning beings may
be looking out, with ours, upon the great
45
sky of stars. So,though we may give little
attention to the attempts so frequently
made by writers on Astronomy to make
man seem nothing by the crushing weight
of numbers about distances and size, there
is no reason for us to take a very high view
of ourselves and our value in the scale of
possible existences.
To come back to the question of the
Sun. If we had no other way of measuring
its distance than that of which an account
has been given, by the help of Venus, we
would have a long time of waiting before
us, and would not be able, till 2004, to take
any steps to get a truer measure of the
distance. Happily, we are not so limited.
There are a number of other ways in which
that distance may be measured. One of
the most interesting is that in which the
rate of motion of light is used as the
measuring-rod. No doubt the reader has
knowledge of the fact that light takes time
to make its journey through space. For
light to get, for example, from the planet
Jupiter to the Earth, from 30 to 40 minutes
will be needed ; we say 30 to 40 because
the distance between the two planets is
naturally different at different times.
Jupiter has a number of smaller bodies
46
going round it, so that by watching
the eclipses of these ‘satellites’ when
they go the other side of Jupiter, it
is seen that these take place 16 minutes
later when Jupiter is on the other side of
the Sun than when the Earth is between
the two. In other words, light takes 16
minutes to go over a line twice the distance
from the Earth to the Sun. In view of the
fact that the distance of light is measured
on the Earth by a certain process, having
nothing to do with any bodies in the sky,
and is seen to be something over 186,000
miles a second, the working out of the
distance of the Sun (8 minutes journey of
light) is a question of simple arithmetic.
There are, as has been said, other
processes for measuring the Sun’s distance,
which is very important, because it is the
base of almost all numbers as to distances
in astronomy.
Between the planets Mars and Jupiter
there are a great number of small bodies
going round the Sun in a place where
another planet might be looked for. How
it is that there is no great planet there, and
what was the reason why a great number
of small bodies were formed in its place, is
a question to which we are not able to
give any certain answer.
47
Some of these small planets have their
motion in curves which are so far from being
a circle that they come much nearer to
the Earth than any other body (not taking
the Moon into account). When one of
these bodies comes near to the Earth in
this way, the process of measuring used
is very like that for the journey of Venus
over the Sun, of which an account has been
given.
We may take as an example the small
planet Eros. This planet, in 1931, came
to as short a distance as 16,000,000 miles
from the Earth.
For the purpose of measuring it,
positions are taken up at two different
points, as far away from one another as
possible, on the Earth. Against the fixed
stars (which are so far away that there is
no need for the effect of the measuring
base between the watchers to be taken
into account) the little planet will, to
one watcher, seem to be in one position
and to the other, in another. This change
of position caused by the distance between
the two watchers, gives the distance of
the body from the Earth.
48
That is got by ' angle-measure ’, which
is used in addition for mapping-work
on the Earth, and in view of the fact that
these words have been used a number of
times in this book, it will, no doubt, be of
use to some readers to have a short account
of their sense.
If it is necessary for anything to be
measured at a distance, this is done by
measuring off a line, named a ‘ base line ’,
with great care, and then looking at the
thing to be measured, first from one and
then from the other end of the base line.
This line is one side of a
triangle1
the positions seen give the two angles, one on
one side of it and one on the other,
and this by a common law of geometry,
gives the complete triangle.
49
In this way the distance of the Moon, for
example, is measured by two persons looking
at it at the same time from two points at
a distance from one another on the Earth
(the true distance -- the base line -- between
them having been fixed with care) and
noting the different position in which it is
seen by the one and the other. This,
again, gives one side of a triangle (the base
line) and the two angles nearest to it, and
it is a question of simple mathematics to
get the other sides of the triangle, at the
top of which is the Moon.
The same thing may be done by one man
only, who, seated at his telescope, takes
the position of the Moon at one time, and
again some hours later. The Earth having
gone round, the effect is the same as if two
positions had been taken by two different
persons. Naturally the Moon’s true
motion in that time has to be taken into
account, but that is done without great
trouble, and this was the process of
measuring used by the old Greek astronomer,
Hipparchus, though naturally he
had no telescope.
We see that it is the same effect as we
get in a moving train, the nearer things
seeming to go by quicker, and those at a
distance more slowly. If the rate of motion
of the train is given, it is quite possible to
give the distance of the different things
seen from the window by their rate of
seeming motion.
So angle-measure is the measuring of the
amount by which the direction of two
lines is different. It is like the different
positions of the hand of a clock, but in
angle-measure the clock face, or circle, has
360 degrees, every degree 60 minutes and
every minute 60 seconds.
50
The relation between distance and angle
is this : however long or short a thing may
be, at a distance 206,265 times as great
as it is long, it will have an angle of one
second; in this way a foot-rule will give
an angle of one second if placed at a
distance of 206,265 feet. At a distance of 3,438
feet the foot-rule will give an angle of one
minute, and at 57 feet
l an angle of one
degree. The same is true of a yard-rule
or of a rule a mile long : they will give an
angle of one second, at distances of a yard,
or a mile, and so on, under the same conditions.
Take, for example, a train going in a
straight line at 60 miles an hour, from the
window of which we see a high building
which goes through an angle of 57 degrees
in one minute of time. In one minute the
train does one mile. One mile, as we have
said, would have to be 57 miles away to be
seen as a line equal to one degree. But
this one is seen as equal to 57 degrees, that
is to say, it is 57 times nearer. In other
words, the high building is one mile away.
2
51
1 mile -- 57 miles = 1°
1 mile -- l mile = 57°
PICTURE 2.
Angle measure dependent on distance.
The reader may say that this still does
not give us the distance of the Sun.
But when we have the distance of any
one planet from the Sun, the distance of
all the others is given to us by a law which
is named Kepler’s third law. This law
gives a clear relation between the time
taken by the different planets to make a
complete journey round the Sun, and
their distances from the Sun.
Let us take, for example, Jupiter and
the Earth. Jupiter takes about 11-6/7 times
longer than the Earth to go round the
Sun. Taking the square of that number
The nearer the pin-points are together, the
we get, roughly, 140. Jupiter is 5-1/5 times
farther from the Sun than the Earth is.
If we take the ‘ cube ’, or third power, of
5-1/5 we get the same number, 140.
52
When in 1781 the discovery of the planet
Uranus was made by Sir William Herschel]
it was not possible to get its distance from
the Sun by measuring. But it was seen
that Uranus took 84 times as long as the
Earth (30,687 days) to make a complete
journey round the Sun.
If we take the square of 82-1/12, we get
7,056. Taking what is named the cube
root of this last number ( ∛7056), we get
about 19-1/5, and that is truly the number
of times which Uranus is farther away
from the Sun than the Earth.
Or again, Neptune is 30 times farther
from the Sun than we are. If we take 30
times 30, and again 30 times the number
got (that is, the cube), we have 27,000.
The time of Neptune’s journey round the
Sun is 165 years and the square of this
number gives us roughly 27,000. So if we
get the distance of one planet and take
note of the time it takes to go round the
Sun, by noting the time of any other
planet, we are able to say what its distance
is.
53
This, the third of Kepler’s laws, is,
though Kepler was not conscious of it, a
statement in a limited form of the great
law of attraction which was the later discovery
of Newton, because the sense of it
clearly is that the quicker a planet is
moving, the nearer it may be to the Sun
without falling into it. Something has
been said earlier about this law, the effect
of which is that every bit of substance
everywhere in space has an attraction for
every other bit of substance, and that the
force of the the attraction is dependent on
the amount of the substance, being in
addition four times smaller when the distance
between the bits of substance becomes
twice as great, and so on.
The other two laws worked out by
Kepler, a great authority without whose
work Newton would not have been able to
make his discoveries, were equally
important. It was he who said that the
motion of the planets takes place in curves
named ‘ ellipses ’ which are almost a circle.
A picture of this may be made by putting
two pins in a bit of paper, placing a bit of
thread with ends knotted together over
them, and making a curve with joined ends
in which the pencil is guided by the thread.
54
PICTURE 3.
How to make an ellipse.
The nearer the pin—points are together
the more will the ellipse be like a circle.
In such ellipses the planets make their
journey, with the Sun at one of the
pin-points. Kepler’s other law, the third,
was that the line joining the Sun and the
planet goes over equal spaces in equal
times, that is, when the planet is nearer
the Sun it goes so much more quickly
that the line from it to the Sun, which is
shorter, goes over the same amount of
space as when it is farther away.
PICTURE 4.
The line from a planet to the Sun goes over
equal amounts of space in equal times.
55
We have seemingly got away from the
thing under discussion, the Sun, but in
fact we have no space to say much more
about it. The power of its light is such
that it is equal to 3.23 x 10
27 normal
candle-power
l, but the amount which
gets to the top of our air, is cut down
by the distance to 576,000 candle-power.
Of this only one-half gets down
to the face of the Earth. Again, as to the
heat given off by the Sun, the statement
of it in numbers has no sense to the mind
of man, because it is so great. One authority
gives a number, starting with 8, and
going to 24 places, as the amount of work
done by the heat of the Sun at its face,
that is, the number of horse-power.
Clearly, such numbers say little to
us; our mind is not able to make any picture
of them, and they have only a crushing
effect. But the Earth only gets one
2,735-millionth part of all the energy
given off by the Sun. All the planets
56
together only get one 229-millionth part
of it, the rest going off into space as a
complete loss. But to the present writer
(writing in 1918) there seemed to be good
reasons for supporting the view (which
later came again from a much greater
authority -- Sir Oliver Lodge) that, far
away in space, a process of building up
is taking place, in which the heat which
is produced by the loss of substance in the
Sun comes together again and is changed
back into mass or substance. This is
only an idea, and is not of much value
till it is possible in some way or other to
put it to the test. But there is one important
thing to be said for this idea : the
destruction and the forming of suns would
be going on at the same time in different
parts of space, while, on any other theory,
time almost without limit would seem to
be necessary for the first forming of an
amount of stored energy such as is present
in the great mass of the Sun.
57
Violent outbursts take place in the Sun
which it is not possible to see without the
help of instruments. Great holes are
formed into which a number of bodies the
size of the Earth would be able to go. In
some minutes great masses of gases go up
from the body of the Sun for thousands
of miles into space. On the great ball of
day which seems to us so quiet, sounds
and noises of such power that our ears
would not be able to put up with them
for a second are all the time coming into
existence, but, happily, we are shut off
from them by the great space of 93,000,000
miles, without any air, through which
sound is not able to go. Now when we
see a quiet red ball going down in peace
in the west, it is hard for us to get the
idea into our minds that what is taking
place over there is not peace, but war, a
war of natural forces worse than anything
which might be put into words or pictured
by our minds.
This will have to be the end of our
account of the Sun, not because there is
not much to be said -- there are a great
number of interesting things -- but because
the space in this book is limited, and
there are other questions needing attention.
58
V . THE PLANETS
The planets are those great round bodies, like our Earth, which go round
the Sun in eclipses.
The great planets are, up to now, 9 in number.
We say "up to now" because,
though hundreds of sharp eyes, armed with telescope of great power, have been
watching the sky every night for a great
number of years, the discovery was made
only three years back, of a new planet
which has now been named Pluto. It is
for that reason quite possible that there
may be other additions to the number.
In early times man only had the know-
ledge of five planets : Mercury, Venus,
Mars, Jupiter and Saturn. Later the
discovery was made that our Earth is no
more than a planet, so making six. Sir
William Herschell's discovery of Uranus
in 1781 made the number seven. Then
the work of Adams and Le Verrier, in
1846, made another addition to the number
in the form of the planet Neptune and
last, in 1930, came the discovery of one
more planet, Pluto, the ninth, by a young
American named Tombaugh.
59
Mercury.
Mercury is the smallest planet, and the
one nearest the Sun. A number of
astronomers have been of the opinion
that there is another planet, between
Mercury and the Sun. So certain were
they, that they gave him a name -- Vulcan
-- a very good name for one placed so near
the Sun's mass of flaming heat, as Pluto is
for the watcher over the dark deeps of
outer space.
The great French astronomer, Le
Verrier, who had, by working out the
details and numbers of the outer planets,
whose motions were not quite those
pointed to by mathematics, been able to
make out the existence and position of
the planet Neptune, had the desire to do
the same thing for Vulcan, a planet which
in his belief was in existence nearer to the
Sun than Mercury. He was working at
this for a number of years, noting all the
times when others said they had seen
Vulcan and all the effects it seemed to
have in changing the motions of Mercury
(through the operation of the Law of
Attraction) he got what was, in his
opinion, its true position, and made a public
statement that probably Vulcan would
make a journey over the face of the Sun
on March 22nd, 1877. A number of
telescopes were turned on the Sun that
day. But the planet was nowhere to be
seen.
60
The idea of a planet nearer the Sun than
Mercury has now almost been given up.
The trouble in the motion of Mercury
which had been put down to Vulcan (the
near-point of Mercury to the Sun is moved
forward by a
greater amount than is
accounted for by the attraction of the
other planets) has now -- so the opinion
goes -- been accounted for by the theory
of relativity.
Although
search for a moon of Mercury continued into the 2000's.
In early times, Mercury was at first
taken for one planet in the morning and
another at night, because it sometimes came
up before the Sun and at other times went
down after it. For this reason it had two
names, Set and Horus among the
Egyptians, and Apollo and Mercury
among the Greeks.
61
Mercury does not go farther away from
the Sun than 28 degrees in the sky. So
it is only to be seen about two hours before
the Sun comes up or after it goes down.
Measured through the middle its size is
3,000 miles. Its distance from the Sun is
36 million miles and its year about 88
,days. It seems to have the same face
turned to the Sun all the time, through
the braking effect of the tides of water
or other substance caused by the great
attraction of the Sun (see page 2l).
It does not seem possible that there are
any living things on Mercury, because the
heat of the Sun is so great (about nine times
greater than on the Earth) ; and it is to
be doubted if there is any air there.
Venus.
The planet which is nearest to the Sun
after Mercury is Venus. She is 67 million
miles from the Sun. Her greatest distance
from it by angle-measure in the sky is
about 47 degrees.
It may be said here that the circle of the
sky measuring, naturally, 360 degrees,
and the day being 24 hours, there are 15
degrees to every hour. For that reason,
as was said, Mercury may sometimes be
seen in the sky for two hours before (or
after) the Sun, and Venus for about three
62
hours. The measure of Venus through the
middle is 7,770 miles. Its year is about
224 days long.
Like Mercury, Venus had two names:
Hesperus and Phosphorus and, for the
same reason, it was taken in early times
for two different planets.
As to the time which Venus takes to go
round itself, that is, its day, there is no
agreement among astronomers. Some
take the view, which was that of the great
Italian astronomer Schiapparelli, that its
day is the same as its year, that is, that
it keeps the same face turned to the Sun
all the time. This is doubted by others,
who have been of the opinion that its day
is a little shorter than ours. It is very
hard to be certain because the planet is
covered with such thick clouds that we
are not able to see anything-on its face.
But the weight of argument seems to be
on the side of a short day, because a long
day would only be caused by the same »
thing by which the day of the Moon and
Mercury have been made shorter -- the
tides. But on the Earth the Moon is the
chief cause of the tides, causing two-thirds I
of the effect, and the Sun one-third. Venus
has no moon, but it is much nearer to the Sun.
63
The effect of this is to make the
Sun's tides on Venus a little greater than
all the tides on the Earth, because the
force of the tide effect is dependent not
on the square but on the cube of the distance.
But all this does not make it at all
certain that the turning motion of Venus
has been stopped and that she has the
same face to the Sun all the time. It
seems probable that her day is about as
long as our day.
There is one point in which Venus is
different, in the writer's opinion, from
every other body (not taking into account
possibly Mars) seen in the sky. It is a
point of the greatest interest, and one
which we may give thought to seriously
for a minute. This is almost the only time
when we get the answer " Yes " to the
question " Are there living things on other
planets ? " At all other times the answer
sent back to us from space is a rough
" No ". But it seems quite possible that
there may be living things on Venus,
though our knowledge of what the air
on that planet is like makes the question
a very complex one. Its body seems
almost of the same thick substance as the
64
Earth, judging by its power of attraction.
It has a great covering air, full of thick
clouds. It gets twice as much heat as
the Earth, it is true, but we only get half
of what comes to the top of our air, and
the air on Venus being much greater
in weight and having more clouds, the
amount of heat which gets through to its
land and water will be far less. A man
taken to Venus would have almost the
same weight ; it would, in fact, be a little
less, but he would probably not be conscious
of it. To say it is improbable that
there are living things on Venus is to say
that it is improbable that there are living
things anywhere, because nowhere will
the conditions be the same in all points as
on the Earth.
But what sort of living things ? It is
a moving thought that the things which
had existence on the Earth in the Silurian
time about 400 million years back, low
forms of sea-animals only (at that time
the heat of the Earth was greater) may
be there today before our eyes, on Venus,
cut off only by distance, and having still to
undergo the cruel stages of development
into higher forms. Or great animals of a
65
later time like the dinosaur, the ichthyosaurus,
and others, may be fighting their
great fights out there. Or it may be that
the development of living forms has taken
a completely different road. But we are
cut off from it, and are kept from stepping
back into what is an earlier stage of our
earth, not by what we are conscious of as
Deep and strange questions, the answers
of which are kept from us by some inner
law of existence, but only by the simple
physical effect of distance. How strange
it would be if simply by bridging the distance
between us and Venus, we were able
to put back the hand of time !
Mars.
Mars is the first of the outer planets, the
other two (Mercury and Venus) being
inner planets, that is, moving on the inner
side of the ellipse in which the Earth makes
its journey.
Its measure across is 4,200 miles, its
year is 687 of our days and its common
distance from the Sun is l4l,000,000
miles. Its weight is T2; that of the Earth.
It has two moons, Deimos and Phobos,
which are very small, and near to it, and
the strange thing about Phobos is that
66
it goes round Mars in about 7 hours, and
Mars makes one turn in 24 hours and 37
minutes (the day of Mars) so that Phobos
goes round him three times every day.
In other words, seen from Mars, and even
taking into account the turning motion
of Mars in the same direction, it goes about
30 angle-minutes, that is about the
diameter of the Moon, in one minute.
Anyone living on Mars would, in this way,
have a good clock at hand at all times in
the sky. Phobos is about 36 miles
across and its distance from Mars about
3,850 miles.
On Mars again we have what is naturally
the most interesting thing to us as
living beings, that is, the fact that the
existence of some form of living things
on it is almost certain. It has air, though
very thin, and what seems to be water,
and its ' canals ' (waterways), as they are
named, are now taken to be land covered
with some form of plants, which are seen
by us when, in the Spring, great ice masses
are turned into water which comes over
the land at a great rate, causing new
growth. But it is very uncertain if any
higher form of existence is possible. The
amount of heat which Mars gets from the
67
Sun is so much smaller than that of the
Earth that it seems most probable that
what we see on Mars are low plant-forms
like those seen some way up the mountains
of the Earth, where existence is not possible
for the higher forms. Such plants,
which have such small needs that existence
is still possible for them under conditions
of this sort, would not be able to give the
food necessary for the support of higher
beings, with powers of reason.
So that, while living things are only now
coming into existence, or are at quite an
early stage on Venus, they are getting
near their full development on the Earth,
and seem to be in process of slow death
on Mars.
The other planets outside Mars are of
but little true interest, being only masses
of dead substance.
68
Small Planets
After Mars, come the very small planets,
some miles to some hundreds of miles
across and numbering more than 1,000.
The only two interesting points about
them are, that they seem to be bits of a
planet which was not rightly formed and
got all broken up, and that some of them
go round the Sun in such long ellipses
that they come very near the Earth, which
as has been said, is of great use for measuring
the Sun’s distance.
There is some reason for looking upon
the small planets taking the place of a
greater planet, as the outcome of the
great attraction of Jupiter. This, being so
near, would have been the cause of great
tides on such a planet, and clearly the
attraction of its parts to one another was
not enough to keep it together.
Jupiter
After the small planets comes the great
planet Jupiter. It is more than five times
as far from the Sun as the Earth. It is
87,000 miles across, and its year is almost
12 times (11.86) as long as ours.
This, the greatest of the planets, is very
thin in substance, being about a quarter
the weight of the Earth in relation to its
mass, and more like a gas than a solid.
It is the seat of violent motions of
clouds and great winds.
It has nine moons, live of them very
small, but the others (Io, Europa, Ganymede
and Callistro) greater than our Moon.
One strange point about Jupiter is that,
though so great in size, it goes round on
itself in less than 10 hours ; so, any one at
its middle-line (equator) would go 26 times
quicker than at the Earth’s middle line.
It is in a very strange condition, with
great forces playing on it, and is very
interesting from that point of view, but
it certainly has no living things on it.
69
Saturn
Saturn, which comes after it, has strange
rings round it made up of thousands of
small bodies, as if a moon had been completely
broken up into little bits before
it was formed. These rings are very
beautiful when seen through a telescope.
It has in addition moons which are not
so small. Again, we seem to see here the
great effect of the power of attraction of
Jupiter ! Saturn goes round in 16¼ hours.
It is 76,000 miles in diameter, and is 92½
times farther from the Sun than the Earth.
Its year is about 29% of our years. There
is again no question of there being living
things on this planet.
Uranus
To get to the planet Uranus we have to
make a great journey, 19 times the distance
from the Earth to the Sun. It is
32,000 miles across, and its year is 84 of
ours. It has four moons, the only use of
which seem to be that we are able, from
them, to get the mass of the planet worked
out. There is nothing interesting about
this planet in itself.
70
Neptune
Neptune, to which we now come, was
the discovery of two great men at the same
time. Adams in England and Le Verrier
in France, noting that the motions of
Uranus were not regular, on working out
these motions, came to the decision that
there was another planet in existence,
and they were even able to say in what
place in the sky it would be. When given
this position in the sky, Professor Salle,
in Berlin, on September 23rd, 1846,
pointing his telescope to the place, saw
the planet there. Neptune is about
2,790,000,000 miles from the Sun. Its
year is about 164 of ours, and it is about
32,000 miles across. It has one moon.
The most interesting thing about it is
the way in which its discovery was made.
71
Neptune was the limit of the Sun’s
system till the year 1930. Then there
came from America the news that a young
American had made the discovery there
of a new planet, to which the name Pluto
was given. As if in punishment for some
bad act, it makes its sad journey at so
great a distance from the Sun that its
feeble light is like that of the last half-hour
before the death of a winter’s day on the
Earth. Its distance from the Sun is no
less than 3,700,000,000 miles and its
journey round the Sun takes 247 years.
The thought of the sad watch kept by
this planet all by itself, in the dark deeps
of space, on the outer limits of our system,
is crushing to the feeble heart of man,
living from his birth in the bright warm
rays of a Sun so near, from the astronomer’s
point of view, that it may almost be
said to be touching us. And a man would
have to be very interested in astronomy
before he got any comfort from the fact
that it would be possible from there to get
a much truer measure of the star distances
with a base-line 7,400,000,000 miles long !
1
72
So that we may make our account of
the Sun’s system complete, a word or two
may now be said about '
comets '. These
are small bodies made up of a small, sometimes
hard head, and a body of gas so thin
that, on looking through one hundreds
of millions of miles thick, the feeblest
stars have been clearly seen. Their curves
of motion may be of almost any form,
and their journeys round the Sun may take
any number of years. They may go to
Neptune or Pluto or even farther, or go
right off into space.
The idea that there is danger to the
Earth from comets has long been given up.
Even if they were made of some poison
gas (the suggestion has been made of an
oxygen [O
2] comet which would put fire to
the Earth, which is very foolish) the
amount of substance is so small that danger
seems quite out of the question. But we are
not able to say what the future may have
in store in the form of some great body
coming in at a great rate from outer space.
73
The last point to be touched upon before
we come to the end of the Sun’s system
is a short account of what are named
'
Falling Stars '.
It is certain that there is at all times a
mass of dust and substance coming down
from the sky on to the face of the Earth.
So great is the amount of this dust rain
that the suggestion has even been made
that a great part of the Earth may have
been formed in this way. Even in times
recorded in history, in a great town like
Rome, it is seen that buildings of old
Roman times are at a level much lower
than that of streets and houses of the
present day. It seems certain that some
part of this, at least, has come from the sky.
These additions to our Earth, which we do
not see, are much greater than the ‘ falling
stars ’ which we do see. These ‘ falling
stars ’, moving in great ‘rivers’ and made
up of small bits of planet substance,
go round the Sun in ellipses like planets,
sometimes cutting across the line of
journey of the Earth. At such times we
may frequently see, high up in the sky, a
bright light as if a star was truly falling,
so quick that it is sometimes gone in a
small part of a second. This is caused by
one of the bits of substance coming into
our air at a great rate, the rubbing of the
air making it white with heat so that it is
burned away in a second. Other ‘ falling
stars ’ are greater and come down on to the
Earth.
74
These ‘ falling stars ’ are seen to come
from the direction of different star-groups
(though naturally they have nothing to
do with the stars) and they are named from
those groups, as the Perseids, from the
star-group Perseus, on lO—12 August, the
Orionids from Orion, in October, and so on.
Where those bits of substance come
from is very uncertain. Some say they
were sent out with great force from fire-mountains
on the Earth in early times;
others that they come from the great
mountains on the Moon. Others again
have different views. When you see
some of the great bits of ‘falling stars',
which, having come down on Earth, are
kept in museums, you will have the strange
feeling that you are looking upon something
which has been far away in space,
and has possibly been near to Mars or to
Venus. If it had a tongue, it might
give us a very interesting story. But,
though this may seem full of strange
interest to you, a little thought will make
it clear to you that we have reason for
looking upon our Earth and ourselves,
a with the same or even greater surprise and
interest, because are we not all equally
the offspring of space, of the stars ?
75
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