The Earth
The earth being our place of abode is, of course, to us the most important
planet in the solar system. It is a curious paradox that the moon's
surface (at least the visible portion) is better known to us than the
surface of the earth. Every spot on the moon's visible surface equal in
size to say Liverpool or Glasgow is well known to lunar observers, whereas
there are thousands of square miles on the earth's surface--for example,
near the poles and in the centre of Australia--which are wholly unknown to
the earth's inhabitants; and are perhaps likely to remain so.
Many attempts have been made by "paradoxers" to show that the earth is a
flat plane and not a sphere. But M. Ricco has found by actual experiment
that the reflected image of the setting sun from a smooth sea is an
elongated ellipse. This proves mathematically beyond all doubt that the
surface of the sea is spherical; for the reflection from a plane surface
would be necessarily _circular_. The theory of a "flat earth" is
therefore proved to be quite untenable, and all the arguments (?) of the
"earth flatteners" have now been--like the French Revolution--"blown into
space."
The pole of minimum temperature in the northern hemisphere, or "the pole
of cold," as it has been termed, is supposed to lie near Werchojansk in
Siberia, where a temperature of nearly -70° has been observed.
From a series of observations made at Annapolis (U.S.A.) on the gradual
disappearance of the blue of the sky after sunset, Dr. See finds that the
extreme height of the earth's atmosphere is about 130 miles. Prof. Newcomb
finds that meteors first appear at a mean height of about 74 miles.[57]
An aurora seen in Canada on July 15, 1893, was observed from stations 110
miles apart, and from these observations the aurora was found to lie at a
height of 166 miles above the earth's surface. It was computed that if the
auroral "arch maintained an equal height above the earth its ends were
1150 miles away, so that the magnificent sight was presented of an auroral
belt in the sky with 2300 miles between its two extremities."[58]
"Luminous clouds" are bright clouds sometimes seen at night near the end
of June and beginning of July. They appear above the northern horizon
over the sun's place about midnight, and evidently lie at a great height
above the earth's surface. Observations made in Germany by Dr. Jesse, and
in England by Mr. Backhouse, in the years 1885-91, show that the height of
these clouds is nearly constant at about 51 miles.[59] The present writer
has seen these remarkable clouds on one or two occasions in County Sligo,
Ireland, during the period above mentioned.
M. Montigny has shown that "the approach of violent cyclones or other
storms is heralded by an increase of scintillation" (or twinkling of the
stars). The effect is also very evident when such storms pass at a
considerable distance. He has also made some interesting observations
(especially on the star Capella), which show that, not only does
scintillation increase in rainy weather, but that "it is very evident, at
such times, in stars situated at an altitude at which on other occasions
it would not be perceptible at all; thus confirming the remark of
Humboldt's with regard to the advent of the wet season in tropical
countries."[60]
In a paper on the subject of "Optical Illusions" in _Popular Astronomy_,
February, 1906, Mr. Arthur K. Bartlett, of Batter Creek, Michigan
(U.S.A.), makes the following interesting remarks:--
"The lunar halo which by many persons is regarded as a remarkable and
unexplained luminosity associated with the moon, is to meteorological
students neither a mysterious nor an anomalous occurrence. It has been
frequently observed and for many years thoroughly understood, and at
the present time admits of an easy scientific explanation. It is an
atmospheric exhibition due to the refraction and dispersion of the
moon's light through very minute ice crystals floating at great
elevations above the earth, and it is explained by the science of
meteorology, to which it properly belongs; for it is not of cosmical
origin, and in no way pertains to astronomy, as most persons suppose,
except as it depends on the moon, whose light passing through the
atmosphere, produces the luminous halo, which as will be seen, is
simply an optical illusion, originating, not in the vicinity of the
moon--two hundred and forty thousand miles away--but just above the
earth's surface, and within the aqueous envelope that surrounds it on
all sides.... A halo may form round the sun as well as the moon ...
but a halo is more frequently noticed round the moon for the reason
that we are too much dazzled by the sun's light to distinguish faint
colours surrounding its disc, and to see them it is necessary to look
through smoked glass, or view the sun by reflection from the surface
of still water, by which its brilliancy is very much reduced."...
"A 'corona' is an appearance of faintly coloured rings often seen around
the sun and moon when a light fleecy cloud passes over them, and should
not be mistaken for a halo, which is much larger and more complicated in
its structure. These two phenomena are frequently confounded by
inexperienced observers." With these remarks the present writer fully
concurs.
Mr. Bartlett adds--
"As a halo is never seen except when the sky is hazy, it indicates
that moisture is accumulating in the atmosphere which will form
clouds, and usually result in a storm. But the popular notion that the
number of bright stars visible within the circle indicates the number
of days before the storm will occur, is without any foundation
whatever, and the belief is almost too absurd to be refuted. In
whatever part of the sky a lunar halo is seen, one or more bright
stars are always sure to be noticed inside the luminous ring, and the
number visible depends entirely upon the position of the moon.
Moreover, when the sky within the circle is examined with even a small
telescope, hundreds of stars are visible where only one, or perhaps
two or three, are perceived with the naked eye."
It is possible to have five Sundays in February (the year must of course
be a "leap year"). This occurred in the year 1880, Sunday falling on
February 1, 8, 15, 22, and 29. But this will not happen again till the
year 1920. No century year (such as 1900, 2000, etc.) could possibly have
five Sundays in February, and the Rev. Richard Campbell, who investigated
this matter, finds the following sequence of years in which five Sundays
occur in February: 1604, 1632, 1660, 1688, 1728, 1756, 1784, 1824, 1852,
1880, 1920, 1948, 1976.[61]
In an article on "The Last Day and Year of the Century: Remarks on Time
Reckoning," in _Nature_, September 10, 1896, Mr. W. T. Lynn, the eminent
astronomer, says, "The late Astronomer Royal, Sir George Airy, once
received a letter requesting him to settle a dispute which had arisen in
some local debating society, as to which would be the first day of the
next century. His reply was, 'A very little consideration will suffice to
show that the first day of the twentieth century will be January 1, 1901.'
Simple as the matter seems, the fact that it is occasionally brought into
question shows that there is some little difficulty connected with it.
Probably, however, this is in a great measure due to the circumstance that
the actual figures are changed on January 1, 1900, the day preceding being
December 31, 1899. A century is a very definite word for an interval
respecting which there is no possible room for mistake or difference of
opinion. But the date of its ending depends upon that of its beginning.
Our double system of backward and forward reckoning leads to a good deal
of inconvenience. Our reckoning supposes (what we know was not the case,
but as an era the date does equally well) that Christ was born at the end
of B.C. 1. At the end of A.D. 1, therefore, one year had elapsed from the
event, at the end of A.D. 100, one century, and at the end of 1900,
nineteen centuries.... It is clear, then, that the year, as we call it, is
an ordinal number, and that 1900 years from the birth of Christ (reckoning
as we do from B.C. 1) will not be completed until the end of December 31
in that year, the twentieth century beginning with January 1, 1901, that
is (to be exact) at the previous midnight, when the day commences by civil
reckoning." With these remarks of Mr. Lynn I fully concur, and, so far as
I know, all astronomers agree with him. As the discussion will probably
again arise at the end of the twentieth century, I would like to put on
record here what the scientific opinion was at the close of the nineteenth
century.
Prof. E. Rutherford, the well-known authority on radium, suggests that
possibly radium is a source of heat from within the earth. Traces of
radium have been detected in many rocks and soils, and even in sea water.
Calculation shows that the total amount distributed through the earth's
crust is enormously large, although relatively small "compared with the
annual output of coal for the world." The amount of radium necessary to
compensate for the present loss of heat from the earth "corresponds to
only five parts in one hundred million millions per unit mass," and the
"observations of Elster and Gertel show that the radio-activity observed
in soils corresponds to the presence of about this proportion of
radium."[62]
The earth has 12 different motions. These are as follows:--
1. Rotation on its axis, having a period of 24 hours.
2. Revolution round the sun; period 365-1/4 days.
3. Precession; period of about 25,765 years.
4. Semi-lunar gravitation; period 28 days.
5. Nutation; period 18-1/2 years.
6. Variation in obliquity of the ecliptic; about 47" in 100 years.
7. Variation of eccentricity of orbit.
8. Change of line of apsides; period about 21,000 years.
9. Planetary perturbations.
10. Change of centre of gravity of whole solar system.
11. General motion of solar system in space.
12. Variation of latitude with several degrees of periodicity.[63]
"An amusing story has been told which affords a good illustration of
the ignorance and popular notions regarding the tides prevailing even
among persons of average intelligence. 'Tell me,' said a man to an
eminent living English astronomer not long ago, 'is it still
considered probable that the tides are caused by the moon?' The man of
science replied that to the best of his belief it was, and then asked
in turn whether the inquirer had any serious reason for questioning
the relationship. 'Well, I don't know,' was the answer; 'sometimes
when there is no moon there seems to be a tide all the same.'"![64]
With reference to the force of gravitation, on the earth and other bodies
in the universe, Mr. William B. Taylor has well said, "With each revolving
year new demonstrations of its absolute precision and of its universal
domination serves only to fill the mind with added wonder and with added
confidence in the stability and the supremacy of the power in which has
been found no variableness neither shadow of turning, but which--the same
yesterday, to-day and for ever--
"Lives through all life, extends through all extent,
Spreads undivided, operates unspent."[65]
With reference to the habitability of other planets, Tennyson has
beautifully said--
"Venus near her! smiling downwards at this earthlier earth of ours,
Closer on the sun, perhaps a world of never fading flowers.
Hesper, whom the poets call'd the Bringer home of all good things;
All good things may move in Hesper; perfect people, perfect kings.
Hesper--Venus--were we native to that splendour, or in Mars,
We should see the globe we groan in fairest of their evening stars.
Could we dream of war and carnage, craft and madness, lust and spite,
Roaring London, raving Paris, in that spot of peaceful light?
Might we not in glancing heavenward on a star so silver fair,
Yearn and clasp the hands, and murmur, 'Would to God that we were
there!'"
The ancient Greek writer, Diogenes Laertius, states that Anaximander
(610-547 B.C.) believed that the earth was a sphere. The Greek words are:
[Greek: misên te tên gên keisthai, kentry taxin epechousan ousan
sphairoeidê].[66]
With reference to the Aurora Borealis, the exact nature of which is not
accurately known, "a good story used to be told some years ago of a
candidate who, undergoing the torture of a _vivâ voce_ examination, was
unable to reply satisfactorily to any of the questions asked. 'Come, sir,'
said the examiner, with the air of a man asking the simplest question,
'explain to me the cause of the aurora borealis.' 'Sir,' said the unhappy
aspirant for physical honours, 'I could have explained it perfectly
yesterday, but nervousness has, I think, made me lose my memory.' 'This is
very unfortunate,' said the examiner; 'you are the only man who could have
explained this mystery, and you have forgotten it.'"[67] This was written
in the year 1899, and probably the phenomenon of the aurora remains
nearly as great a mystery to-day. In 1839, MM. Bravais and Lottin made
observations on the aurora in Norway in about N. latitude 70°. Bravais
found the height to be between 62 and 93 miles above the earth's surface.
The cause of the so-called Glacial Epoch in the earth's history has been
much discussed. The Russian physicist, Rogovsky, has advanced the
following theory--
"If we suppose that the temperature of the sun at the present time is
still increasing, or at least has been increasing until now, the
glacial epoch can be easily accounted for. Formerly the earth had a
high temperature of its own, but received a lesser quantity of heat
from the sun than now; on cooling gradually, the earth's surface
attained such a temperature as caused a great part of the surface of
the northern and southern hemispheres to be covered with ice; but the
sun's radiation increasing, the glaciers melted, and the climatic
conditions became as they are now. In a word, the temperature of the
earth's surface is a function of two quantities: one decreasing (the
earth's own heat), and the other increasing (the sun's radiation), and
consequently there may be a minimum, and this minimum was the glacial
epoch, which, as shown by recent investigations, those of Luigi de
Marchi (Report of _G. Schiaparelli, Meteorolog. Zeitschr._, 30,
130-136, 1895), are not local, but general for the whole earth" (see
also M. Neumahr, _Erdegeschicht_).[68]
Prof. Percival Lowell thinks that the life of geological palæozoic times
was supported by the earth's internal heat, which maintained the ocean at
a comparatively warm temperature.[69]
The following passage in the Book of the Maccabees may possibly refer to
an aurora--
"Now about this time Antiochus made his second inroad into Egypt. And
it _so_ befell that throughout all the city, for the space of almost
forty days, there appeared in the midst of the sky horsemen in swift
motion, wearing robes inwrought with gold and _carrying_ spears,
equipped in troops for battle; and drawing of swords; and _on the
other side_ squadrons of horse in array; and encounters and pursuits
of both armies; and shaking of shields, and multitudes of lances, and
casting of darts, and flashing of golden trappings, and girding on of
all sorts of armour. Wherefore all men besought that the vision might
have been given for food."[70]
According to Laplace "the decrease of the mean heat of the earth during a
period of 2000 years has not, taking the extremist limits, diminished as
much as 1/300th of a degree Fahrenheit."[71]
From his researches on the cause of the Precession of the Equinoxes,
Laplace concluded that "the motion of the earth's axis is the same as if
the whole sea formed a solid mass adhering to its surface."[72]
Laplace found that the major (or longer) axis of the earth's orbit
coincided with the line of Equinoxes in the year 4107 B.C. The earth's
perigee then coincided with the autumnal equinox. The epoch at which the
major axis was perpendicular to the line of equinoxes fell in the year
1250 A.D.[73]
Leverrier has found the minimum eccentricity of the earth's orbit round
the sun to be 0·0047; so that the orbit will never become absolutely
circular, as some have imagined.
Laplace says--
"Astronomy considered in its entirety is the finest monument of the
human mind, the noblest essay of its intelligence. Seduced by the
illusions of the senses and of self-pride, for a long time man
considered himself as the centre of the movement of the stars; his
vain-glory has been punished by the terrors which his own ideas have
inspired. At last the efforts of several centuries brushed aside the
veil which concealed the system of the world. We discover ourselves
upon a planet, itself almost imperceptible in the vast extent of the
solar system, which in its turn is only an insensible point in the
immensity of space. The sublime results to which this discovery has
led should suffice to console us for our extreme littleness, and the
rank which it assigns to the earth. Let us treasure with solicitude,
let us add to as we may, this store of higher knowledge, the most
exquisite treasure of thinking beings."[74]
With reference to probable future changes in climate, the great physicist,
Arrhenius, says--
"We often hear lamentation that the coal stored up in the earth is
wasted by the present generation without any thought of the future,
and we are terrified by the awful destruction of life and property
which has followed the volcanic eruptions of our days. We may find a
kind of consolation in the consideration that here, as in every other
case, there is good mixed with evil. By the influence of the
increasing percentage of carbonic acid in the atmosphere, we may hope
to enjoy ages with more equable and better climates, especially as
regards the colder regions of the earth, ages when the earth will
bring forth much more abundant crops than at present, for the benefit
of rapidly propagating mankind."[75]
The night of July 1, 1908, was unusually bright. This was noticed in
various parts of England and Ireland, and by the present writer in Dublin.
Humboldt states that "at the time of the new moon at midnight in 1743, the
phosphorescence was so intense that objects could be distinctly recognized
at a distance of more than 600 feet."[76]
An interesting proof of the earth's rotation on its axis has recently been
found.
"In a paper in the _Proceedings_ of the Vienna Academy (June, 1908) by
Herr Tumlirz, it is shown mathematically that if a liquid is flowing
outwards between two horizontal discs, the lines of flow will be
strictly straight only if the discs and vessel be at rest, and will
assume certain curves if that vessel and the discs are in rotation,
as, for example, due to the earth's rotation. An experimental
arrangement was set up with all precautions, and the stream lines were
marked with coloured liquids and photographed. These were in general
accord with the predictions of theory and the supposition that the
earth is rotating about an axis."[77]
In a book published in 1905 entitled _The Rational Almanac_, by Moses B.
Cotsworth, of York, the author states that (p. 397), "The explanation is
apparent from the Great Pyramid's Slope, which conclusively proves that
when it was built the latitude of that region was 7°·1 more than at
present. Egyptian Memphis now near Cairo was then in latitude 37°·1, where
Asia Minor now ranges, whilst Syria would then be where the Caucasus
regions now experience those rigorous winters formerly experienced in
Syria." But the reality of this comparatively great change of latitude in
the position of the Great Pyramid can be easily disproved. Pytheas of
Marseilles--who lived in the time of Alexander the Great, about 330
B.C.--measured the latitude of Marseilles by means of a gnomon, and found
it to be about 42° 56'-1/2. As the present latitude of Marseilles is 43°
17' 50", no great change in the latitude could have taken place in over
2000 years.[78] From this we may conclude that the latitude of the Great
Pyramid has _not_ changed by 7°·1 since its construction. There is, it is
true, a slow diminution going on in the obliquity of the ecliptic (or
inclination of the earth's axis), but modern observations show that this
would not amount to as much as one degree in 6000 years. Eudemus of
Rhodes--a disciple of Aristotle (who died in 322 B.C.)--found the
obliquity of the ecliptic to be 24°, which differs but little from its
present value, 23° 27'. Al-Sufi in the tenth century measured the latitude
of Schiraz in Persia, and found it 29° 36'. Its present latitude is 29°
36' 30",[79] so that evidently there has been no change in the latitude in
900 years.