THE EARTH.
To us, as its inhabitants, the Earth appeals in two characters, and in
writing a book on astronomy it is necessary, yet difficult, to keep
these two characters separate. The Earth is an ordinary planet member of
the solar system, amenable to the same laws, impelled by the same
forces, and going through the same movements as the other members of the
Sun’s _entourage_. Yet, by reason of the fact that we are ourselves on
the Earth and are not spectators of it looking at it from at a distance,
there are many phenomena coming under our notice which require special
treatment, and it is often very difficult to say where the province of
the astronomer ends and that of the geographer begins. This volume being
specially designed to deal with astronomical matters, I shall pass over
many subjects which may be said to be on the border line, and which some
of my readers may therefore be disappointed not to find discussed.
Besides the geographer, the geologist and his scientific brother the
mineralogist are concerned with the Earth regarded as a planet moving
through space as the other planets do. The geologist studies the actual
structure of the Earth, its circumstances and history so far as they
have been revealed to us, whilst the mineralogist investigates and names
the materials of which it is composed, and classifies such materials
with the assistance of the geologist on the one hand and of the chemist
on the other. All these subordinate sciences—subordinate I mean from an
astronomer’s point of view—open up very varied, instructive, and
interesting fields of study, but they are of course foreign to the
purpose of the present volume.
Though the Earth is commonly regarded as a sphere it is not that in
reality, because it is not of identical dimensions from east to west and
from north to south. It is somewhat flattened at the poles; its polar
diameter is less than its equatorial diameter, in the ratio of about 298
to 299, or, expressed in miles, its polar diameter is about 26 miles
less than its equatorial diameter. If a globe 3 feet in diameter be
taken to represent the Earth, then the polar diameter will, on this
scale, be ⅛ inch too long. This flattening of the poles of the Earth
finds its counterpart, so far as we know, in most, and probably in all
of the planets. It is most considerable and therefore most conspicuous
in the case of Jupiter. It ought here to be added that a suspicion
exists that the equatorial section of the Earth is not a perfect circle,
but that the diameter of the Earth, taken through the points on the
equator marked by the meridians 13° 58′ and 193° 58′ east of Greenwich,
is one mile longer than the diameter at right angles to these two
points.
The science which inquires into matters of this kind, including besides
the figure of the Earth, the length of the degree at different
latitudes, and the distances of places from one another, alike in
angular measure and in time, is called Geodesy; it is, in point of fact,
land-surveying on a very large scale, in which instruments and processes
of astronomical origin are brought into operation, and in which
astronomers are more or less required to take the lead.
Although we all of us now perfectly understand that the Earth is a
planet moving round the Sun as a centre, it is, comparatively speaking,
but recently that this fact has become generally recognised and
understood. It is true that we can discover here and there in ancient
writings some trace of the idea, yet it is doubtful whether 2000 years
ago more than a few “advanced” thinkers thoroughly and clearly accepted
it as a distinct truth. It was much more in consonance with popular
thought and the actual appearance of things that the Earth should be the
centre round which the Sun revolved and on which the planets depended;
and accordingly, sometimes in one shape and sometimes in another, the
notion of the Earth being the centre of the universe was generally
accepted. The contrary opinion had, however, a few sympathisers. For
instance, Aristarchus of Samos, who lived in the third century before
the Christian era, supposed, if we may trust the testimony of Archimedes
and Plutarch, that the Earth revolved round the Sun; this, however, was
regarded as a “heresy,” in respect of which he was accused of “impiety.”
Some few years elapsed and a certain Cleanthes of Assos is said by
Plutarch to have suggested that the great phenomena of the universe
might be explained by assuming that the Earth was endued with a motion
of translation round the Sun together with one of rotation on its own
axis. The historian states that this idea was so contrary to the
received opinions that it was proposed to put Cleanthes on his trial for
impiety.
In former times the philosophers who studied the solar system ranged
themselves in several “schools of thought,” to use a modern hackneyed
phrase. Some upheld the Ptolemaic system, which took its name from a
great Egyptian astronomer, Claudius Ptolemy, though it does not appear
that he was actually the first to suggest it. The Ptolemaic system
regarded the Earth as the centre, with the following bodies, all called
planets, revolving round it in the order stated:—the Moon, Mercury,
Venus, the Sun, Mars, Jupiter, and Saturn. It will be observed that
there are seven bodies here named, and as seven was regarded as the
“number of perfection,” it was in later times considered that only these
seven bodies (neither more nor less) could really be the Earth’s
celestial attendants. Though Ptolemy was in one sense an Egyptian, there
yet prevailed amongst the Egyptians at large another theory slightly
different from Ptolemy’s. According to the “Egyptian theory,” Mercury
and Venus were regarded as satellites of the Sun, and not as primary
planets appurtenant to the Earth.
After Ptolemy’s era many centuries elapsed, during which the whole
subject of the solar system lay practically dormant, and it continued so
until the revival of learning brought new theorists upon the scene. The
most important of these was Copernicus, who, in the sixteenth century,
propounded a theory which eventually superseded all others, and, with
slight modifications, is the one now accepted. Copernicus placed the Sun
in the centre of the system, and treated it as the point around which
all the primary planets revolved. So far, so good; but Copernicus went
astray on the question of the orbits of the planets. He failed to
realise the true character of the curves which they follow and treated
these curves as “epicycles,” which word may be described as representing
a complicated combination of little circles which taken together form a
big one. It was left to Kepler and Newton to settle all such details on
a true and firm basis. But before this stage was reached a man of the
highest astronomical attainments and practical experience, Tycho Brahe,
made shipwreck of his reputation as an astronomer by solemnly reviving
the idea of the Earth being the immovable centre of everything. He
treated the Moon as revolving round the Earth at no great distance and
the Sun as doing the same thing a little farther off; the five planets
revolving round the sun as solar satellites. The “Tychonic system,” as
it is called, has something in common with the Ptolemaic system without
being by any means as logical as the latter. That such far-fetched ideas
as Tycho’s should have been palmed off on the world of science so
recently as 300 years ago is passing strange; but the explanation
appears to be that his action arose out of a misconception of certain
passages of Holy Scripture, which seemed irreconcilable with the
Copernican theory. It must not be forgotten that Copernicus’s famous
book, published in 1543, in which he had announced his views, had been
condemned by the Papal “Congregation of the Index;” and therefore Tycho
might have had as a further motive a desire to curry favour with the
authorities of the Church of Rome, and to gratify his own vanity at the
same time.
With these explanations it will no longer be misleading if, for
convenience sake, I speak of a certain great circle of the heavens as
apparently traversed by the Sun every year, owing to the revolution of
our Earth round that body. This circle is called the “Ecliptic,” and its
plane is usually employed by astronomers as a fixed plane of reference.
It must be distinguished from that other great circle called the
“celestial equator,” which is the plane of the Earth’s equator extended
towards the stars. The plane of the equator is inclined to the ecliptic
at an angle of about 23½°, which angle is known as the “obliquity of the
ecliptic.” It is this inclination which gives rise to the seasons which
follow one another in succession during our annual journey round the
Sun. The two points where the celestial equator and the ecliptic
intersect are called the “equinoxes,” of spring or autumn as the case
may be; the points midway between these being the “solstices,” of summer
or winter as the case may be. These words need but little explanation,
at any rate, as regards those persons who are able to trace the Latin
origin of the words. “Equinox” is simply the place occupied by the Sun
twice every year (namely about March 20 and September 22), when day and
night are theoretically equal throughout the world, when also the sun
rises exactly in the east and sets exactly in the west. The “solstices”
represent the standing still of the sun at the given times and places,
and are the neutral points where the Sun attains its greatest northern
or southern declination. This usually occurs about June 21 and December
21. It must not be forgotten by the way, that the above application of
the words “summer” and “winter” to the solstices is only correct so far
as concerns places in northern terrestrial latitudes—Europe and the
United States, for instance. In southern terrestrial latitudes—for
instance, when speaking of what happens at the Cape of Good Hope and in
Australia—the words must be reversed.
We have seen in a previous chapter that whilst the orbits of the planets
are nearly true circles, none of them are quite such: and the departure
from the truly circular form results in some important consequences.
Whilst some of these are too technical to be explained in detail here,
one at least must be referred to because of what it involves. Not only
is the Earth’s orbit eccentric in form, but its eccentricity varies
within narrow limits; and besides this the orbit itself, as a whole, is
subject to a periodical shift of place, from the joint effect of all
which changes it comes about that our seasons are now of unequal length,
the spring and summer quarters of the year unitedly extending to 186
days, whilst the autumn and winter quarters comprise only 178 days. The
sun therefore has the chance of shining for a longer absolute period of
time over the northern hemisphere than over the southern hemisphere;
hence the northern is the warmer of the two hemispheres, because it has
a better, because a longer, chance of storing up an accumulation of
solar radiant heat. Probably it is one result of this that the north
polar regions of the Earth are easier of access than the south polar
regions. In the northern hemisphere navigators have reached to 81° of
latitude, whereas 71° is the highest limit yet attained in the southern
hemisphere. Readers who have studied the history of explorations in the
Arctic regions will not need to be reminded of the controversy which has
so often arisen respecting the existence or nonexistence of an “Open
Polar Sea.”
It has already been hinted that it is not an easy matter to determine,
when dealing with the Earth, where astronomy and its allied sciences,
geography, geodesy and geology respectively, begin and end. But as
certain topics connected with these sciences, such as the rotundity of
the Earth and its rotation on its axis, will come more conveniently
under consideration in other volumes of this series, I shall pass them
over and only treat of a few things which more directly concern the
student of nature observing either with or without the assistance of a
telescope.
The fact that the Earth is surrounded by a considerable atmosphere
largely composed of aqueous vapour has a material bearing on the success
or failure of observations made on the Earth of bodies situated at a
distance. It may be taken as a general rule that the nearer an observer
is to the surface of the sea, or otherwise to the surface of the land at
the sea-level, the greater will be the difficulty which will confront
him in carrying on astronomical observations. Hence such observations
are generally made with unsatisfactory results on the sea coast or on
the banks of rivers. An interesting but rather ancient illustration of
this last-named fact is to be found in the circumstance that Copernicus,
who died at the age of 70, complained in his last moments that much as
he had tried he had never succeeded in detecting the planet Mercury, a
failure due, as Gassendi supposed, to the vapours prevailing near the
horizon at the town of Thorn on the banks of the Vistula where the
illustrious philosopher lived.
The phenomena depending on the presence of aqueous vapour in the
atmosphere which especially under the notice of the astronomer are
Refraction, Twilight, and the Twinkling of the Stars.
Refraction is what it professes to be, a bending, and what is bent is
the ray of light coming from a celestial object to a terrestrial
station. Olmsted has put the matter in this way:—“We must consider that
any such object always appears in the direction in which the _last_ ray
of light comes to the eye. If the light which comes from a star were
bent into fifty directions before it reached the eye, the star would
nevertheless appear in a line described by the ray nearest the eye. The
operation of this principle is seen when an oar, or any stick, is thrust
into the water. As the rays of light by which the oar is seen have their
direction changed as they pass out of water into air, the apparent
direction in which the body is seen is changed in the same degree,
giving it a bent appearance—the part below the water having apparently a
different direction from the part above.” The direction of this
refraction is determined by the general law of optics that when a ray of
light passes out of a rarer into a denser medium (for instance out of
air into water, or out of space into the Earth’s atmosphere) it is bent
_towards_ a perpendicular to the surface of the medium; but when it
passes out of a denser into a rarer medium it is bent _from_ the
perpendicular. The effect of refraction is to make a heavenly body
appear to have an apparent altitude greater than its true altitude, so
that, for example, an object situated actually _in_ the horizon will
appear _above_ it. Indeed it sometimes happens that objects which are
actually below the horizon and which otherwise would be invisible were
it not for refraction are thus brought into sight. It was in consequence
of this that on April 20, 1837, the Moon rose eclipsed before the Sun
had set.
Sir Henry Holland thus alludes to the phenomenon:—“I am tempted to
notice a spectacle, having a certain association with this science,
which I do not remember to have seen recorded either in prose or poetry,
though well meriting description in either way. This spectacle requires,
however, a combination of circumstances rarely occurring—a perfectly
clear Eastern and Western horizon, and an entirely level intervening
surface, such as that of the sea or the African desert—the former
rendering the illusion, if such it may be called, most complete to the
eye. The view I seek to describe embraces the orb of the setting Sun,
and that of the full Moon rising in the East—_both above the horizon at
the same time_. The spectator on the sea between, if he can discard from
mental vision the vessel on which he stands, and regard only these two
great globes of Heaven and the sea-horizon circling unbroken around him,
gains a conception through this spectacle clearer than any other
conjunction can give, of those wonderful relations which it is the
triumph of astronomy to disclose. All objects are excluded save the Sun,
the Moon, and our own Globe between, but these objects are such in
themselves that their very simplicity and paucity of number enhances the
sense of the sublime. Only twice or thrice, however, have I witnessed
the sight in its completeness—once on a Mediterranean voyage between
Minorca and Sardinia—once in crossing the desert from Suez to Cairo,
when the same full Moon showed me, a few hours later, the very different
but picturesque sight of one of the annual caravans of Mecca pilgrims,
with a long train of camels making their night march towards the Red
Sea.”[3]
It is due to the same cause that the Sun and the Moon when very near the
horizon may often be noticed to exhibit a distorted oval outline. The
fact simply is, that the upper and the lower limbs undergo a different
degree of refraction. The lower limb being nearer the horizon is more
affected and is consequently raised to a greater extent than the upper
limb, the resulting effect being that the two limbs are seemingly
squeezed closer together by the difference of the two refractions. The
vertical diameter is compressed and the circular outline becomes thereby
an oval outline with the lesser axis vertical and the greater axis
horizontal.
Though the foregoing information merely embraces a few general
principles and facts, the reader will have no difficulty in
understanding that refraction exercises a very inconvenient disturbing
influence on observations which relate to the exact places of celestial
objects. No such observations are available for mutual comparison,
however great the skill of the observer, or the perfection of his
instrument, unless, and until certain corrections are applied to the
observed positions in order to neutralise the disturbing effects of
refraction. In practice this is usually done by means of tables of
corrections, those in most general use being Bessel’s. Inasmuch as
refraction depends upon the aqueous vapour in the atmosphere, its amount
at any given moment is affected by the height of the barometer and the
temperature of the air. Accordingly when, for any purpose, the utmost
precision is required, it is necessary to take into account the height
of the barometer and the position of the mercury in the thermometer at
the moment in question. At the zenith there is no refraction whatever,
objects appearing projected on the background of the sky exactly in the
position they would occupy were the earth altogether destitute of an
atmosphere at all. The amount of the refraction increases gradually, but
in accordance with a very complex law, from the zenith to the horizon.
Thus the displacement due to refraction which at the zenith is nothing
and at an altitude of 45° is only 57″ becomes at the horizon more than
½°. One very curious consequence is involved in the fact that the
displacement due to refraction is at the horizon what it is; the
diameter both of the Sun and Moon may be said to be ½°, more or less, so
that when we see the lower edge of either of these luminaries just
_touching_ the horizon _in reality_ the whole disc is completely _below_
it, and would be altogether hidden by the convexity of the earth were it
not for the existence of the earth’s atmosphere and the consequent
refraction of the rays of light passing through it from the Sun (or
Moon) to the observer.
Twilight is another phenomenon associated with astronomical principles
and effects which depends in some degree on the Earth’s atmosphere and
on the laws which regulate the reflection and refraction of light. After
the Sun has set it continues to illuminate the clouds and upper strata
of the air just as it may often be seen shining on the tops of hills
long after it has disappeared from the view of the inhabitants of the
plains below, and indeed may illuminate the chimneys of a house when it
is no longer visible to a person standing in the garden below. The air
and clouds thus illuminated reflect some of the Sun’s light to the
surface of the earth lying immediately underneath, and thus produce
after sun-set and before sun-rise, in a degree more or less considerable
according as the Sun is only a little or is much depressed below the
horizon, that luminous glow which we call “twilight.” This word is of
Saxon origin and implies the presence of a twin, or double, light. As
soon as the Sun has disappeared below the horizon all the clouds
overhead continue for a few minutes so highly illuminated as to reflect
scarcely less light than the direct light of the Sun. As, however, the
Sun gradually sinks lower and lower, less and less of the visible
atmosphere receives any portion of its light, and consequently less and
less is reflected minute by minute to the Earth at the observer’s
station until at length the time comes when there is no sunlight to be
reflected—and it is night. The converse of all this happens before and
up to sun-rise; night ceases, twilight ensues, gradually becoming more
definite; the dawn appears, and finally the full Sun bursts forth. It
may here be stated as a note by the way that the circumstances under
which the Sun first shows itself after it has risen above the horizon
has some bearing on the probable character of the weather which is at
hand. When the first indications of day-light are seen above a bank of
clouds it is thought to be a sign of wind; but if the first streaks of
light are discovered low down, that is in, or very near the horizon,
fair weather may be expected.
Twilight is usually reckoned to last until the Sun has sunk 18° below
the horizon, but the question of its duration depends on where the
observer is stationed, on the season of the year, and (in a slight
degree) on the condition of the atmosphere. The general rule is that the
twilight is least in the tropics and increases as the observer moves
away from the equator towards either pole. Whilst in the tropics a
depression of 16° or 17° is sufficient to put an end to the phenomenon,
in the latitude of England a depression of from 17° to 20° is required.
As implied above, it varies with the latitude; and as regards the
different seasons of the year, it is least on March 1 and October 12,
being three weeks before the vernal equinox and three weeks after the
autumnal equinox. The duration at the equator may be about 1 hour 12
minutes; it amounts to nearly 2 hours at the latitude of Greenwich, and
so on towards the pole. At each pole in turn the Sun is below the
horizon for 6 months, but as it is less than 18° below the horizon for
about 3½ of those 6 months it may be said that there is a continual
twilight for those 3½ months. Something of the same sort of thing as
this occurs in the latitude of Greenwich, for there is no true night at
Greenwich from May 22 to July 21, but constant twilight from sunset to
sunrise, or 2 months of twilight in all. Though twilight at the equator
is commonly set down as lasting about an hour, this period is there, as
elsewhere, affected by the elevation of the observer above the
sea-level. Where the air is very rarified, as at places situated as
Quito and Lima are, the twilight is said to last no more than 20
minutes, and this would accord with the theory that where there is no
air at all (_e.g._, on the Moon) there is no twilight at all. The
greater purity and clearness of mountain air, rarified as it is, is
another cause which contributes to vary by reducing the duration of
twilight.
It is sometimes stated that a secondary twilight may be noticed, and Sir
John Herschel has spoken of it as “consequent on a re-reflection of the
rays dispersed through the atmosphere in the primary one. The phenomenon
seen in the clear atmosphere of the Nubian Desert, described by
travellers under the name of the ‘afterglow,’ would seem to arise from
this cause.” I am not acquainted with any records which throw light on
these remarks of Sir John Herschel.
The phenomenon of twinkling is a subject which has been much neglected,
possibly on account of its apparent, but only apparent, simplicity. The
familiar verse of our days of childhood—
“Twinkle, twinkle little star,
How I wonder what you are,
Up above the earth so high,
Like a diamond in the sky,”
contains even in this simple form a good deal of food for reflection;
whilst the new version—
“Twinkle, twinkle little star,
Now we’ve found out what you are,
When unto the midnight sky
We the spectroscope apply,”
does so yet more.
As an optical phenomenon the twinkling, or to use the more scientific
phrase, the scintillation, of the stars is a matter which has been
strangely ignored by physicists. Indeed, the only investigators who seem
to have dealt with it in any sort of detail are two Italians, Secchi and
Respighi, Dufour, a Frenchman, Montigny, a Belgian, and the Rev. E.
Ledger, an Englishman. Secchi has truly remarked that the twinkling of
the stars is one of the most beautiful of the minor phenomena of the
heavens. Light, sometimes bright, sometimes feeble, sometimes white,
sometimes red, darts about in intermittent gleams, like the sparkling
flashes of a well-cut diamond, and works upon the feelings of even the
most stolid spectator. The theory of twinkling is still surrounded by
many difficulties. One thing, however, is certain—it has nothing to do
with recurrent changes in the intrinsic light or physical condition of
the star itself, but arises during the passage of its rays through our
atmosphere; it depends, therefore, in some way or other on the varying
conditions of the atmosphere. On the summit of high mountains, according
to the observations of all careful observers (notably Tacchini, who
studied the subject on Mount Etna), the light of the stars is steady,
like that of the planets; and it is so likewise during the hours of calm
which often precede terrestrial storms. The vibrations are usually more
frequent near the horizon, and diminish with the elevation of the star
above the horizon; in other words, with the lessening of the thickness
of the atmospheric strata which the rays of light have to traverse.
Nevertheless, during windy weather, and specially with northerly wind,
it may be noticed that the stars twinkle high up above the horizon, and
even as far as the zenith. From these and other similar considerations
we are justified in drawing the conclusion that twinkling largely
depends on the condition and movements of the atmosphere.
Secchi further points out that it is impossible to study carefully with
the naked eye all the features of twinkling, and that telescopic
assistance is imperatively necessary. When, with the aid of a telescope,
we scrutinise a star during a disturbed evening marked by much twinkling
we see an image diffused and undefined and surrounded by rays, as if
several images were superposed, and were jumping about rapidly. On such
occasions we do not see that little defined disc surrounded by
motionless diffraction rings, ordinarily indicative of a tranquil
atmosphere. With a telescope armed with a medium power, the field of
view of which is more extensive than that of a high power, we find that
if a light tap is given to the telescope, the ordinary simple image is
changed into a luminous curve, the perimeter of which is formed entirely
of a succession of arcs exhibiting the colours of the rainbow. This
coloured curve does not, in principle, differ from what one sees on
swinging round and round in the air such a thing as a stick, the end of
which is alight, having been freshly taken from a fire. The glowing tip
produces in appearance a continuous arc, the result of the persistence
of the image of the tip on the retina. In such a case the colour is
constant, because the illumination resulting from the blazing wood does
not vary; but in the case of a star the arcs are differently coloured
during the very brief space of time in which the vibrating telescope
transports the image from one side to another of the visible field. This
experiment is from its nature very crude, but the idea was improved upon
and reduced to a systematic shape by Montigny, who introduced into his
telescope, at a certain distance from the eyepiece, a concave lens
eccentrically placed with respect to the axis of the instrument, and
endued with a rapid movement of rotation imparted by suitable mechanism.
He thus obtained images which revolved with regularity, and so was able
to submit certain features of the phenomenon to a definite system of
measurement. To cut a long story short, Montigny started with the
assumption (made good by the sequel) that possibly stars were affected
in their twinkling by intrinsic constitutional differences; and that
possibly Secchi’s classification of stars into four types (a
classification which depends on the spectra which they yield) might put
him on the track of some intelligible conclusions with respect to the
theory of twinkling.[4]
The results he ultimately arrived at were, that the yellow and red stars
of the IInd and IIIrd types twinkle less rapidly than the white stars of
the Ist type. Whilst the average number of scintillations per second of
the stars of type III. were 56, those of type II. were 69, and those of
type I. 86. These differences may be confidently said to depend upon too
many observations of too many different stars to be fortuitous. Montigny
also arrived at a number of incidental conclusions of considerable
interest. The one main thread running through them, is that there is a
connection between the twinkling of a star and its spectrum, which had
never before been thought of. We are justified, indeed, in going so far
as to say, that Montigny’s observations point distinctly to a law on
this subject, the law being that the more the spectrum of a star is
interrupted by dark lines, the less frequent are its scintillations. The
individual character of the light, therefore, emitted by any given star
appears to affect its twinkling, both as regards the frequency thereof
and the colours displayed.
Montigny collected some other interesting facts with reference to
twinkling, which may here be stated in a concise form. There is a
greater display of twinkling in showery weather, than when the
atmosphere is in a normal condition; and in winter than in summer,
whatever may be the weather. In dry weather in Spring and Autumn the
twinkling is about the same, but wet has more effect in Autumn than in
Spring in developing the phenomenon. Variations in the barometric
pressure and in the humidity of the air also affect the amount of
twinkling; there is more before a rainy period, likely to last 2 or 3
days, than before a single, or, so to speak, casual rainy day. Twinkling
also varies with the aggregate total rain-fall of any group of days,
being more pronounced as the rain-fall is greater, but decreasing
suddenly and considerably as soon as the rainy condition of the
atmosphere has passed away. The number of scintillations found to be
observable with the aid of Montigny’s instrument (which he called a
“_scintillomètre_”), varied from a minimum of 50 during June and July,
to 97 in January, and 101 in February, increasing and decreasing in
regular sequence from month to month. When an Aurora Borealis is
visible, there is a marked increase in the amount of twinkling. It would
be interesting to follow up this last named discovery by an endeavour to
ascertain whether the fluctuations which are coincident in point of time
with an Auroral display depend upon optical considerations connected
with the Aurora, or on physical considerations having any relation to
the increased development of terrestrial magnetism.
I have been thus particular in unfolding somewhat fully the present
state of our knowledge concerning the twinkling of the stars, because it
is evident that there are many interesting points connected with it,
which may be studied by any patient and attentive star-gazer, and which
do not need the instrumental appliances and technical refinements which
are only to be found in fully-equipped public and private observatories.
It should be mentioned in conclusion that the planets twinkle very
little, or, more often, not at all. This is mainly due to the fact that
they exhibit discs of sensible diameter and therefore that there is, as
Young puts it, “a general unchanging average of brightness for the sum
total of all the luminous points of which the disc is composed. When,
for instance, point A of the disc becomes dark for a moment, point B,
very near to it, is just as likely to become bright; the interference
conditions being different for the 2 points. The different points of the
disc _do not keep step_, so to speak, in their twinkling.” The
non-twinkling of planets because they possess sensible discs is often
available as a means for determining when a planet is looked for, which,
of several objects looked at, is the planet wanted and which are merely
stars.