The Story of the Solar System :: CHAPTER V. THE EARTH.

CHAPTER V. 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 ? 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.

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