From these considerations it will be seen at once that the inferior planets show various phases comparable to the waxing and waning of our moon in its monthly round. Superior conjunction is, in fact, similar to full moon, and inferior conjunction to new moon; while the eastern and western elongations may be compared respectively to the moon"s first and last quarters. It will be recollected how, when these phases were first seen by the early telescopic observers, the Copernican theory was felt to be immensely strengthened; for it had been pointed out that if this system were the correct one, the planets Venus and Mercury, were it possible to see them more distinctly, would of necessity present phases like these when viewed from the earth. It should here be noted that the telescope was not invented until nearly seventy years after the death of Copernicus.
The apparent swing of an inferior planet from side to side of the sun, at one time on the east side, then pa.s.sing into and lost in the sun"s rays to appear once more on the west side, is the explanation of what is meant when we speak of an _evening_ or a _morning star_. An inferior planet is called an evening star when it is at its eastern elongation, that is to say, on the left-hand of the sun; for, being then on the eastern side, it will set after the sun sets, as both sink in their turn below the western horizon at the close of day. Similarly, when such a planet is at its western elongation, that is to say, to the right-hand of the sun, it will go in advance of him, and so will rise above the eastern horizon before the sun rises, receiving therefore the designation of morning star. In very early times, however, before any definite ideas had been come to with regard to the celestial motions, it was generally believed that the morning and evening stars were quite distinct bodies. Thus Venus, when a morning star, was known to the ancients under the name of Phosphorus, or Lucifer; whereas they called it Hesperus when it was an evening star.
Since an inferior planet circulates between us and the sun, one would be inclined to expect that such a body, each time it pa.s.sed on the side nearest to the earth, should be seen as a black spot against the bright solar disc. Now this would most certainly be the case were the orbit of an inferior planet in the same plane with the orbit of the earth. But we have already seen how the orbits in the solar system, whether those of planets or of satellites, are by no means in the one plane; and that it is for this very reason that the moon is able to pa.s.s time after time in the direction of the sun, at the epoch known as new moon, and yet not to eclipse him save after the lapse of several such pa.s.sages. Transits, then, as the pa.s.sages of an inferior planet across the sun"s disc are called, take place, for the same reason, only after certain regular lapses of time; and, as regards the circ.u.mstances of their occurrence, are on a par with eclipses of the sun. The latter, however, happen much more frequently, because the moon pa.s.ses in the neighbourhood of the sun, roughly speaking, once a month, whereas Venus comes to each inferior conjunction at intervals so long apart as a year and a half, and Mercury only about every four months. From this it will be further gathered that transits of Mercury take place much oftener than transits of Venus.
Until recent years _Transits of Venus_ were phenomena of great importance to astronomers, for they furnished the best means then available of calculating the distance of the sun from the earth. This was arrived at through comparing the amount of apparent displacement in the planet"s path across the solar disc, when the transit was observed from widely separated stations on the earth"s surface. The last transit of Venus took place in 1882, and there will not be another until the year 2004.
_Transits of Mercury_, on the other hand, are not of much scientific importance. They are of no interest as a popular spectacle; for the dimensions of the planet are so small, that it can be seen only with the aid of a telescope when it is in the act of crossing the sun"s disc. The last transit of Mercury took place on November 14, 1907, and there will be another on November 6, 1914.
The first person known to have observed a transit of an inferior planet was the celebrated French philosopher, Ga.s.sendi. This was the transit of Mercury which took place on the 7th of December 1631.
The first time a transit of Venus was ever seen, so far as is known, was on the 24th of November 1639. The observer was a certain Jeremiah Horrox, curate of Hoole, near Preston, in Lancashire. The transit in question commenced shortly before sunset, and his observations in consequence were limited to only about half-an-hour. Horrox happened to have a great friend, one William Crabtree, of Manchester, whom he had advised by letter to be on the look out for the phenomenon. The weather in Crabtree"s neighbourhood was cloudy, with the result that he only got a view of the transit for about ten minutes before the sun set.
That this transit was observed at all is due entirely to the remarkable ability of Horrox. According to the calculations of the great Kepler, no transit could take place that year (1639), as the planet would just pa.s.s clear of the lower edge of the sun. Horrox, however, not being satisfied with this, worked the question out for himself, and came to the conclusion that the planet would _actually_ traverse the lower portion of the sun"s disc. The event, as we have seen, proved him to be quite in the right. Horrox is said to have been a veritable prodigy of astronomical skill; and had he lived longer would, no doubt, have become very famous. Unfortunately he died about two years after his celebrated transit, in his _twenty-second_ year only, according to the accounts.
His friend Crabtree, who was then also a young man, is said to have been killed at the battle of Naseby in 1645.
There is an interesting phenomenon in connection with transits which is known as the "Black Drop." When an inferior planet has just made its way on to the face of the sun, it is usually seen to remain for a short time as if attached to the sun"s edge by what looks like a dark ligament (see Fig. 12, p. 153). This gives to the planet for the time being an elongated appearance, something like that of a pear; but when the ligament, which all the while keeps getting thinner and thinner, has at last broken, the black body of the planet is seen to stand out round against the solar disc.
[Ill.u.s.tration: FIG. 12.--The "Black Drop."]
This appearance may be roughly compared to the manner in which a drop of liquid (or, preferably, of some glutinous substance) tends for a while to adhere to an object from which it is falling.
When the planet is in turn making its way off the face of the sun, the ligament is again seen to form and to attach it to the sun"s edge before its due time.
The phenomenon of the black drop, or ligament, is entirely an illusion, and, broadly speaking, of an optical origin. Something very similar will be noticed if one brings one"s thumb and forefinger _slowly_ together against a very bright background.
This peculiar phenomenon has proved one of the greatest drawbacks to the proper observation of transits, for it is quite impossible to note the exact instant of the planet"s entrance upon and departure from the solar disc in conditions such as these.
The black drop seems to bear a family resemblance, so to speak, to the phenomenon of Baily"s beads. In the latter instance the lunar peaks, as they approach the sun"s edge, appear to lengthen out in a similar manner and bridge the intervening s.p.a.ce before their time, thus giving prominence to an effect which otherwise should scarcely be noticeable.
The last transit of Mercury, which, as has been already stated, took place on November 14, 1907, was not successfully observed by astronomers in England, on account of the cloudiness of the weather. In France, however, Professor Moye, of Montpellier, saw it under good conditions, and mentions that the black drop remained very conspicuous for fully a minute. The transit was also observed in the United States, the reports from which speak of the black drop as very "troublesome."
Before leaving the subject of transits it should be mentioned that it was in the capacity of commander of an expedition to Otaheite, in the Pacific, to observe the transit of Venus of June 3, 1769, that Captain Cook embarked upon the first of his celebrated voyages.
In studying the surfaces of Venus and Mercury with the telescope, observers are, needless to say, very much hindered by the proximity of the sun. Venus, when at the greatest elongations, certainly draws some distance out of the glare; but her surface is, even then, so dazzlingly bright, that the markings upon it are difficult to see. Mercury, on the other hand, is much duller in contrast, but the disc it shows in the telescope is exceedingly small; and, in addition, when that planet is left above the horizon for a short time after sunset, as necessarily happens after certain intervals, the mists near the earth"s surface render observation of it very difficult.
Until about twenty-five years ago, it was generally believed that both these planets rotated on their axes in about twenty-four hours, a notion, no doubt, originally founded upon an unconscious desire to bring them into some conformity with our earth. But Schiaparelli, observing in Italy, and Percival Lowell, in the clear skies of Arizona and Mexico, have lately come to the conclusion that both planets rotate upon their axes in the same time as they revolve in their orbits,[12] the result being that they turn one face ever towards the sun in the same manner that the moon turns one face ever towards the earth--a curious state of things, which will be dealt with more fully when we come to treat of our satellite.
The marked difference in the brightness between the two planets has already been alluded to. The surface of Venus is, indeed, about five times as bright as that of Mercury. The actual brightness of Mercury is about equivalent to that of our moon, and astronomers are, therefore, inclined to think that it may resemble her in having a very rugged surface and practically no atmosphere. This probable lack of atmosphere is further corroborated by two circ.u.mstances. One of these is that when Mercury is just about to transit the face of the sun, no ring of diffused light is seen to encircle its disc as would be the case if it possessed an atmosphere. Such a lack of atmosphere is, indeed, only to be expected from what is known as the _Kinetic Theory of Gases_.
According to this theory, which is based upon the behaviour of various kinds of gas, it is found that these elements tend to escape into s.p.a.ce from the surface of bodies whose force of gravitation is weak. Hydrogen gas, for example, tends to fly away from our earth, as any one may see for himself when a balloon rises into the air. The gravitation of the earth seems, however, powerful enough to hold down other gases, as, for instance, those of which the air is chiefly composed, namely, oxygen and nitrogen. In due accordance with the Kinetic theory, we find the moon and Mercury, which are much about the same size, dest.i.tute of atmospheres. Mars, too, whose diameter is only about double that of the moon, has very little atmosphere. We find, on the other hand, that Venus, which is about the same size as our earth, clearly possesses an atmosphere, as just before the planet is in transit across the sun, the outline of its dark body is seen to be surrounded by a bright ring of light.
The results of telescopic observation show that more markings are visible on Mercury than on Venus. The intense brilliancy of Venus is, indeed, about the same as that of our white clouds when the sun is shining directly upon them. It has, therefore, been supposed that the planet is thickly enveloped in cloud, and that we do not ever see any part of its surface, except perchance the summit of some lofty mountain projecting through the fleecy ma.s.s.
With regard to the great brilliancy of Venus, it may be mentioned that she has frequently been seen in England, with the naked eye in full sunshine, when at the time of her greatest brightness. The writer has seen her thus at noonday. Needless to say, the sky at the moment was intensely blue and clear.
The orbit of Mercury is very oval, and much more so than that of any other planet. The consequence is that, when Mercury is nearest to the sun, the heat which it receives is twice as great as when it is farthest away. The orbit of Venus, on the other hand, is in marked contrast with that of Mercury, and is, besides, more nearly of a circular shape than that of any of the other planets. Venus, therefore, always keeps about the same distance from the sun, and so the heat which she receives during the course of her year can only be subject to very slight variations.
[11] In employing the terms Inferior and Superior the writer bows to astronomical custom, though he cannot help feeling that, in the circ.u.mstances, Interior and Exterior would be much more appropriate.
[12] This question is, however, uncertain, for some very recent spectroscopic observations of Venus seem to show a rotation period of about twenty-four hours.
CHAPTER XV
THE EARTH
We have already seen (in Chapter I.) how, in very early times, men naturally enough considered the earth to be a flat plane extending to a very great distance in every direction; but that, as years went on, certain of the Greek philosophers suspected it to be a sphere. One or two of the latter are, indeed, said to have further believed in its rotation about an axis, and even in its revolution around the sun; but, as the ideas in question were founded upon fancy, rather than upon any direct evidence, they did not generally attract attention. The small effect, therefore, which these theories had upon astronomy, may well be gathered from the fact that in the Ptolemaic system the earth was considered as fixed and at the centre of things; and this belief, as we have seen, continued unaltered down to the days of Copernicus. It was, indeed, quite impossible to be certain of the real shape of the earth or the reality of its motions until knowledge became more extended and scientific instruments much greater in precision.
We will now consider in detail a few of the more obvious arguments which can be put forward to show that our earth is a sphere.
If, for instance, the earth were a plane surface, a ship sailing away from us over the sea would appear to grow smaller and smaller as it receded into the distance, becoming eventually a tiny speck, and fading gradually from our view. This, however, is not at all what actually takes place. As we watch a vessel receding, its hull appears bit by bit to slip gently down over the horizon, leaving the masts alone visible.
Then, in their turn, the masts are seen to slip down in the same manner, until eventually every trace of the vessel is gone. On the other hand, when a ship comes into view, the masts are the first portions to appear.
They gradually rise up from below the horizon, and the hull follows in its turn, until the whole vessel is visible. Again, when one is upon a ship at sea, a set of masts will often be seen sticking up alone above the horizon, and these may shorten and gradually disappear from view without the body of the ship to which they belong becoming visible at all. Since one knows from experience that there is no _edge_ at the horizon over which a vessel can drop down, the appearance which we have been describing can only be explained by supposing that the surface of the earth is always curving gradually in every direction.
The distance at which what is known as the _horizon_ lies away from us depends entirely upon the height above the earth"s surface where we happen at the moment to be. A ship which has appeared to sink below the horizon for a person standing on the beach, will be found to come back again into view if he at once ascends a high hill. Experiment shows that the horizon line lies at about three miles away for a person standing at the water"s edge. The curving of the earth"s surface is found, indeed, to be at the rate of eight inches in every mile. Now it can be ascertained, by calculation, that a body curving at this rate in every direction must be a globe about 8000 miles in diameter.
Again, the fact that, if not stopped by such insuperable obstacles as the polar ice and snow, those who travel continually in any one direction upon the earth"s surface always find themselves back again at the regions from which they originally set out, is additional ground for concluding that the earth is a globe.
We can find still further evidence. For instance, in an eclipse of the moon the earth"s shadow, when seen creeping across the moon"s face, is noted to be _always_ circular in shape. One cannot imagine how such a thing could take place unless the earth were a sphere.
Also, it is found from observation that the sun, the planets, and the satellites are, all of them, round. This roundness cannot be the roundness of a flat plate, for instance, for then the objects in question would sometimes present their thin sides to our view. It happens, also, that upon the discs which these bodies show, we see certain markings shifting along continually in one direction, to disappear at one side and to reappear again at the other. Such bodies must, indeed, be spheres in rotation.
The crescent and other phases, shown by the moon and the inferior planets, should further impress the truth of the matter upon us, as such appearances can only be caused by the sunlight falling from various directions upon the surfaces of spherical bodies.
Another proof, perhaps indeed the weightiest of all, is the continuous manner in which the stars overhead give place to others as one travels about the surface of the earth. When in northern regions the Pole Star and its neighbours--the stars composing the Plough, for instance--are over our heads. As one journeys south these gradually sink towards the northern horizon, while other stars take their place, and yet others are uncovered to view from the south. The regularity with which these changes occur shows that every point on the earth"s surface faces a different direction of the sky, and such an arrangement would only be possible if the earth were a sphere. The celebrated Greek philosopher, Aristotle, is known to have believed in the globular shape of the earth, and it was by this very argument that he had convinced himself that it was so.
The idea of the sphericity of the earth does not appear, however, to have been generally accepted until the voyages of the great navigators showed that it could be sailed round.
The next point we have to consider is the rotation of the earth about its axis. From the earliest times men noticed that the sky and everything in it appeared to revolve around the earth in one fixed direction, namely, towards what is called the West, and that it made one complete revolution in the period of time which we know as twenty-four hours. The stars were seen to come up, one after another, from below the eastern horizon, to mount the sky, and then to sink in turn below the western horizon. The sun was seen to perform exactly the same journey, and the moon, too, whenever she was visible. One or two of the ancient Greek philosophers perceived that this might be explained, either by a movement of the entire heavens around the earth, or by a turning motion on the part of the earth itself. Of these diverse explanations, that which supposed an actual movement of the heavens appealed to them the most, for they could hardly conceive that the earth should continually rotate and men not be aware of its movement. The question may be compared to what we experience when borne along in a railway train. We see the telegraph posts and the trees and buildings near the line fly past us one after another in the contrary direction. Either these must be moving, or we must be moving; and as we happen to _know_ that it is, indeed, we who are moving, there can be no question therefore about the matter. But it would not be at all so easy to be sure of this movement were one unable to see the objects close at hand displacing themselves.
For instance, if one is shut up in a railway carriage at night with the blinds down, there is really nothing to show that one is moving, except the jolting of the train. And even then it is hard to be sure in which direction one is actually travelling.
The way we are situated upon the earth is therefore as follows. There are no other bodies sufficiently near to be seen flying past us in turn; our earth spins without a jolt; we and all things around us, including the atmosphere itself, are borne along together with precisely the same impetus, just as all the objects scattered about a railway carriage share in the forward movement of the train. Such being the case, what wonder that we are unconscious of the earth"s rotation, of which we should know nothing at all, were it not for that slow displacement of the distant objects in the heavens, as we are borne past them in turn.
If the night sky be watched, it will be soon found that its apparent turning movement seems to take place around a certain point, which appears as if fixed. This point is known as the north pole of the heavens; and a rather bright star, which is situated very close to this hub of movement, is in consequence called the Pole Star. For the dwellers in southern lat.i.tudes there is also a point in their sky which appears to remain similarly fixed, and this is known as the south pole of the heavens. Since, however, the heavens do not turn round at all, but the earth does, it will easily be seen that these apparently stationary regions in the sky are really the points towards which the axis of the earth is directed. The positions on the earth"s surface itself, known as the North and South Poles, are merely the places where the earth"s axis, if there were actually such a thing, would be expected to jut out. The north pole of the earth will thus be situated exactly beneath the north pole of the heavens, and the south pole of the earth exactly beneath the south pole of the heavens.
We have seen that the earth rotates upon its imaginary axis once in about every twenty-four hours. This means that everything upon the surface of the earth is carried round once during that time. The measurement around the earth"s equator is about 24,000 miles; and, therefore, an object situated at the equator must be carried round through a distance of about 24,000 miles in each twenty-four hours.
Everything at the equator is thus moving along at the rapid rate of about 1000 miles an hour, or between sixteen and seventeen times as fast as an express train. If, however, one were to take measurements around the earth parallel to the equator, one would find these measurements becoming less and less, according as the poles were approached. It is plain, therefore, that the speed with which any point moves, in consequence of the earth"s rotation, will be greatest at the equator, and less and less in the direction of the poles; while at the poles themselves there will be practically no movement, and objects there situated will merely turn round.
The considerations above set forth, with regard to the different speeds at which different portions of a rotating globe will necessarily be moving, is the foundation of an interesting experiment, which gives us further evidence of the rotation of our earth. The measurement around the earth at any distance below the surface, say, for instance, at the depth of a mile, will clearly be less than a similar measurement at the surface itself. The speed of a point at the bottom of a mine, which results from the actual rotation of the earth, must therefore be less than the speed of a point at the surface overhead. This can be definitely proved by dropping a heavy object down a mine shaft. The object, which starts with the greater speed of the surface, will, when it reaches the bottom of the mine, be found, as might be indeed expected, to be a little ahead (_i.e._ to the east) of the point which originally lay exactly underneath it. The distance by which the object gains upon this point is, however, very small. In our lat.i.tudes it amounts to about an inch in a fall of 500 feet.
The great speed at which, as we have seen, the equatorial regions of the earth are moving, should result in giving to the matter there situated a certain tendency to fly outwards. Sir Isaac Newton was the first to appreciate this point, and he concluded from it that the earth must be _bulged_ a little all round the equator. This is, indeed, found to be the case, the diameter at the equator being nearly twenty-seven miles greater than it is from pole to pole. The reader will, no doubt, be here reminded of the familiar comparison in geographies between the shape of the earth and that of an orange.
In this connection it is interesting to consider that, were the earth to rotate seventeen times as fast as it does (_i.e._ in one hour twenty-five minutes, instead of twenty-four hours), bodies at the equator would have such a strong tendency to fly outwards that the force of terrestrial gravity acting upon them would just be counterpoised, and they would virtually have _no weight_. And, further, were the earth to rotate a little faster still, objects lying loose upon its surface would be shot off into s.p.a.ce.
The earth is, therefore, what is technically known as an _oblate spheroid_; that is, a body of spherical shape flattened at the poles. It follows of course from this, that objects at the polar regions are slightly nearer to the earth"s centre than objects at the equatorial regions. We have already seen that gravitation acts from the central parts of a body, and that its force is greater the nearer are those central parts. The result of this upon our earth will plainly be that objects in the polar regions will be pulled with a slightly stronger pull, and will therefore _weigh_ a trifle more than objects in the equatorial regions. This is, indeed, found by actual experiment to be the case. As an example of the difference in question, Professor Young, in his _Manual of Astronomy_, points out that a man who weighs 190 pounds at the equator would weigh 191 at the pole. In such an experiment the weighing would, however, have to be made with a _spring balance_, and _not with scales_; for, in the latter case, the "weights" used would alter in their weight in exactly the same degree as the objects to be weighed.