Astronomy of To-day Part 3

Light travels at the tremendous speed of about 186,000 miles a second.

It therefore takes only about a second and a quarter to come to us from the moon. It traverses the 93,000,000 of miles which separate us from the sun in about eight minutes. It travels from the sun out to Neptune in about four hours, which means that it would cross the solar system from end to end in eight. To pa.s.s, however, across the distance which separates us from Alpha Centauri it would take so long as about four and a quarter years!

Astronomers, therefore, agree in estimating the distances of the stars from the point of view of the time which light would take to pa.s.s from them to our earth. They speak of that distance which light takes a year to traverse as a "light year." According to this notation, Alpha Centauri is spoken of as being about four and a quarter light years distant from us.

Now as the rays of light coming from Alpha Centauri to us are chasing one another incessantly across the gulf of s.p.a.ce, and as each ray left that star some four years before it reaches us, our view of the star itself must therefore be always some four years old. Were then this star to be suddenly removed from the universe at any moment, we should continue to see it still in its place in the sky for some four years more, after which it would suddenly disappear. The rays which had already started upon their journey towards our earth must indeed continue travelling, and reaching us in their turn until the last one had arrived; after which no more would come.

We have drawn attention to Alpha Centauri as the nearest of the stars.

The majority of the others indeed are ever so much farther. We can only hazard a guess at the time it takes for the rays from many of them to reach our globe. Suppose, for instance, we see a sudden change in the light of any of these remote stars, we are inclined to ask ourselves when that change did actually occur. Was it in the days of Queen Elizabeth, or at the time of the Norman Conquest; or was it when Rome was at the height of her glory, or perhaps ages before that when the Pyramids of Egypt were being built? Even the last of these suppositions cannot be treated lightly. We have indeed no real knowledge of the distance from us of those stars which our giant telescopes have brought into view out of the depths of the celestial s.p.a.ces.

CHAPTER VI

CELESTIAL MEASUREMENT

Had the telescope never been invented our knowledge of astronomy would be trifling indeed.

Prior to the year 1610, when Galileo first turned the new instrument upon the sky, all that men knew of the starry realms was gathered from observation with their own eyes unaided by any artificial means. In such researches they had been very much at a disadvantage. The sun and moon, in their opinion, were no doubt the largest bodies in the heavens, for the mere reason that they looked so! The mighty solar disturbances, which are now such common-places to us, were then quite undreamed of.

The moon displayed a patchy surface, and that was all; her craters and ring-mountains were surprises as yet in store for men. Nothing of course was known about the surfaces of the planets. These objects had indeed no particular characteristics to distinguish them from the great host of the stars, except that they continually changed their positions in the sky while the rest did not. The stars themselves were considered as fixed inalterably upon the vault of heaven. The sun, moon, and planets apparently moved about in the intermediate s.p.a.ce, supported in their courses by strange and fanciful devices. The idea of satellites was as yet unknown. Comets were regarded as celestial portents, and meteors as small conflagrations taking place in the upper air.

In the entire absence of any knowledge with regard to the actual sizes and distances of the various celestial bodies, men naturally considered them as small; and, concluding that they were comparatively near, a.s.signed to them in consequence a permanent connection with terrestrial affairs. Thus arose the quaint and erroneous beliefs of astrology, according to which the events which took place upon our earth were considered to depend upon the various positions in which the planets, for instance, found themselves from time to time.

It must, however, be acknowledged that the study of astrology, fallacious though its conclusions were, indirectly performed a great service to astronomy by reason of the accurate observations and diligent study of the stars which it entailed.

We will now inquire into the means by which the distances and sizes of the celestial orbs have been ascertained, and see how it was that the ancients were so entirely in the dark in this matter.

There are two distinct methods of finding out the distance at which any object happens to be situated from us.

One method is by actual measurement.

The other is by moving oneself a little to the right or left, and observing whether the distant object appears in any degree altered in position by our own change of place.

One of the best ill.u.s.trations of this relative change of position which objects undergo as a result of our own change of place, is to observe the landscape from the window of a moving railway carriage. As we are borne rapidly along we notice that the telegraph posts which are set close to the line appear to fly past us in the contrary direction; the trees, houses, and other things beyond go by too, but not so fast; objects a good way off displace slowly; while some spire, or tall landmark, in the far distance appears to remain unmoved during a comparatively long time.

Actual change of position on our own part is found indeed to be invariably accompanied by an apparent displacement of the objects about us, such apparent displacement as a result of our own change of position being known as "parallax." The dependence between the two is so mathematically exact, that if we know the amount of our own change of place, and if we observe the amount of the consequent displacement of any object, we are enabled to calculate its precise distance from us.

Thus it comes to pa.s.s that distances can be measured without the necessity of moving over them; and the breadth of a river, for instance, or the distance from us of a s.h.i.+p at sea, can be found merely by such means.

It is by the application of this principle to the wider field of the sky that we are able to ascertain the distance of celestial bodies. We have noted that it requires a goodly change of place on our own part to s.h.i.+ft the position in which some object in the far distance is seen by us. To two persons separated by, say, a few hundred yards, a s.h.i.+p upon the horizon will appear pretty much in the same direction. They would require, in fact, to be much farther apart in order to displace it sufficiently for the purpose of estimating their distance from it. It is the same with regard to the moon. Two observers, standing upon our earth, will require to be some thousands of miles apart in order to see the position of our satellite sufficiently altered with regard to the starry background, to give the necessary data upon which to ground their calculations.

The change of position thus offered by one side of the earth's surface at a time is, however, not sufficient to displace any but the nearest celestial bodies. When we have occasion to go farther afield we have to seek a greater change of place. This we can get as a consequence of the earth's movement around the sun. Observations, taken several days apart, will show the effect of the earth's change of place during the interval upon the positions of the other bodies of our system. But when we desire to sound the depths of s.p.a.ce beyond, and to reach out to measure the distance of the nearest star, we find ourselves at once thrown upon the greatest change of place which we can possibly hope for; and this we get during the long journey of many millions of miles which our earth performs around the sun during the course of each year. But even this last change of place, great as it seems in comparison with terrestrial measurements, is insufficient to show anything more than the tiniest displacements in a paltry forty-three out of the entire host of the stars.

We can thus realise at what a disadvantage the ancients were. The measuring instruments at their command were utterly inadequate to detect such small displacements. It was reserved for the telescope to reveal them; and even then it required the great telescopes of recent times to show the slight changes in the position of the nearer stars, which were caused by the earth's being at one time at one end of its...o...b..t, and some six months later at the other end--stations separated from each other by a gulf of about one hundred and eighty-six millions of miles.

The actual distances of certain celestial bodies being thus ascertainable, it becomes a matter of no great difficulty to determine the actual sizes of the measurable ones. It is a matter of everyday experience that the size which any object appears to have, depends exactly upon the distance it is from us. The farther off it is the smaller it looks; the nearer it is the bigger. If, then, an object which lies at a known distance from us looks such and such a size, we can of course ascertain its real dimensions. Take the moon, for instance. As we have already shown, we are able to ascertain its distance. We observe also that it looks a certain size. It is therefore only a matter of calculation to find what its actual dimensions should be, in order that it may look that size at that distance away. Similarly we can ascertain the real dimensions of the sun. The planets, appearing to us as points of light, seem at first to offer a difficulty; but, by means of the telescope, we can bring them, as it were, so much nearer to us, that their broad expanses may be seen. We fail, however, signally with regard to the stars; for they are so very distant, and therefore such tiny points of light, that our mightiest telescopes cannot magnify them sufficiently to show any breadth of surface.

Instead of saying that an object looks a certain breadth across, such as a yard or a foot, a statement which would really mean nothing, astronomers speak of it as measuring a certain angle. Such angles are estimated in what are called "degrees of arc"; each degree being divided into sixty minutes, and each minute again into sixty seconds. Popularly considered the moon and sun _look_ about the same size, or, as an astronomer would put it, they measure about the same angle. This is an angle, roughly, of thirty-two minutes of arc; that is to say, slightly more than half a degree. The broad expanse of surface which a celestial body shows to us, whether to the naked eye, as in the case of the sun and moon, or in the telescope, as in the case of other members of our system, is technically known as its "disc."

CHAPTER VII

ECLIPSES AND KINDRED PHENOMENA

Since some members of the solar system are nearer to us than others, and all are again much nearer than any of the stars, it must often happen that one celestial body will pa.s.s between us and another, and thus intercept its light for a while. The moon, being the nearest object in the universe, will, of course, during its motion across the sky, temporarily blot out every one of the others which happen to lie in its path. When it pa.s.ses in this manner across the face of the sun, it is said to _eclipse_ it. When it thus hides a planet or star, it is said to _occult_ it. The reason why a separate term is used for what is merely a case of obscuring light in exactly the same way, will be plain when one considers that the disc of the sun is almost of the same apparent size as that of the moon, and so the complete hiding of the sun can last but a few minutes at the most; whereas a planet or a star looks so very small in comparison, that it is always _entirely swallowed up for some time_ when it pa.s.ses behind the body of our satellite.

The sun, of course, occults planets and stars in exactly the same manner as the moon does, but we cannot see these occultations on account of the blaze of sunlight.

By reason of the small size which the planets look when viewed with the naked eye, we are not able to note them in the act of pa.s.sing over stars and so blotting them out; but such occurrences may be seen in the telescope, for the planetary bodies then display broad discs.

There is yet another occurrence of the same cla.s.s which is known as a _transit_. This takes place when an apparently small body pa.s.ses across the face of an apparently large one, the phenomenon being in fact the exact reverse of an occultation. As there is no appreciable body nearer to us than the moon, we can never see anything in transit across her disc. But since the planets Venus and Mercury are both nearer to us than the sun, they will occasionally be seen to pa.s.s across his face, and thus we get the well-known phenomena called Transits of Venus and Transits of Mercury.

As the satellites of Jupiter are continually revolving around him, they will often pa.s.s behind or across his disc. Such occultations and transits of satellites can be well observed in the telescope.

There is, however, a way in which the light of a celestial body may be obscured without the necessity of its being hidden from us by one nearer. It will no doubt be granted that any opaque object casts a shadow when a strong light falls directly upon it. Thus the earth, under the powerful light which is directed upon it from the sun, casts an extensive shadow, though we are not aware of the existence of this shadow until it falls upon something. The shadow which the earth casts is indeed not noticeable to us until some celestial body pa.s.ses into it.

As the sun is very large, and the earth in comparison very small, the shadow thrown by the earth is comparatively short, and reaches out in s.p.a.ce for only about a million miles. There is no visible object except the moon, which circulates within that distance from our globe, and therefore she is the only body which can pa.s.s into this shadow. Whenever such a thing happens, her surface at once becomes dark, for the reason that she never emits any light of her own, but merely reflects that of the sun. As the moon is continually revolving around the earth, one would be inclined to imagine that once in every month, namely at what is called _full moon_, when she is on the other side of the earth with respect to the sun, she ought to pa.s.s through the shadow in question.

But this does not occur every time, because the moon's...o...b..t is not quite _upon the same plane_ with the earth's. It thus happens that time after time the moon pa.s.ses clear of the earth's shadow, sometimes above it, and sometimes below it. It is indeed only at intervals of about six months that the moon can be thus obscured. This darkening of her light is known as an _eclipse of the moon_. It seems a great pity that custom should oblige us to employ the one term "eclipse" for this and also for the quite different occurrence, an eclipse of the sun; in which the sun's face is hidden as a consequence of the moon's body coming directly _between_ it and our eyes.

The popular mind seems always to have found it more difficult to grasp the causes of an eclipse of the moon than an eclipse of the sun. As Mr.

J.E. Gore[4] puts it: "The darkening of the sun's light by the interposition of the moon's body seems more obvious than the pa.s.sing of the moon through the earth's shadow."

Eclipses of the moon furnish striking spectacles, but really add little to our knowledge. They exhibit, however, one of the most remarkable evidences of the globular shape of our earth; for the outline of its shadow when seen creeping over the moon's surface is always circular.

[Ill.u.s.tration: FIG. 3.--Total and Partial Eclipses of the Moon. The Moon is here shown in two positions; i.e. _entirely_ plunged in the earth's shadow and therefore totally eclipsed, and only _partly_ plunged in it or partially eclipsed.]

_Eclipses of the Moon_, or Lunar Eclipses, as they are also called, are of two kinds--_Total_, and _Partial_. In a total lunar eclipse the moon pa.s.ses entirely into the earth's shadow, and the whole of her surface is consequently darkened. This darkening lasts for about two hours. In a partial lunar eclipse, a portion only of the moon pa.s.ses through the shadow, and so only _part_ of her surface is darkened (see Fig. 3). A very striking phenomenon during a total eclipse of the moon, is that the darkening of the lunar surface is usually by no means so intense as one would expect, when one considers that the sunlight at that time should be _wholly_ cut off from it. The occasions indeed upon which the moon has completely disappeared from view during the progress of a total lunar eclipse are very rare. On the majority of these occasions she has appeared of a coppery-red colour, while sometimes she has a.s.sumed an ashen hue. The explanations of these variations of colour is to be found in the then state of the atmosphere which surrounds our earth. When those portions of our earth's atmosphere through which the sun's rays have to filter on their way towards the moon are free from watery vapour, the lunar surface will be tinged with a reddish light, such as we ordinarily experience at sunset when our air is dry. The ashen colour is the result of our atmosphere being laden with watery vapour, and is similar to what we see at sunset when rain is about. Lastly, when the air around the earth is thickly charged with cloud, no light at all can pa.s.s; and on such occasions the moon disappears altogether for the time being from the night sky.

_Eclipses of the Sun_, otherwise known as Solar Eclipses, are divided into _Total_, _Partial_, and _Annular_. A total eclipse of the sun takes place when the moon comes between the sun and the earth, in such a manner that it cuts off the sunlight _entirely_ for the time being from a _portion_ of the earth's surface. A person situated in the region in question will, therefore, at that moment find the sun temporarily blotted out from his view by the body of the moon. Since the moon is a very much smaller body than the sun, and also very much the nearer to us of the two, it will readily be understood that the portion of the earth from which the sun is seen thus totally eclipsed will be of small extent. In places not very distant from this region, the moon will appear so much s.h.i.+fted in the sky that the sun will be seen only partially eclipsed. The moon being in constant movement round the earth, the portion of the earth's surface from which an eclipse is seen as total will be always a comparatively narrow band lying roughly from west to east. This band, known as the _track of totality_, can, at the utmost, never be more than about 165 miles in width, and as a rule is very much less. For about 2000 miles on either side of it the sun is seen partially eclipsed. Outside these limits no eclipse of any kind is visible, as from such regions the moon is not seen to come in the way of the sun (see Fig. 4 (i.), p. 67).

It may occur to the reader that eclipses can also take place in the course of which the positions, where the eclipse would ordinarily be seen as total, will lie outside the surface of the earth. Such an eclipse is thus not dignified with the name of total eclipse, but is called a partial eclipse, because from the earth's surface the sun is only seen _partly eclipsed at the utmost_ (see Fig. 4 (ii.), p. 67).

[Ill.u.s.tration: (i.) Total Eclipse of the Sun.]

[Ill.u.s.tration: (ii.) Partial Eclipse of the Sun.

FIG. 4.--Total and Partial Eclipses of the Sun. From the position A the Sun cannot be seen, as it is entirely blotted out by the Moon. From B it is seen partially blotted out, because the Moon is to a certain degree in the way. From C no eclipse is seen, because the Moon does not come in the way.

It is to be noted that in a Partial Eclipse of the Sun, the position A lies _outside_ the surface of the Earth.]

An _Annular eclipse_ is an eclipse which just fails to become total for yet another reason. We have pointed out that the orbits of the various members of the solar system are not circular, but oval. Such oval figures, it will be remembered, are technically known as ellipses. In an elliptic orbit the controlling body is situated not in the middle of the figure, but rather towards one of the ends; the actual point which it occupies being known as the _focus_. The sun being at the focus of the earth's...o...b..t, it follows that the earth is, at times, a little nearer to him than at others. The sun will therefore appear to us to vary a little in size, looking sometimes slightly larger than at other times.

It is so, too, with the moon, at the focus of whose orbit the earth is situated. She therefore also appears to us at times to vary slightly in size. The result is that when the sun is eclipsed by the moon, and the moon at the time appears the larger of the two, she is able to blot out the sun completely, and so we can get a total eclipse. But when, on the other hand, the sun appears the larger, the eclipse will not be quite total, for a portion of the sun's disc will be seen protruding all around the moon like a ring of light. This is what is known as an annular eclipse, from the Latin word _annulus_, which means a ring. The term is consecrated by long usage, but it seems an unfortunate one on account of its similarity to the word "annual." The Germans speak of this kind of eclipse as "ring-formed," which is certainly much more to the point.

There can never be a year without an eclipse of the sun. Indeed there must be always two such eclipses _at least_ during that period, though there need be no eclipse of the moon at all. On the other hand, the greatest number of eclipses which can ever take place during a year are seven; that is to say, either five solar eclipses and two lunar, or four solar and three lunar. This general statement refers merely to eclipses in their broadest significance, and informs us in no way whether they will be total or partial.

Of all the phenomena which arise from the hiding of any celestial body by one nearer coming in the way, a total eclipse of the sun is far the most important. It is, indeed, interesting to consider how much poorer modern astronomy would be but for the extraordinary coincidence which makes a total solar eclipse just possible. The sun is about 400 times farther off from us than the moon, and enormously greater than her in bulk. Yet the two are relatively so distanced from us as to look about the same size. The result of this is that the moon, as has been seen, can often blot out the sun entirely from our view for a short time. When this takes place the great blaze of sunlight which ordinarily dazzles our eyes is completely cut off, and we are thus enabled, unimpeded, to note what is going on in the immediate vicinity of the sun itself.