Astronomy

ASTRONOMY. The earth on which we live is the fifth largest planet belonging to one of the lesser stars. Perhaps it is less necessary now than it used to be to insist on the smallness of our planet. Scientific inventions and ease of travel seem to have brought different parts of the earth near together, and we no longer hold an exaggerated idea of its immensity. But it is when we look up into the vault of the heavens that we realize the insignificance of the earth in the scheme of the material uni verse. Our sight penetrates space beyond space revealing world beyond world of unimaginable grandeur ; and the greatest of these orbs is but as a speck in the vast intervening void. All this world beyond the earth is the field of the science of astronomy.

Of the objects of the sky the sun and moon stand out from the rest in prominence. Both appear to us of much the same size. In fact the phenomenon of eclipses gives a delicate test showing that the apparent angular sizes are almost identical; for when the moon passes between us and the sun, sometimes it is just able to cover it completely, sometimes it just fails to cover it and leaves a narrow ring of the sun showing all round it. But this apparent agreement is only a coincidence ; the sun and moon are bodies of altogether different size and cosmical importance.

The moon is a smaller globe than the earth, of no particular importance except to the earth which it attends as satellite ; it is probable that in the remote past its material formed part of the earth, that it broke away in a great convulsion, and has gradually receded to its present distance of 240,00o miles. Thus the first halting point in our journey through space tends rather to en hance our idea of the dignity of the earth by showing that a sub ordinate globe attends it; but that is unique, and when we pass beyond this comparatively small distance over which the earth's domain extends, the "proud father" is seen to be a very humble member of a great community.

The other luminary, the sun, stands to our earth in much the same relation as the earth does to the moon. The sun is the ruler and the earth a subordinate globe travelling round nearly in a circle (but strictly an ellipse) under the controlling force of the sun's gravitational attraction. The sun is of a size that befits the dignity of a ruler. The amount of matter constituting it is equiv alent to 300,00o earths rolled into one. This great mass is main tained, by means which are still very largely a mystery, at enor mously high temperature so that it pours forth the unceasing stream of heat and light of so much importance to terrestrial life. Our average distance from the sun is 92,870,00o miles; but since the earth's orbit is not exactly circular its distance varies over a range of about 3,000,00o miles according to the time of year.

When we pass beyond the moon all astronomical distances be come inconceivably great, and the reader may be inclined to group them together without distinction as virtually infinite. But it has to be realized that we have passed to a new scale of extension where we must make a distinction of comparatively smaller and greater distances. Great as may be the distance from the earth to the sun it is traversed by light in about eight minutes; or a radio telegram would take the same time to travel. We see the sun not as it is now but as it was eight minutes ago. This gives a con venient way of realizing how much further our journey through space must extend. There is little doubt that the most remote object in the heavens which can be seen without telescopic aid is a small fuzzy patch of light in the constellation Andromeda. At first glance this would be taken for one of the fainter stars, but the diffuseness of the light is distinctive, and telescopes show it to be a great spiral nebula. The light which we see to-day left that nebula 900,00o years ago. This is more or less the limit of our exploration ; the telescope shows other spiral nebulae, smaller and presumably in many cases more remote, but their distances are at present a matter of conjecture.

The Solar System.

Just as the earth's supremacy comes to an end beyond the moon, so there are limits to the sun's supremacy. The solar system, as it is called, may be regarded roughly as lim ited by the orbit of the outermost planet, Neptune ; though many of the comets obeying the sun's attraction have elongated paths which take them some way out into the space beyond this limit for part of their course. Neptune's distance from the sun is 3o times as great as that of the earth ; and as seen from Neptune the brightness and heat would be diminished to about a thousandth; but even so the sun would appear far more brilliant than any of the stars. The isolation of this system from other astronomical bodies or systems can be best realized from the fact that the sun's light takes 4 hours to reach the limits of its domain (Nep tune), but 4 years to reach its nearest neighbour among the stars.

For this reason the study of the heavenly bodies falls naturally into two divisions : the solar system and the stellar universe, the latter comprising all that is beyond the solar system.

To the solar system belong, besides the sun and the earth with its moon, the planets or "wandering stars." Such of the planets as are visible to the naked eye are ordinarily mistaken for true or "fixed" stars; they can usually be distinguished by the fact that their light does not twinkle, but that is by no means an infallible test since it depends a great deal on atmospheric conditions. Their special characteristic is that they move from place to place in the sky—not like the stars which form fixed constellations. For this reason it is impossible to give simple directions as to where they are to be found, or to insert positions for them in a map of the heavens. A planet of unusual brilliancy seen towards the west after sunset or the east before sunrise may fairly safely be identified as Venus; after the sun and moon it is much the brightest object of the heavens. Jupiter is also much brighter than any of the stars ; its position in the sky is not so circumscribed as that of Venus and it may be visible at any time of the night. Mars is easily distinguished from other planets by its red colour, and when near the earth is conspicuous by its brilliancy; at other times it may easily be mistaken for one of the reddish fixed stars. Saturn can scarcely be distinguished from the bright (first magnitude) stars except by those who have enough familiarity with the normal appearance of the heavens to spot "a bright star in the wrong place." Mercury is rarely seen except in tropical latitudes; at favourable times it is visible as a brilliant point of light in the glow of sunset or sunrise. The other planets, Uranus and Nep tune, are telescopic objects. These planets are bodies revolving round the sun and having the same status as the earth. Telescopes show that some of them are attended by satellites as the earth is attended by the moon. A planet more remote than Neptune was discovered at the Lowell observatory in Feb. 193o in a posi tion agreeing closely with a prediction made by the late Percival Lowell as the result of calculation from the perturbations of Uranus. Its present distance from the sun is between 3,700 and 4,000 million miles. The period is unknown at present but is expected to be about 30o years.

To complete the enumeration of the bodies of the solar system we must mention: (z) The minor planets, of which more than a thousand have been discovered, which in some way seem to rep resent what ought to have been an ordinary planet between Mars and Jupiter where there is a hiatus in the regular spacing of the successive orbits; (2) the comets, bodies of no great mass, which suffer extremes of heat and cold owing to their very elongated orbits and display "fireworks" at each return to proximity to the sun's heat; (3) swarms of meteors which the earth ploughs through on its way round the sun; these bodies (mostly of very small mass) become incandescent on striking the earth's atmos phere and show themselves as "falling stars"; (4) diffuse gaseous or meteoric matter seen in the zodiacal light and evidenced in other more indirect ways.

The Stellar Universe.

As already stated we have to adapt our minds to a transcendently large scale of size and distance in studying the solar system; but this is only a prelude to another million-fold leap in studying the scale of the stellar universe. Passing outside its own particular domain, the sun is just one of some thousands of millions of stars. It is not a particularly large star. In fact, most of the stars which we see in the night sky are in reality much more brilliant ; but this is not quite a fair compar ison to make, because naturally it is the more brilliant members of the community that have the best chance of being noticed. Whilst we must recognize that there are numerous stars which outshine the sun, we must also recognize that feebler stars are still more numerous. This part of astronomy includes the study of individ ual stars and their associations, star clusters, and nebulae. The last term includes three distinct types of objects.

The two main divisions of astronomical knowledge—the solar system and the stellar universe—cannot be kept entirely disso ciated. Thus our study of the sun as the leading member of the solar system must be supplemented by a comparative study of him in relation to his compeers in the stellar universe. Moreover, much of astronomical research is directed not so much towards knowledge of individual objects as to the discovery of physical laws and truths of universal application; the problem of the laws of motion of the planets round the sun repeats itself in the mo tion of the components of a double star ; the configuration which in our own system gives rise to the awe-inspiring spectacle of an eclipse is also the secret of the winking of the "demon" star Algol.

Some Practical Uses of Astronomy.

What is the use of astronomy? It is not necessary here to defend the pursuit of pure science whether or not it leads to results which can be used (or, as commonly, misused) for modifying the conditions of human exist ence. Therefore it is not in any spirit of apology or defence that reference may be made to some of the practical reasons why a study of astronomy is necessary. To begin with, it provides the explanation of certain phenomena of immediate importance to us such as the seasons and the tides. The exact time circulated eral times a day by radio is derived from continued astronomical observations. It is of fundamental importance in navigation. Greenwich observatory was founded in 1675 to meet urgent needs of navigation; and in particular its long series of observations of the moon from 175o onwards has been the basis by which longi tudes were determined up to recent times. The radio time-signals have now rendered the moon obsolete for this purpose ; but even after r goo the ownership of many hundreds of square miles of territory hung in the balance until the boundary commissioners were furnished with the latest observations of the moon's position.

All this, it will be said, belongs to the old fashioned type of astronomy, and is no justification of the usefulness of much of the present domain of research which seems to be inspired with the idea of getting as far away as possible from the earth and all terrestrial things. The answer is that scientific knowledge forms a single whole, and if astronomy lags behind, the sister science of physics will suffer. Present-day astronomy has a very definite part to play in the general advance. The stars and diffuse nebulae are physical laboratories where we can watch experiments per formed on matter under the most extreme conditions of tem perature and density. They supply the gaps in knowledge which the experimental physicist is unable to cover in the limited condi tions of a terrestrial laboratory. The element helium, of great prac tical use to-day, was first discovered on the sun. The theory of relativity has revolutionized the conceptions of physics and is fundamental in our modern knowledge of the atom ; but it largely owes its general acceptance to the astronomical tests which were applied. Numerous technical illustrations could be given of the way in which the stars have been invoked to supplement the ter restrial laboratory. At the time of writing (1928) comes the news that the most prominent lines in the spectra of the nebulae are now identified as "forbidden lines" of the spectrum of the oxy gen atom in a certain state—f orbidden, that is to say, in anything approaching terrestrial conditions but possible at the extremely low density of a nebula. We cannot foresee what will be the next practical application of the rapidly advancing physical knowledge wrung by joint effort from the laboratory and the stars ; but that there will be applications the whole history of science and invention assures us. One thing may be pointed out. The sun and stars hold the secret of releasing from matter vast quantities of energy compared with which all our commercial sources of energy are insignificant. Few scientific men would venture to hold out any expectation that by quest of this secret we may be able to provide the world with a source that will supersede all present fuel; but it would certainly be remiss not to make every effort to learn what we can of the conditions of release of this transcendent power. Observatories and Their Work.—Numerous observatories have been set up in most countries to carry out astronomical obser vations. It may be well first to remove the rather amusing popular misconceptions as to the kind of work that goes on in these. The astronomer does not spend his nights with his eye "glued to the telescope." He is not bound to keep a continuous watch over the sky lest something should turn up when he happened to be asleep. Very little of his time is spent in gazing at the show objects of the heavens—the moon, planets, nebulae, etc.—which for the most part can show him nothing more to-night than they have shown him for many years past. He does not relapse into inactivity in those intervals when there is nothing particularly new or strik ing going on in the sky. Whilst we are on the subject of popular misconceptions, it may be added that the chief preoccupation of astronomers is not the question whether there is intelligent life on Mars or elsewhere in the universe ; although an occasional crumb of information that might possibly bear on this subject may happen to be picked up, astronomy can take no responsibility for the speculations that may ensue. Also astronomers do not predict the weather; and, their work being at. the mercy of the clouds, they have more cause than most people to rail against failure to foretell the state of the sky a few hours in advance.

What, then, is the work of an observatory? The question is rather like asking what is the work of a factory; no summary can cover all the different kinds of investigation that the different insti tutions may take up. A few years ago a central feature of every observatory was an accurate astronomical clock, which had to be kept corrected by frequent observations of stars. But now lines of work have developed in which accurate knowledge of time is inessential; and there is at least one famous observatory where no one worries about the time to a minute or so. But it would be nearly true to say that all astronomical work consists of exact measurement. If the eye of the observer is gazing fixedly into the telescope for a few minutes, his fingers are all the time twid dling screws to move cross-wires or other devices; and in the course of the night he has to look as much at terrestrial microm eter-readings as at celestial phenomena. Part, but by no means all, of the work consists of photography, the exposures ranging from less than a minute to many hours according to the nature of the investigation. These photographs are afterwards measured up under the microscope. Many measures are made photographi cally and visually of the brightness of different stars. Or the light may be split up by a spectroscope before photographing, and the spectra subsequently measured. What is learnt from all these measures forms the subject matter of many separate articles on celestial objects included in this encyclopaedia. The reader will there find evidence that the work is often fruitful; but he may not easily be convinced that there is need for such an enormous amount of measurement. There are several lines of explanation which may help us to understand this.

The Demand for Data.

Few people realize how big the vault of the sky is. If the moon is photographed with an ordinary as trographic telescope its image seems disappointingly small. The usual size of plate is 64x6+in. ; and the beginner perhaps ex pects that this will cover only a limited area of the moon. Instead of that the full moon is only a circle just over an inch in diam eter. Yet even on this unambitious scale it takes 1o,000 plates to cover the whole sky. With more powerful telescopes giving a larger scale the task is correspondingly increased. The number of stars that might be studied is overwhelming, and indeed it is necessary to impose some systematic limitation on the survey; but proper motions determined by the ten thousand, parallaxes and radial velocities by the thousand, fail to satisfy the demands of the theorist for more and more data. Much of our knowledge of the stars has come from statistical studies, and work of that kind swallows up data by the thousand before it can assert results with confidence and accuracy.

It is by spreading our net wide that we catch the objects which will repay an intensive study. The minor planets give a good ex ample of this. After the first few were discovered there seemed little object in continuing to find hundreds more, calculating their orbits, and generally keeping track of them. There was some chance that statistics of their distribution in distance, eccentricity, inclination, etc., might prove interesting; but otherwise they were an unmitigated nuisance in astronomy. But of ter Ceres, Pallas, Juno, Vesta, the work went on, each new planet being as unin teresting as its predecessor. Then in 1898, No. 433, Eros, was dis covered, a body of the greatest astronomical importance which was the subject of thousands of observations at its close approach to the earth in 1901, and which will no doubt be watched with equal zeal at the still closer approach in 1931. Another dull accu mulation of these planets followed until No. 588, Achilles, started the interesting Trojan group, which if it has done nothing else has set a new and difficult problem to the dynamical astronomers. Another illustration may be given. One of the greatest needs in stellar astronomy is a knowledge of the masses of the stars; many important conclusions turn on evidence as to the mass. The only chance of making a direct determination of the mass is when the star is a binary system ; but it is rare to find all conditions favour able. Burnham's General Catalogue lists 13,600 visual double stars and to these may be added more than i,000 spectroscopic binaries. From these we are able to scrape together just about 3o reasonably well-determined masses. It seems a meagre amount of grain to extract from so much chaff; yet the advance that this knowledge of stellar mass makes possible is so great that we should have no reason to feel dissatisfied even if this were the only result of double star observation. When in these astronomi cal articles a star is mentioned as having given occasion for some new extension of knowledge, it should be recalled that in most cases the observer had no initial reason to suspect that it would prove more fruitful than a hundred other stars on his programme.

Astronomical Telescopes.

A telescope of some kind is em ployed in nearly all observations. There are two entirely distinct functions which it may fulfil—(1) to collect enough light from extremely faint objects to make them visible or capable of meas urement; (2) to resolve and magnify brighter objects. These functions are independent, and any particular telescope is usually specialized for one purpose or the other. Having, for example, employed a wide aperture and long exposure to collect enough light from a faint extension of a nebula or the end of a comet's tail we do not want to squander it over a large area on the pho tographic plate. A wide dumpy instrument gathering much light on to a small scale picture is best for detecting the faintest objects. On the other hand, when the amount of light permits, we profit by employing a long telescope with high magnification; the accu racy of measurements is enhanced by the increased scale. For these and other reasons there is a tendency to specialization of telescopes for different kinds of work. Owing to conditions of achromatism a refracting telescope cannot ordinarily be used both for visual and photographic work ; attempts to combine both func tions in one instrument involve extra lenses and considerable sac rifice of light. In some applications of the largest telescopes the perfect figuring of the mirror would seem to be rather wasted. For measurement of the heat of the stars the rays have to be con centrated on a disc enormously greater than the ordinary size of a star-image ; and in photo-electric work they merely have to enter the wide mouth of a photo-electric cell. In the spectroscopy of nebulae the perfection of the focus is unimportant. There would seem to be room in an observatory nowadays for a very big, very "bad" telescope.

Besides the largest telescopes mounted equatorially, i.e., so as to keep the same stars in view notwithstanding the apparent diurnal rotation of the sky, there are instruments (useless for prolonged scrutiny of objects) with which observations of position of the stars and planets are snapped as they traverse the field of view. The transit circle (or meridian circle), the altazimuth and the zenith telescope are the most important of these. They are used for measuring the positions, and hence ultimately the motions of heavenly bodies. With the equatorial telescopes positions can only be measured relative to the stars in the same field. This suffices for some kinds of work : but, for example, in following the motion of the moon and planets round the sky the whole sys tem of reference stars will need to be connected together. This liaison is provided mainly by the transit-circle and forms a very important branch of practical work known as "Fundamental Astronomy." Finally reference may be made to kinds of work which may per haps give some colour to the popular misconceptions mentioned at the beginning of this section. There are two things which come on an astronomer without warning—the outbreak of a Nova or tem porary star, and the arrival of a comet. The arrival of comets can be predicted sometimes, but the biggest comets come unan nounced; outbreaks of Novae are never predicted. The discovery of either is likely to fall to someone who makes a practice of searching the sky night after night, whether with deliberate inten tion or from delight in its wonders. It is not likely to fall to those who are occupied with the intensive measurements above mentioned. Af ter a comet has been observed on three nights its orbit is worked out, and it is possible to judge whether it is likely, by close approach to the sun or the earth, to give favourable op portunity for studying outstanding questions of cometary struc ture. The result is generally disappointing and it passes out of notice so far as the majority of astronomers are concerned, though the comet specialists will continue to keep an eye on its behaviour. The announcement of a Nova most decidedly disturbs the even life of an observatory ; for these stars present some of the most perplexing problems of present day astronomy and there is great need for observation especially in the earliest stages of the outbreak. At least for spectroscopic workers it is well worth while to suspend other problems and make concentrated efforts to explore the mysterious phenomena.