Distinction.—These objects, though classed together in catalogues, offer some great distinctions which the observer will not be long in recognizing. It was thought at one period that all nebulæ were resolvable into stars, and that their nebulous aspect was merely due to the confused light of remote star-clusters. But modern telescopes, backed up by the unequivocal testimony of the spectroscope, have shown that purely nebulous matter really exists in space. The largest instruments cannot resolve it into stars, and it yields a gaseous spectrum. The conjecture has been thrown out that it may be considered as the unformed material of which suns and planets are made.

Large Number visible.—D'Arrest once said that nebulæ are so numerous as to be infinite, and his opinion is supported by the rapid increase in the number known. Let us make a comparison. Messier inserted in the Connaissances des Temps for 1783 and 1784 (published in 1781) a catalogue containing 103 nebulæ and star-clusters. Of these 68 were new. In 1888 a new edition of Sir J. Herschel's catalogue of 1864 (revised and extended by Dreyer) was printed by the Royal Astronomical Society, and this includes 7840 objects! The labours of the Herschels, of Lord Rosse, D'Arrest, Marth, Tempel, Stephan, and Swift have vastly augmented our knowledge in this branch since the time of Messier.

Varieties of Form and Grouping.—A telescope reveals all grades of condensation in stellar groups.

Of all the planets, Jupiter is the most interesting for study by the amateur. It is true that Saturn forms an exquisite object, and that his wonderful ring-system is well calculated to incite admiration as a feature unique in the solar system. But when the two planets come to be repeatedly observed, and the charm of first impressions has worn away, the observer must admit that Jupiter, with his broad disk and constantly changing markings, affords the materials for prolonged study and sustained interest. With Saturn the case is different. His features are apparently quiescent; usually there are no definite spots upon the belts or rings. There is a sameness in the telescopic views; and this ultimately leads to a feeling of monotony, which causes the object to be neglected in favour of another where active changes are in visible progress.

Brightness and Position.—Jupiter is a brilliant object in the heavens, his lustre exceeding that of Mars or Saturn, though not equal to that of Venus. I have occasionally seen the planet with the naked eye in the daytime, about half an hour after sunrise; and it has been frequently observed by Bond, in America, with the Sun at a considerable altitude. Humboldt and Bonpland, at Cumana, 10° N. lat., saw Jupiter distinctly with the naked eye, 18 minutes after the Sun had appeared in the horizon, on Sept. 26, 1830.

The number of large telescopes having so greatly increased in recent years, and there being every prospect that the demand for such instruments will continue, it may be well to consider their advantages as compared with those of much inferior size. Object-glasses and specula will probably soon be made of a diameter not hitherto attained; for it is palpably one of the ambitions of the age to surpass all previous efforts in the way of telescopic construction. There are some who doubt that such enormous instruments are really necessary, and question whether the results obtained with them are sufficient return for the great expense involved in their erection. Large instruments require large observatories; and the latter must be at some distance from a town, and in a locality where the atmosphere is favourable. Nothing can be done with great aperture in the presence of smoke and other vapours, which, as they cross the field, become ruinous to definition. Moreover, a big instrument is not to be manipulated with the same facility as a small one: and when anything goes wrong with it, its rectification may be a serious matter, owing to the size. Such telescopes need constant attention if they would be kept in thorough working order. On the other hand, small instruments involve little outlay, they are very portable, and require little space. They may be employed in or out of doors, according to the inclination and convenience of the observer.

The instrument which has so vastly extended our knowledge of the Universe, which has enabled us to acquire observations of remarkable precision, and supplied the materials for many sublime speculations in Astronomy, was invented early in the seventeenth century. Apart from its special application as a means of exploring the heavens with a capacity that is truly marvellous, it is a construction which has also been utilized in certain other departments with signal success. It provided mankind with a medium through which to penetrate far beyond the reach of natural vision, and to grasp objects and phenomena which had either eluded detection altogether or had only been seen in dim and uncertain characters. It has also proved a very efficient instrument for various minor purposes of instruction and recreation. The invention of the telescope formed a new era in astronomy; and though, with a few exceptions, men were slow at first in availing themselves of its far-seeing resources, scepticism was soon swept aside and its value became widely acknowledged.

But though the telescope was destined to effect work of the utmost import, and to reach a very high degree of excellence in after times, the result was achieved gradually. Step by step its powers were enlarged and its qualities perfected, and thus the stream of astronomical discovery has been enabled to flow on, stimulated by every increase in its capacity.

Early in autumn, when the evenings are frequently clear, many persons are led with more force than usual to evince an interest in our satellite, and to desire information which may not be conveniently obtained at the time. The aspect of the Moon at her rising, near the time of the full, during the months of August, September, and October, is more conspicuously noticeable than at any other season of the year, on account of the position she then assumes on successive nights, enabling her to rise at closely identical times for several evenings together. The appearance of her large, ruddy globe at near the same hour, and her increasing brilliancy as her horizontal rays give way under a more vertical position, originated the title of “Harvest Moon,” to commemorate the facility afforded by her light for the ingathering of the corn preceding the time of the autumnal equinox.

It will be universally admitted that the Moon possesses special attractions for us, as being situated nearer than any other celestial body, and forming the inseparable companion or tributary world to the Earth. The many important influences she exercises have led to her becoming the object of close investigation; so that her motions and physical appearances have been ascertained with a remarkable degree of exactness and amplitude.

Appearance of the Planet.—Mars is the fourth planet in the order of distance from the Sun. He revolves in an orbit outside that of the Earth, and is the smallest of the superior planets. His brilliancy is sometimes considerable when be occupies a position near to the Earth, and he emits an intense red light, which renders his appearance all the more striking. Ordinarily his lustre does not equal that of Jupiter, though when favourably placed he becomes a worthy rival of that orb. In 1719 he shone so brightly and with such a fiery aspect as to cause a panic. The superstitious notions and belief in astrological influences prevailing at that time no doubt gave rise to the popular apprehension that the ruddy star was an omen of disaster, and thus it was regarded with feelings of terror. Fortunately the light of science has long since removed such ideas from amongst us, and celestial objects, in all their various forms, are contemplated without misgiving. They are rather welcomed as affording the means of advancing our knowledge of God's wonderful works as displayed in the heavens.

Period &c.—Mars revolves round the Sun in 686d 23h 30m 41a, and his mean distance from that luminary is 141,500,000 miles. The orbit is one of considerable eccentricity, the distance varying between 154,700,000 and 128,360,000 miles.

Superstitious ideas with regard to comets as the harbingers of disaster have long since been discarded for more rational opinions. They are no longer looked upon as ill-omened presages of evil, or as

Many references are to be found among old writings to the supposed evil influence of these bodies, and to the dread which their appearance formerly incited in the popular mind. Shakespeare makes an allusion to the common belief :—

and in relation to the habit of connecting historical events with their apparition, he further says :—

But, happily, the notions prevalent in former times have been superseded by the more enlightened views naturally resulting from the acquirement and diffusion of knowledge; so that comets, though still surrounded by a good deal of mystery, are now regarded with considerable interest, and welcomed, not only as objects devoid of malevolent character, but as furnishing many useful materials for study. Mere superstition has been put aside as an impediment to real progress, and a more intelligent age has recognized the necessity of dealing only with facts and explaining them according to the laws of nature; for it is on facts, and their just interpretation, that all true searchers after knowledge must rely.

Choice of Telescopes.—The subject of the choice of telescopes has exercised every astronomer more or less, and the question as to the best form of instrument is one which has occasioned endless controversy. The decision is an important one to amateurs, who at the outset of their observing careers require the most efficient instruments obtainable at reasonable cost. It is useless applying to scientific friends who, influenced by different tastes, will give an amount of contradictory advice that will be very perplexing. Some invariably recommend a small refractor and unjustly disparage reflectors, as not only unfitted for very delicate work, but as constantly needing re-adjustment and resilvering.

Others will advise a moderate-sized reflector as affording wonderfully fine views of the Moon and planets. The question of cost is greatly in favour of the latter construction, and, all things considered, it may claim an unquestionable advantage. A man who has decided to spend a small sum for the purpose not merely of gratifying his curiosity but of doing really serviceable work, must adopt the reflector, because refractors of, say, 5 inches and upwards are far too costly, and become enormously expensive as the diameter increases. This is not the case with reflectors; they come within the reach of all, and may indeed be constructed by the observer himself with a little patience and ingenuity.

Refractors and Reflectors.—The relative merits of refractors and reflectors have been so frequently compared and discussed that we have no desire to re-open the question here.

The planetary observer has to accept such opportunities as are given him; he must use his telescope at the particular seasons when his objects are well presented. These are limited in number, and months may pass without one of them coming under favourable review. In stellar work no such irregularities can affect the progress of observations. The student of sidereal astronomy has a vast field to explore, and a diversity of objects of infinite extent. They are so various in their lustre, in their grouping, and in their colours, that the observer's interest is actively retained in his work, and we often find him pursuing it with unflagging diligence through many years. No doubt there would be many others employing their energies in this rich field of labour but for the uninteresting character of star-disks, which are mere points of light, and therefore incapable of displaying any detail. Those who study the Sun, Moon, or planets have a large amount of surface-configuration to examine and delineate, and this is ever undergoing real or apparent changes. But this is wholly wanting in the telescopic images of stars, which exhibit a sameness and lack of detail that is not satisfying to the tastes of every observer.

Number.—These bodies, also called minor planets, and, formerly, asteroids, comprise a very numerous class, and they are extremely small, being quite invisible to the naked eye except in one or two special cases. They all revolve in orbits situated between Mars and Jupiter. The total number discovered is about 300, of which Prof. J. Palisa of Vienna has found more than 70, and the late Dr. C. H. F. Peters of Clinton, N.Y., 49. I have not given exact numbers in the two former cases, because these discoveries are still rapidly progressing.

History of their Discovery.—The first known planetoid (Ceres) was sighted by Piazzi on Jan. 1, 1801. The following year, on March 28, Olbers found another (Pallas). In 1804, on Sept. 1, Harding discovered a third (Juno); and in 1807, March 29, Olbers was a second time successful (Vesta). Then for thirty-eight years no additions were made to the number. The host of planetoids circulating between Mars and Jupiter preserved their incognito without disturbance from the prying and wakeful eyes of astronomers.

But in 1845 Hencke, of Driessen, after years of watching, at length broke the spell of tranquillity by finding another small planet; and his example was emulated by many other observers in subsequent years. Hind, De Gasparis, and Goldschmidt were amongst the earliest and most successful of those who gathered new planets from amongst the stars of the zodiacal constellations.

THE MOVEMENTS OF A COMET

The planets are all massive globes, more or less flattened at the poles; but now we have to talk about a multitude of objects of the most irregular shapes, and of the most flimsy description. We call them comets, and they exist in such numbers that an old astronomer has said “there were more comets in the sky than fishes in the sea,” though I think we cannot quite believe him. There is also another wide difference between planets and comets: planets move round in nearly circular ellipses, and not only do we know where a planet is to-night, but we know where it was a month ago, or a hundred years ago, or where it will be a hundred years or a thousand years to come. All such movements are conducted with conspicuous regularity and order; but now we are to speak of bodies which generally come in upon us in the most uncertain and irregular fashion. They visit us we hardly know from whence, except that it is from outer space, and they are adorned in a glittering raiment, almost spiritual in its texture. They are always changing their appearance in a baffling, but still very fascinating manner. If an artist tries to draw a comet, he will hardly have finished his picture of it in one charming robe before he finds it arrayed in another.

Everyone who wishes to learn something about astronomy should make a determined effort to become acquainted with the principal constellations, and to find out the names of the brighter and more interesting stars. I have therefore added to Star-land this little chapter; in which I have tried to make the study of the stars so simple, that by taking advantage of a few clear nights, there ought to be no difficulty in obtaining knowledge of a few constellations.

The first step is to become familiar with the Great Bear, or Ursa Major, as astronomers more generally call the group. We begin with this, because after it has been once recognised, then you will find it quite easy to make out the other constellations and stars. It may save you some trouble if you can get someone to point out to you the Great Bear, but even without such aid, I think you will be able to make out the seven bright stars which form this remarkable group, from the figure here given (Fig. 90). Of course, the position of this constellation, as of every other in the heavens, changes with the hour of the night, and changes with the seasons of the year. About April the constellation at 11 o'clock at night is high over your head. In September at the same hour, the Great Bear is low down in the north.

It has long been the custom at the Royal Institution of Great Britain to provide each Christmastide a course of Lectures specially addressed to a juvenile audience.

On two occasions, namely, in 1881 and in 1887, the Managers entrusted this honourable duty to me. The second course was in the main a repetition of the first; and on my notes and recollections of both the present little volume has been founded.

I am indebted to my friends Rev. Maxwell Close, Mr. Arthur Rambaut, and Dr. John Todhunter for their kindness in reading the proofs.

THE PHASES OF OUR ATTENDANT THE MOON

The first day of the week is related to the greatest body in the heavens—the sun—and accordingly we call that day Sun-day. The second day of the week is similarly called after the next most important celestial body—the moon—and though we do not actually say Moon-day, we do say Monday, which is very nearly the same. In French, too, we have lune for moon, and Lundi signifies our Monday. The other days of the week also have names derived from the heavens, but of these we shall speak here after. We are now going to talk about the moon.

We can divide the objects in this room into two classes. There are the bright faces in front of me, and there are the bright electric lights above. The electric lights give light, and the faces receive it. I can see both lights and faces; but I see the lights by the light which they themselves give. I see the faces by the illumination which they have received from the electric lights. This is a very simple distinction, but it is a very important one in Star-land. Among all these bodies which glitter in the heavens there are some which shine by their own light like the lamps. There are others only brilliant by reflected light like the faces.

THE HEAT AND BRIGHTNESS OF THE SUN

We can all feel that the sun is very hot, and we know that it is very big and a long way off. Let us first talk about the heat from the sun. On a cold day it is pleasant to go into a room with a good fire, and everybody knows that the nearer we go to the fire, the more strongly we feel the heat. The boy who is at the far end of the room may be shivering with cold, while those close to the fire are as hot as they find to be pleasant. If we could draw nearer to the sun than we are at present, we should find ourselves to be warmer than we are here. Indeed, if we went close enough, the temperature would rise so much that we could not endure it; we should be roasted. On the other hand, we should certainly be frozen to death if we were transported much further away from the sun than we are now. We are able to live comfortably, because our bodies are just arranged to suit the warmth at that distance from the sun at which the earth is actually placed.

Suppose you were able to endure any degree of heat, and that you had some way of setting out on a voyage to the sun.