(777.) Besides the bodies we have described in the foregoing chapters, the heavens present us with an innumerable multitude of other objects, which are called generally by the name of stars. Though comprehending individuals differing from each other, not merely in brightness, but in many other essential points, they all agree in one attribute, — a high degree of permanence as to apparent relative situation. This has procured them the title of “fixed stars;” an expression which is to be understood in a comparative and not an absolute sense, it being certain that many, and probable that all, are in a state of motion, although too slow to be perceptible unless by means of very delicate observations, continued during a long series of years.

(778.) Astronomers are in the habit of distinguishing the stars into classes, according to their apparent brightness. These are termed magnitudes. The brightest stars are said to be of the first magnitude; those which fall so far short of the first degree of brightness as to make a strongly marked distinction are classed in the second; and so on down to the sixth or seventh, which comprise the smallest stars visible to the naked eye, in the clearest and darkest night.

(1.) Every student who enters upon a scientific pursuit, especially if at a somewhat advanced period of life, will find not only that he has much to learn, but much also to unlearn. Familiar objects and events are far from presenting themselves to our senses in that aspect and with those connections under which science requires them to be viewed, and which constitute their rational explanation. There is, therefore, every reason to expect that those objects and relations which, taken together, constitute the subject he is about to enter upon will have been previously apprehended by him, at least imperfectly, because much has hitherto escaped his notice which is essential to its right understanding: and not only so, but too often also erroneously, owing to mistaken analogies, and the general prevalence of vulgar errors. As a first preparation, therefore, for the course he is about to commence, he must loosen his hold on all crude and hastily adopted notions, and must strengthen himself, by something of an effort and a resolve, for the unprejudiced admission of any conclusion which shall appear to be supported by careful observation and logical argument, even should it prove of a nature adverse to notions he may have previously formed for himself, or taken up, without examination, on the credit of others. Such an effort is, in fact, a commencement of that intellectual discipline which forms one of the most important ends of all science. It is the first movement of approach towards that state of mental purity which alone can fit us for a full and steady perception of moral beauty as well as physical adaptation.

(401.) The moon, like the sun, appears to advance among the stars with a movement contrary to the general diurnal motion of the heavens, but much more rapid, so as to be very readily perceived (as we have before observed) by a few hours’ cursory attention on any moonlight night. By this continual advance, which, though sometimes quicker, sometimes slower, is never intermitted or reversed, it makes the tour of the heavens in a mean or average period of 27d 7h 43m 11s°.5, returning, in that time, to a position among the stars nearly coincident with that it had before, and which would be exactly so, but for reasons presently to be stated.

(402.) The moon, then, like the sun, apparently describes an orbit round the earth, and this orbit cannot be very different from a circle, because the apparent angular diameter of the full moon is not liable to any great extent of variation.

(403.) The distance of the moon from the earth is concluded from its horizontal parallax, which may be found either directly, by observations at remote geographical stations, exactly similar to those described in art. 355., in the case of the sun, or by means of the phenomena called occultations, from which also its apparent diameter is most readily and correctly found. From such observations it results that the mean or average distance of the center of the moon from that of the earth is 59.9643 of the earth's equatorial radii, or about 237,000 miles.

On Nebulæ

Although the cause is utterly unknown, and in the present stage of human cognoscence appears to be inscrutable, it is surmised that the exceptional bodies designated Nebulæ have a connection with double-stars (see Arago's Popular Astronomy, book xi. chapter xxiv.) while, as to colours, I have noticed in them pale tints of white, creamy white, yellow, green, and blue. It therefore follows that these incomprehensible but palpable evidences of Omnipotent power and design are not unnecessarily hauled in and appended to our dissertation upon Sidereal Chromatics.

It will be recollected by all who are really concerned about the matter, that, when the wondrous revelations of Lord Rosse were communicated to the public, certain buzzing popinjays, who hang about and obstruct the avenues to the temple of science, vociferously proclaimed that the Nebular Theory had received its coup de grace from the castle at Parsonstown. Now this crude conceit was assuredly not imbibed from his Lordship's statement, he having most pointedly said, that “now, as has always been the case, an increase of instrumental power has added to the number of clusters at the expense of the nebula; properly so called; still it would be very unsafe to conclude that such will always be the case, and thence to draw the obvious inference that all nebulosity is but the glare of stars too remote to be separated by the utmost power of our instruments.”

(130.) Our first chapters have been devoted to the acquisition chiefly of preliminary notions respecting the globe we inhabit, its relation to the celestial objects which surround it, and the physical circumstances under which all astronomical observations must be made, as well as to provide ourselves with a stock of technical words and elementary ideas of most frequent and familiar use in the sequel. We might now proceed to a more exact and detailed statement of the facts and theories of astronomy: but, in order to do this with full effect, it will be desirable that the reader be made acquainted with the principal means which astronomers possess, of determining, with the degree of nicety their theories require, the data on which they ground their conclusions; in other words, of ascertaining by measurement the apparent and real magnitudes with which they are conversant. It is only when in possession of this knowledge that he can fully appretiate either the truth of the theories themselves, or the degree of reliance to be placed on any of their conclusions antecedent to trial: since it is only by knowing what amount of error can certainly be perceived and distinctly measured, that he can satisfy himself whether any theory oifers so close an approximation, in its numerical results, to actual phenomena, as will justify him in receiving it as a true representation of nature.

(131.) Astronomical instrument-making may be justly regarded as the most refined of the mechanical arts, and that in which the nearest approach to geometrical precision is required, and has been attained.

DOUBLE STAR COLOURS CONTINUED

Opening remarks

In consecution of this fascinating subject, we will now follow the period of 1844, the date of the “Cycle,” by that of 1860, the year in which the Hartwell Continuation appeared; after a further advertence to most of its matter, and considerable correspondence on the several points connected therewith. It has been suggested to me that some colours may undergo pulsations, but the adduced instances are mostly at low altitudes, where atmospheric influences are prevalent; and there may be a want of rigorous correction of the residual spectrum of the refracting telescope. But embarrassments in the outset of any enterprize only enhance the value of proven examples.

It may be noted—en passant—that a slight perturbation pervaded the minds of observing neophytes on the averment pronouncing that there are only three primary colours, namely— red, yellow, and blue; and that the other four—orange, green, indigo, and violet—are de facto produced by combinations of the former, and are therefore secondary or compound colours. This is, however, comparatively easy, as well to suggest as to adopt; but when an arrogant Goethe—unversed even in first principles—steps forward in the pride and panoply of popularity to explain the physiological and chemical qualities of the same, in order to demolish the “nauseous precepts of Newton,” we are really taken aback by his temerarious effrontery.

(346.) In the foregoing chapters, it has been shown that the apparent path of the sun is a great circle of the sphere, which it performs in a period of one sidereal year. From this it follows, that the line joining the earth and sun lies constantly in one plane; and that, therefore, whatever be the real motion from which this apparent motion arises, it must be confined to one plane, which is called the plane of the ecliptic.

(347.) We have already seen (art. 146.) that the sun's motion in right ascension among the stars is not uniform. This is partly accounted for by the obliquity of the ecliptic, in consequence of which equal variations in longitude do not correspond to equal changes of right ascension. But if we observe the place of the sun daily throughout the year, by the transit and circle, and from these calculate the longitude for each day, it will still be found that, even in its own proper path, its apparent angular motion is far from uniform. The change of longitude in twenty-four mean solar hours averages 0° 59′ 8″°.33; but about the 31st of December it amounts to 1° 1′ 9″°.9, and about the 1st of July is only 0° 57′ 11″°.5. Such are the extreme limits, and such the mean value of the sun's apparent angular velocity in its annual orbit.

(348.) This variation of its angular velocity is accompanied with a corresponding change of its distance from us.

(456.) The sun and moon are not the only celestial objects which appear to have a motion independent of that by which the great constellation of the heavens is daily carried round the earth. Among the stars there are several,—and those among the brightest and most conspicuous,—which, when attentively watched from night to night, are found to change their relative situations among the rest; some rapidly, others much more slowly. These are called planets. Four of them—Venus, Mars, Jupiter, and Saturn—are remarkably large and brilliant; another, Mercury, is also visible to the naked eye as a large star, but, for a reason which will presently appear, is seldom conspicuous; a sixth, Uranus, is barely discernible without a telescope; and nine others—Neptune, Ceres, Pallas, Vesta, Juno, Astræa, Hebe, Iris, Flora—are never visible to the naked eye. Besides these fifteen, others yet undiscovered may exist; and it is extremely probable that such is the case,—the multitude of telescopic stars being so great that only a small fraction of their number has been sufficiently noticed to ascertain whether they retain the same places or not, and the ten last-mentioned planets having all been discovered within little more than half a century from the present time.

Prelude

As this object constitutes a remarkable case of Sidereal Chromatics, in which Nature seems to have been caught at her work, some account of the decided mutations of colour recently detected in it becomes necessary; and the story may be related in a few words. It should, however, be premised that we are told (and it is highly probable), that light, when first emitted from the photospheres of the sun and stars “should be in all cases identical, the differences of colour depending upon the differences of constitution of the investing atmosphere :” but the variability of the hues in question is still to be explained, since they cannot be satisfactorily accounted for by any prevailing scientific theory. It is truly wonderful; and at the present status of the phenomenon, we can only ejaculate St. Augustine's rem vidi, causam non vidi.

Still it behoves us, like humble neophytes, not merely to wonder and worship, for that line of conduct would not advance us in the adoration due to our omniscient and omnipresent Creator. We must endeavour to understand the glorious phenomena so benignantly opened out to us as portions of a vast design. Some features of nature may be more important for our comprehension than others, yet in our present nescience we should patiently regard them all with equal care, and scrutinise them until their apparent mystery is unveiled.

(11.) The magnitudes, distances, arrangement, and motions of the great bodies which make up the visible universe, their constitution and physical condition, so far as they can be known to us, with their mutual influences and actions on each other, so far as they can be traced by the effects produced, and established by legitimate reasoning, form the assemblage of objects to which the attention of the astronomer is directed. The term astronomy itself, which denotes the law or rule of the astra (by which the ancients understood not only the stars properly so called, but the sun, the moon, and all the visible constituents of the heavens), sufficiently indicates this; and, although the term astrology, which denotes the reason, theory, or interpretation of the stars, has become degraded in its application, and confined to superstitious and delusive attempts to divine future events by their dependence on pretended planetary influences, the same meaning originally attached itself to that epithet.

(12.) But, besides the stars and other celestial bodies, the earth itself, regarded as an individual body, is one principal object of the astronomer's consideration, and, indeed, the chief of all. It derives its importance, in a practical as well as theoretical sense, not only from its proximity, and its relation to us as animated beings, who draw from it the supply of all our wants, but as the station from, which we see all the rest, and as the only one among them to which we can, in the first instance, refer for any determinate marks and measures by which to recognize their changes of situation, or with which to compare their distances.

ON THE COLOURS OF DOUBLE STARS

The Author's intent

Under this title we are not about to inflict on the reader any of the extravagant notions of the ancients—aye, not even the best of them—concerning the essence of hues and tints, they being utterly unsupported either by reason or experiment. Nor is it intended to theorize under modern advantages upon light and rays, the seductions of harmony and contrast, nor to jump at any of the remaining uncertainties of the question. The present light sketch is merely handed forth to investigate that property of colours which affects the sight only: and, if the absolute tints of double stars are determined at special epochs, much will be contributed towards the mysterious laws which regulate such variations as may be observed. Upon this understanding, we will open our fire with the following extract from the Prolegomena to my

CYCLE OF CELESTIAL OBJECTS

Initiatory Remarks

For the sake of the tyro, it may be proper to premise that the light of the two stars composing the double one is frequently found to be of very dissimilar intensity, and often of very dissimilar colours; and no one who has ever directed a telescope to the heavens can have failed to be struck with the brilliant hues they present, especially such lovely objects as γ Andromedæ, α Herculis, and ε Boötis.

St. John's Lodge, 1—8—64.

My dear Friend,

Although the discoveries in sidereal astronomy since the advent of the elder Herschel, have been equally gratifying and marvellous, there is every prospect of still further triumphs for assiduity and observational skill to obtain. For this most desirable consummation, every department to which practical vigilance can render its aid, ought to be furnished to the utmost with sifted materials, in order that, from a mass of recorded facts, safe conclusions may be arrived at. This impression, together with the present number and efficiency of achromatic telescopes, induces me again to call attention to the still vague and indeterminate state in which the COLOURS of multiple stars are registered, with a word or two as to the steps for bettering it. In our confabulations on this material point we agreed, that a reprint of my former suggestions in a portable form, would probably attract the notice and suit the convenience of many observers; the present brochure was therefore undertaken to carry out this view.

The method which has hitherto been pursued in these examinations is both trite and inexact, having been mostly practised when the eye, at all times a delicate organ, has been fatigued by taking and reading-off measures made with an illuminated field; and some observers register the position and distance with the magnitudes of the objects, without a word as to their colours.