Great Astronomers - Robert Stawell Ball (the little red hen ebook .TXT) 📗

He saw

clearly that, when sufficient observations of any particular

binary star had been accumulated, it would then be within the

power of the mathematician to elicit from those observations the

shape and the position in space of the path which each of the

revolving stars described around the other. Indeed, in some cases

he would be able to perform the astonishing feat of determining

from his calculations the weight of these distant suns, and thus

be enabled to compare them with the mass of our own sun.

 

[PLATE: NEBULA IN SOUTHERN HEMISPHERE, drawn by Sir John

Herschel.]

 

But this work must follow the observations, it could not precede

them. The first step was therefore to observe and to measure with

the utmost care the positions and distances of those particular

double stars which appear to offer the greatest promise in this

particular research. In 1821, Herschel and a friend of his, Mr.

James South, agreed to work together with this object. South was

a medical man with an ardent devotion to science, and possessed of

considerable wealth. He procured the best astronomical

instruments that money could obtain, and became a most

enthusiastic astronomer and a practical observer of tremendous

energy.

 

South and John Herschel worked together for two years in the

observation and measurement of the double stars discovered by Sir

William Herschel. In the course of this time their assiduity was

rewarded by the accumulation of so great a mass of careful

measurements that when published, they formed quite a volume in

the “Philosophical Transactions.” The value and accuracy of the

work, when estimated by standards which form proper criteria for

that period, is universally recognised. It greatly promoted the

progress of sidereal astronomy, and the authors were in

consequence awarded medals from the Royal Society, and the

Royal Astronomical Society, as well as similar testimonials from

various foreign institutions.

 

This work must, however, be regarded as merely introductory to the

main labours of John Herschel’s life. His father devoted the

greater part of his years as an observer to what he called his

“sweeps” of the heavens. The great reflecting telescope, twenty

feet long, was moved slowly up and down through an arc of about

two degrees towards and from the pole, while the celestial

panorama passed slowly in the course of the diurnal motion before

the keenly watching eye of the astronomer. Whenever a double star

traversed the field Herschel described it to his sister Caroline,

who, as we have already mentioned, was his invariable assistant in

his midnight watches. When a nebula appeared, then he estimated

its size and its brightness, he noticed whether it had a nucleus,

or whether it had stars disposed in any significant manner with

regard to it. He also dictated any other circumstance which he

deemed worthy of record. These observations were duly committed

to writing by the same faithful and indefatigable scribe, whose

business it also was to take a memorandum of the exact position of

the object as indicated by a dial placed in front of her desk, and

connected with the telescope.

 

John Herschel undertook the important task of re-observing the

various double stars and nebulae which had been discovered during

these memorable vigils. The son, however, lacked one inestimable

advantage which had been possessed by the father. John Herschel

had no assistant to discharge all those duties which Caroline had

so efficiently accomplished. He had, therefore, to modify the

system of sweeping previously adopted in order to enable all the

work both of observing and of recording to be done by himself.

This, in many ways, was a great drawback to the work of the

younger astronomer. The division of labour between the observer

and the scribe enables a greatly increased quantity of work to be

got through. It is also distinctly disadvantageous to an observer

to have to use his eye at the telescope directly after he has been

employing it for reading the graduations on a circle, by the light

of a lamp, or for entering memoranda in a note book. Nebulae,

especially, are often so excessively faint that they can only

be properly observed by an eye which is in that highly sensitive

condition which is obtained by long continuance in darkness. The

frequent withdrawal of the eye from the dark field of the

telescope, and the application of it to reading by artificial

light, is very prejudicial to its use for the more delicate

purpose. John Herschel, no doubt, availed himself of every

precaution to mitigate the ill effects of this inconvenience as

much as possible, but it must have told upon his labours as

compared with those of his father.

 

But nevertheless John Herschel did great work during his “sweeps.”

He was specially particular to note all the double stars which

presented themselves to his observation. Of course some little

discretion must be allowed in deciding as to what degree of

proximity in adjacent stars does actually bring them within the

category of “double stars.” Sir John set down all such objects as

seemed to him likely to be of interest, and the results of his

discoveries in this branch of astronomy amount to some thousands.

Six or seven great memoirs in the TRANSACTIONS of the Royal

Astronomical Society have been devoted to giving an account of his

labours in this department of astronomy.

 

[PLATE: THE CLUSTER IN THE CENTAUR, drawn by Sir John Herschel.]

 

One of the achievements by which Sir John Herschel is best known

is his invention of a method by which the orbits of binary stars

could be determined. It will be observed that when one star

revolves around another in consequence of the law of gravitation,

the orbit described must be an ellipse. This ellipse, however,

generally speaking, appears to us more or less foreshortened, for

it is easily seen that only under highly exceptional circumstances

would the plane in which the stars move happen to be directly

square to the line of view. It therefore follows that what we

observe is not exactly the track of one star around the other; it

is rather the projection of that track as seen on the surface of

the sky. Now it is remarkable that this apparent path is still

an ellipse. Herschel contrived a very ingenious and simple method

by which he could discover from the observations the size and

position of the ellipse in which the revolution actually takes

place. He showed how, from the study of the apparent orbit of the

star, and from certain measurements which could easily be

effected upon it, the determination of the true ellipse in which

the movement is performed could be arrived at. In other words,

Herschel solved in a beautiful manner the problem of finding the

true orbits of double stars. The importance of this work may be

inferred from the fact that it has served as the basis on which

scores of other investigators have studied the fascinating subject

of the movement of binary stars.

 

The labours, both in the discovery and measurement of the double

stars, and in the discussion of the observations with the object

of finding the orbits of such stars as are in actual revolution,

received due recognition in yet another gold medal awarded by the

Royal Society. An address was delivered on the occasion by the

Duke of Sussex (30th November, 1833), in the course of which,

after stating that the medal had been conferred on Sir John

Herschel, he remarks:—

 

“It has been said that distance of place confers the same

privilege as distance of time, and I should gladly avail myself

of the privilege which is thus afforded me by Sir John Herschel’s

separation from his country and friends, to express my

admiration of his character in stronger terms than I should

otherwise venture to use; for the language of panegyric, however

sincerely it may flow from the heart, might be mistaken for that

of flattery, if it could not thus claim somewhat of an historical

character; but his great attainments in almost every department of

human knowledge, his fine powers as a philosophical writer, his

great services and his distinguished devotion to science, the high

principles which have regulated his conduct in every relation of

life, and, above all, his engaging modesty, which is the crown of

all his other virtues, presenting such a model of an accomplished

philosopher as can rarely be found beyond the regions of

fiction, demand abler pens than mine to describe them in adequate

terms, however much inclined I might feel to undertake the task.”

 

The first few lines of the eulogium just quoted allude to

Herschel’s absence from England. This was not merely an episode

of interest in the career of Herschel, it was the occasion of one

of the greatest scientific expeditions in the whole history of

astronomy.

 

Herschel had, as we have seen, undertaken a revision of his

father’s “sweeps” for new objects, in those skies which are

visible from our latitudes in the northern hemisphere. He had

well-nigh completed this task. Zone by zone the whole of the

heavens which could be observed from Windsor had passed under his

review. He had added hundreds to the list of nebulae discovered

by his father. He had announced thousands of double stars. At

last, however, the great survey was accomplished. The contents of

the northern hemisphere, so far at least as they could be

disclosed by his telescope of twenty feet focal length, had been

revealed.

 

[PLATE: SIR JOHN HERSCHEL’S OBSERVATORY AT FELDHAUSEN,

Cape of Good Hope.]

 

But Herschel felt that this mighty task had to be supplemented by

another of almost equal proportions, before it could be said that

the twenty-foot telescope had done its work. It was only the

northern half of the celestial sphere which had been fully

explored. The southern half was almost virgin territory, for no

other astronomer was possessed of a telescope of such power as

those which the Herschels had used. It is true, of course, that

as a certain margin of the southern hemisphere was visible from

these latitudes, it had been more or less scrutinized by observers

in northern skies. And the glimpses which had thus been obtained

of the celestial objects in the southern sky, were such as to make

an eager astronomer long for a closer acquaintance with the

celestial wonders of the south. The most glorious object in the

sidereal heavens, the Great Nebula in Orion, lies indeed in that

southern hemisphere to which the younger Herschel’s attention now

became directed. It fortunately happens, however, for votaries of

astronomy all the world over, that Nature has kindly placed her

most astounding object, the great Nebula in Orion, in such a

favoured position, near the equator, that from a considerable

range of latitudes, both north and south, the wonders of the

Nebula can be explored. There are grounds for thinking that the

southern heavens contain noteworthy objects which, on the whole,

are nearer to the solar system than are the noteworthy objects in

the northern skies. The nearest star whose distance is known,

Alpha Centauri, lies in the southern hemisphere, and so also does

the most splendid cluster of stars.

 

Influenced by the desire to examine these objects, Sir John

Herschel determined to take his great telescope to a station in

the southern hemisphere, and thus complete his survey of the

sidereal heavens. The latitude of the Cape of Good Hope is such

that a suitable site could be there found for his purpose. The

purity of the skies in South Africa promised to provide for the

astronomer those clear nights which his delicate task of surveying

the nebulae would require.

 

On November 13, 1833, Sir John Herschel, who had by this time

received the honour of knighthood from William IV., sailed from

Portsmouth for the Cape of Good Hope, taking with him his

gigantic