Great Astronomers by Robert Stawell Ball (best ereader for students .txt) 📖

which it was his special delight to resort

for relaxation during the course of his arduous career. In spite

of the defects of his school training he seems to have manifested

such remarkable abilities that his uncle decided to enter him in

Cambridge University. He accordingly joined Trinity College as a

sizar in 1819, and after a brilliant career in mathematical and

physical science he graduated as Senior Wrangler in 1823. It may

be noted as an exceptional circumstance that, notwithstanding the

demands on his time in studying for his tripos, he was able, after

his second term of residence, to support himself entirely by

taking private pupils. In the year after he had taken his degree

he was elected to a Fellowship at Trinity College.

 

Having thus gained an independent position, Airy immediately

entered upon that career of scientific work which he prosecuted

without intermission almost to the very close of his life. One of

his most interesting researches in these early days is on the

subject of Astigmatism, which defect he had discovered in his own

eyes. His investigations led him to suggest a means of correcting

this defect by using a pair of spectacles with lenses so shaped as

to counteract the derangement which the astigmatic eye impressed

upon the rays of light. His researches on this subject were of

a very complete character, and the principles he laid down are

to the present day practically employed by oculists in the

treatment of this malformation.

 

On the 7th of December, 1826, Airy was elected to the Lucasian

Professorship of Mathematics in the University of Cambridge, the

chair which Newton’s occupancy had rendered so illustrious. His

tenure of this office only lasted for two years, when he exchanged

it for the Plumian Professorship. The attraction which led him to

desire this change is doubtless to be found in the circumstance

that the Plumian Professorship of Astronomy carried with it at

that time the appointment of director of the new astronomical

observatory, the origin of which must now be described.

 

Those most interested in the scientific side of University life

decided in 1820 that it would be proper to found an astronomical

observatory at Cambridge. Donations were accordingly sought for

this purpose, and upwards of 6,000 pounds were contributed by

members of the University and the public. To this sum 5,000

pounds were added by a grant from the University chest, and in

1824 further sums amounting altogether to 7,115 pounds were given

by the University for the same object. The regulations as to the

administration of the new observatory placed it under the

management of the Plumian Professor, who was to be provided with

two assistants. Their duties were to consist in making meridian

observations of the sun, moon, and the stars, and the observations

made each year were to be printed and published. The observatory

was also to be used in the educational work of the University, for

it was arranged that smaller instruments were to be provided by

which students could be instructed in the practical art of making

astronomical observations.

 

The building of the Cambridge Astronomical Observatory was

completed in 1824, but in 1828, when Airy entered on the discharge

of his duties as Director, the establishment was still far from

completion, in so far as its organisation was concerned. Airy

commenced his work so energetically that in the next year after

his appointment he was able to publish the first volume of

“Cambridge Astronomical Observations,” notwithstanding that every

part of the work, from the making of observations to the revising

of the proof-sheets, had to be done by himself.

 

It may here be remarked that these early volumes of the

publications of the Cambridge Observatory contained the first

exposition of those systematic methods of astronomical work which

Airy afterwards developed to such a great extent at Greenwich, and

which have been subsequently adopted in many other places. No

more profitable instruction for the astronomical beginner can be

found than that which can be had by the study of these volumes, in

which the Plumian Professor has laid down with admirable clearness

the true principles on which meridian work should be conducted.

 

[PLATE: SIR GEORGE AIRY.

From a Photograph by Mr. E.P. Adams, Greenwich.]

 

Airy gradually added to the instruments with which the observatory

was originally equipped. A mural circle was mounted in 1832, and

in the same year a small equatorial was erected by Jones. This

was made use of by Airy in a well-known series of observations of

Jupiter’s fourth satellite for the determination of the mass of

the great planet. His memoir on this subject fully ex pounds the

method of finding the weight of a planet from observations of the

movements of a satellite by which the planet is attended. This

is, indeed, a valuable investigation which no student of astronomy

can afford to neglect. The ardour with which Airy devoted himself

to astronomical studies may be gathered from a remarkable report

on the progress of astronomy during the present century, which he

communicated to the British Association at its second meeting in

1832. In the early years of his life at Cambridge his most famous

achievement was connected with a research in theoretical astronomy

for which consummate mathematical power was required. We can only

give a brief account of the Subject, for to enter into any full

detail with regard to it would be quite out of the question.

 

Venus is a planet of about the same size and the same weight as

the earth, revolving in an orbit which lies within that described

by our globe. Venus, consequently, takes less time than the earth

to accomplish one revolution round the sun, and it happens that

the relative movements of Venus and the earth are so proportioned

that in the time in which our earth accomplishes eight of her

revolutions the other planet will have accomplished almost exactly

thirteen. It, therefore, follows that if the earth and Venus are

in line with the sun at one date, then in eight years later both

planets will again be found at the same points in their orbits.

In those eight years the earth has gone round eight times, and

has, therefore, regained its original position, while in the same

period Venus has accomplished thirteen complete revolutions, and,

therefore, this planet also has reached the same spot where it was

at first. Venus and the earth, of course, attract each other, and

in consequence of these mutual attractions the earth is swayed

from the elliptic track which it would otherwise pursue. In like

manner Venus is also forced by the attraction of the earth to

revolve in a track which deviates from that which it would

otherwise follow. Owing to the fact that the sun is of such

preponderating magnitude (being, in fact, upwards of 300,000 times

as heavy as either Venus or the earth), the disturbances induced

in the motion of either planet, in consequence of the attraction

of the other, are relatively insignificant to the main controlling

agency by which each of the movements is governed. It is,

however, possible under certain circumstances that the disturbing

effects produced upon one planet by the other can become so

multiplied as to produce peculiar effects which attain measurable

dimensions. Suppose that the periodic times in which the earth

and Venus revolved had no simple relation to each other, then the

points of their tracks in which the two planets came into line

with the sun would be found at different parts of the orbits, and

consequently the disturbances would to a great extent neutralise

each other, and produce but little appreciable effect. As,

however, Venus and the earth come back every eight years to nearly

the same positions at the same points of their track, an

accumulative effect is produced. For the disturbance of one

planet upon the other will, of course, be greatest when those two

planets are nearest, that is, when they lie in line with the sun

and on the same side of it. Every eight years a certain part of

the orbit of the earth is, therefore, disturbed by the attraction

of Venus with peculiar vigour. The consequence is that, owing to

the numerical relation between the movements of the planets to

which I have referred, disturbing effects become appreciable which

would otherwise be too small to permit of recognition. Airy

proposed to himself to compute the effects which Venus would have

on the movement of the earth in consequence of the circumstance

that eight revolutions of the one planet required almost the same

time as thirteen revolutions of the other. This is a mathematical

inquiry of the most arduous description, but the Plumian Professor

succeeded in working it out, and he had, accordingly, the

gratification of announcing to the Royal Society that he had

detected the influence which Venus was thus able to assert on the

movement of our earth around the sun. This remarkable investigation

gained for its author the gold medal of the Royal Astronomical

Society in the year 1832.

 

In consequence Of his numerous discoveries, Airy’s scientific fame

had become so well recognised that the Government awarded him a

special pension, and in 1835, when Pond, who was then Astronomer

Royal, resigned, Airy was offered the post at Greenwich. There

was in truth, no scientific inducement to the Plumian Professor to

leave the comparatively easy post he held at Cambridge, in which

he had ample leisure to devote himself to those researches which

specially interested him, and accept that of the much more arduous

observatory at Greenwich. There were not even pecuniary

inducements to make the change; however, he felt it to be his duty

to accede to the request which the Government had made that he

would take up the position which Pond had vacated, and accordingly

Airy went to Greenwich as Astronomer Royal on October 1st, 1835.

 

He immediately began with his usual energy to organise the

systematic conduct of the business of the National Observatory.

To realise one of the main characteristics of Airy’s great work at

Greenwich, it is necessary to explain a point that might not

perhaps be understood without a little explanation by those who

have no practical experience in an observatory. In the work of an

establishment such as Greenwich, an observation almost always

consists of a measurement of some kind. The observer may, for

instance, be making a measurement of the time at which a star

passes across a spider line stretched through the field of view;

on another occasion his object may be the measurement of an angle

which is read off by examining through a microscope the lines of

division on a graduated circle when the telescope is so pointed

that the star is placed on a certain mark in the field of view.

In either case the immediate result of the astronomical

observation is a purely numerical one, but it rarely happens,

indeed we may say it never happens, that the immediate numerical

result which the observation gives expresses directly the quantity

which we are really seeking for. No doubt the observation has

been so designed that the quantity we want to find can be obtained

from the figures which the measurement gives, but the object

sought is not those figures, for there are always a multitude of

other influences by which those figures are affected. For

example, if an observation were to be perfect, then the telescope

with which the observation is made should be perfectly placed in

the exact position which it ought to occupy; this is, however,

never the case, for no mechanic can ever construct or adjust a

telescope so perfectly as the wants of the astronomer demand. The

clock also by which we determine the time of the observation

should be correct, but this is rarely if ever the case. We have

to correct our observations for such errors,