Great Astronomers by Robert Stawell Ball (best ereader for students .txt) 📖
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be adjusted relatively to the average value of the solar
disturbance, must also be gradually declining. In other words,
the moon must be approaching nearer to the earth in consequence
of the alterations in the eccentricity of the earth’s orbit
produced by the attraction of the other planets. It is true that
the change in the moon’s position thus arising is an extremely
small one, and the consequent effect in accelerating the moon’s
motion is but very slight. It is in fact almost imperceptible,
except when great periods of time are involved. Laplace undertook
a calculation on this subject. He knew what the efficiency of the
planets in altering the dimensions of the earth’s orbit amounted
to; from this he was able to determine the changes that would be
propagated into the motion of the moon. Thus he ascertained, or
at all events thought he had ascertained, that the acceleration of
the moon’s motion, as it had been inferred from the observations
of the ancient eclipses which have been handed down to us, could
be completely accounted for as a consequence of planetary
perturbation. This was regarded as a great scientific triumph.
Our belief in the universality of the law of gravitation would, in
fact, have been seriously challenged unless some explanation of
the lunar acceleration had been forthcoming. For about fifty
years no one questioned the truth of Laplace’s investigation.
When a mathematician of his eminence had rendered an explanation
of the remarkable facts of observation which seemed so complete,
it is not surprising that there should have been but little
temptation to doubt it. On undertaking a new calculation of the
same question, Professor Adams found that Laplace had not pursued
this approximation sufficiently far, and that consequently there
was a considerable error in the result of his analysis. Adams,
it must be observed, did not impugn the value of the lunar
acceleration which Halley had deduced from the observations,
but what he did show was, that the calculation by which Laplace
thought he had provided an explanation of this acceleration was
erroneous. Adams, in fact, proved that the planetary influence
which Laplace had detected only possessed about half the
efficiency which the great French mathematician had attributed to
it. There were not wanting illustrious mathematicians who came
forward to defend the calculations of Laplace. They computed the
question anew and arrived at results practically coincident with
those he had given. On the other hand certain distinguished
mathematicians at home and abroad verified the results of
Adams. The issue was merely a mathematical one. It had only one
correct solution. Gradually it appeared that those who opposed
Adams presented a number of different solutions, all of them
discordant with his, and, usually, discordant with each other.
Adams showed distinctly where each of these investigators had
fallen into error, and at last it became universally admitted that
the Cambridge Professor had corrected Laplace in a very
fundamental point of astronomical theory.
Though it was desirable to have learned the truth, yet the breach
between observation and calculation which Laplace was believed to
have closed thus became reopened. Laplace’s investigation, had it
been correct, would have exactly explained the observed facts. It
was, however, now shown that his solution was not correct, and
that the lunar acceleration, when strictly calculated as a
consequence of solar perturbations, only produced about half the
effect which was wanted to explain the ancient eclipses
completely. It now seems certain that there is no means of
accounting for the lunar acceleration as a direct consequence of
the laws of gravitation, if we suppose, as we have been in the
habit of supposing, that the members of the solar system concerned
may be regarded as rigid particles. It has, however, been
suggested that another explanation of a very interesting kind may
be forthcoming, and this we must endeavour to set forth.
It will be remembered that we have to explain why the period of
revolution of the moon is now shorter than it used to be. If we
imagine the length of the period to be expressed in terms of days
and fractions of a day, that is to say, in terms of the rotations
of the earth around its axis, then the difficulty encountered is,
that the moon now requires for each of its revolutions around the
earth rather a smaller number of rotations of the earth around its
axis than used formerly to be the case. Of course this may be
explained by the fact that the moon is now moving more swiftly
than of yore, but it is obvious that an explanation of quite a
different kind might be conceivable. The moon may be moving just
at the same pace as ever, but the length of the day may be
increasing. If the length of the day is increasing, then, of
course, a smaller number of days will be required for the moon to
perform each revolution even though the moon’s period was itself
really unchanged. It would, therefore, seem as if the phenomenon
known as the lunar acceleration is the result of the two causes.
The first of these is that discovered by Laplace, though its value
was overestimated by him, in which the perturbations of the earth
by the planets indirectly affect the motion of the moon. The
remaining part of the acceleration of our satellite is apparent
rather than real, it is not that the moon is moving more quickly,
but that our timepiece, the earth, is revolving more slowly, and
is thus actually losing time. It is interesting to note that we
can detect a physical explanation for the apparent checking of the
earth’s motion which is thus manifested. The tides which ebb and
flow on the earth exert a brake-like action on the revolving
globe, and there can be no doubt that they are gradually reducing
its speed, and thus lengthening the day. It has accordingly been
suggested that it is this action of the tides which produces the
supplementary effect necessary to complete the physical
explanation of the lunar acceleration, though it would perhaps be
a little premature to assert that this has been fully
demonstrated.
The third of Professor Adams’ most notable achievements was
connected with the great shower of November meteors which
astonished the world in 1866. This splendid display concentrated
the attention of astronomers on the theory of the movements of the
little objects by which the display was produced. For the
definite discovery of the track in which these bodies revolve, we
are indebted to the labours of Professor Adams, who, by a
brilliant piece of mathematical work, completed the edifice whose
foundations had been laid by Professor Newton, of Yale, and other
astronomers.
Meteors revolve around the sun in a vast swarm, every individual
member of which pursues an orbit in accordance with the well-known
laws of Kepler. In order to understand the movements of these
objects, to account satisfactorily for their periodic recurrence,
and to predict the times of their appearance, it became necessary
to learn the size and the shape of the track which the swarm
followed, as well as the position which it occupied. Certain
features of the track could no doubt be readily assigned. The
fact that the shower recurs on one particular day of the year,
viz., November 13th, defines one point through which the orbit
must pass. The position on the heavens of the radiant point from
which the meteors appear to diverge, gives another element in the
track. The sun must of course be situated at the focus, so that
only one further piece of information, namely, the periodic time,
will be necessary to complete our knowledge of the movements of
the system. Professor H. Newton, of Yale, had shown that the
choice of possible orbits for the meteoric swarm is limited to
five. There is, first, the great ellipse in which we now know the
meteors revolve once every thirty three and one quarter years.
There is next an orbit of a nearly circular kind in which the
periodic time would be a little more than a year. There is a
similar track in which the periodic time would be a few days short
of a year, while two other smaller orbits would also be
conceivable. Professor Newton had pointed out a test by which it
would be possible to select the true orbit, which we know must be
one or other of these five. The mathematical difficulties which
attended the application of this test were no doubt great, but
they did not baffle Professor Adams.
There is a continuous advance in the date of this meteoric shower.
The meteors now cross our track at the point occupied by the
earth on November 13th, but this point is gradually altering.
The only influence known to us which could account for the
continuous change in the plane of the meteor’s orbit arises from
the attraction of the various planets. The problem to be solved
may therefore be attacked in this manner. A specified amount of
change in the plane of the orbit of the meteors is known to
arise, and the changes which ought to result from the attraction
of the planets can be computed for each of the five possible
orbits, in one of which it is certain that the meteors must
revolve. Professor Adams undertook the work. Its difficulty
principally arises from the high eccentricity of the largest of
the orbits, which renders the more ordinary methods of
calculation inapplicable. After some months of arduous labour the
work was completed, and in April, 1867, Adams announced his
solution of the problem. He showed that if the meteors revolved
in the largest of the five orbits, with the periodic time of
thirty three and one quarter years, the perturbations of Jupiter
would account for a change to the extent of twenty minutes of arc
in the point in which the orbit crosses the earth’s track. The
attraction of Saturn would augment this by seven minutes, and
Uranus would add one minute more, while the influence of the Earth
and of the other planets would be inappreciable. The
accumulated effect is thus twenty-eight minutes, which is
practically coincident with the observed value as determined by
Professor Newton from an examination of all the showers of which
there is any historical record. Having thus showed that the great
orbit was a possible path for the meteors, Adams next proved that
no one of the other four orbits would be disturbed in the same
manner. Indeed, it appeared that not half the observed amount of
change could arise in any orbit except in that one with the long
period. Thus was brought to completion the interesting research
which demonstrated the true relation of the meteor swarm to the
solar system.
Besides those memorable scientific labours with which his
attention was so largely engaged, Professor Adams found time for
much other study. He occasionally allowed himself to undertake as
a relaxation some pieces of numerical calculation, so tremendously
long that we can only look on them with astonishment. He has
calculated certain important mathematical constants accurately to
more than two hundred places of decimals. He was a diligent
reader of works on history, geology, and botany, and his arduous
labours were often beguiled by novels, of which, like many other
great men, he was very fond. He had also the taste of a
collector, and he brought together about eight hundred volumes of
early printed works, many of considerable rarity and value. As to
his personal character, I may quote the words of Dr. Glaisher when
he says, “Strangers who first met him were invariably struck by
his simple and unaffected manner. He was a delightful companion,
always cheerful and genial, showing in society but few traces of
his really shy and retiring disposition. His nature was
sympathetic and generous, and in few men have the moral and
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