circumstances of the recent attempt to correct our estimate of the
sun's distance by Delisle's method. Our Government astronomers
bethought themselves of this method as likely to give the best
possible means for correcting, by observations of Venus in transit,
the estimate of the sun's distance which had been deduced by Newcomb,
and confirmed by Leverrier. While as yet their plans were not finally
decided upon, reasons for questioning this conclusion were indicated to
those officials by unofficial astronomers entertaining very friendly
feelings towards them. Retaining, however, their reliance on the
method thus called in question, they carried out their purpose, though
fortunately making provision, very nearly sufficient, for the use of
another method. Now, instead of the estimate of the sun's distance
obtained from the observations by Delisle's method agreeing closely
with Newcomb's mean value,--about 92,350,000 miles,--it exceeds this
value by about a million miles. (See, however, note on the last page
of this article.) According to various ways of considering the results
sent in by his observers, the chief official astronomer obtains a mean
value ranging from about 93,300,000 miles to about 93,375,000 miles.
The last named estimate seems preferred on the whole; but if we take
93,350,000 miles, we shall probably give about the fairest final
mean value. We have seen, however, that the results of observations
by seven distinct methods give values ranging only between 92,250,000
miles and 92,850,000 miles,--the six best methods giving values ranging
only between 92,250,000 miles and about 92,480,000 miles. The new
value thus lies 500,000 miles above the largest and admittedly the
least trustworthy of the seven results, 870,000 miles above the next
largest, a million miles above the mean value, and 1,100,000 miles
above the least value. It certainly ranges 500,000 miles above the
largest admissible value from those seven trusted methods, dealt with
most skilfully, cautiously, and laboriously, by such mathematicians as
Newcomb and Leverrier.
Can we hesitate as to the inference we should deduce from this result?
We need not for a moment call in question the skill or care with which
the British observing parties carried out their operations. Nor need we
doubt that the results obtained have been most skilfully and cautiously
investigated by those to whom the work of supervision and of reduction
has been entrusted. We need not even question the policy of devoting
so large a share of labour and expense to the employment of a method
held in little favour by most experienced Continental and American
astronomers, and objected to by many in England, including some even
among official astronomers. It was perhaps well that the method should
have one fair and full trial. And it is certain that all who have
taken part in the work have done their duty zealously and skilfully.
Captain Tupman, to whom Sir George Airy, our chief official astronomer,
entrusted the management of the calculations, has received, and justly,
from his official superior, the highest commendation for his energy and
discrimination. But beyond all manner of doubt the method employed has
failed under the test thus applied to it. I do not say that hereafter
the method may not succeed. Some of the conditions which at present
render it untrustworthy are such as may be expected to be modified
with the progress of improvement in the construction of scientific
instruments. But as yet the method is certainly not trustworthy.
This might be safely concluded from the wide discrepancy between
the new result and the mean of those before obtained. Yet if all
the various observations made by the British observing parties
agreed closely together, the circumstance, though it could hardly
shake our inference on this point, would yet cause some degree
of perplexity, since, of itself, it would seem to imply that the
method was trustworthy. Fortunately we are not thus troubled by
conflicting evidence. The indications of the untrustworthy nature of
the method, derived from the discordance between the results obtained
by it and those before inferred, are not a whit clearer, clear and
convincing though they are, than are the indications afforded by their
discordance _inter se_. The distance derived from northern and southern
observations of the beginning of transit ought of course to be the
same as that derived from northern and southern observations of the
end of transit. If both sets of observations were exactly correct, the
agreement between the results would be exact. The discordance between
them could only be wide as a consequence of some serious imperfection
in this method of observing a transit. But the discordance is _very_
wide. The observations of the beginning of transit by the British
parties give a distance of the sun exceeding by rather more than a
million miles that deduced from the observations of the end of transit.
I am well assured that neither Continental nor American astronomers
will accept the new estimate of the sun's distance, unless--which
I venture to predict will not be the case--the entire series of
transit observations should seem to point to the same value as the
most probable mean. Even then most astronomers will, I believe, think
rather that transits of Venus do not afford such satisfactory means
of determining the sun's distance as had been supposed. This opinion,
it is well known, was held by Leverrier, insomuch that he declined
to support with the weight of his influence the proposals for heavy
expenditure by France upon expeditions for observing the recent
transit and the approaching transit of the year 1882.
I doubt whether many, even among British astronomers, will accept
the new value. Already the Superintendent of the _Nautical Almanac_
has given his opinion upon it in terms which cannot be regarded as
favourable. 'It is well known,' he says (I quote at least from an
article which has been attributed to him without contradiction on
his part), 'that some astronomers have not expected our knowledge
of the sun's distance to be greatly improved from the observations
of the transit of Venus. Many, we can imagine, will regard with
some suspicion' so great a value as 93,300,000 miles (I substitute
these words for technical expressions identical in real meaning).
'Nevertheless, whatever degree of doubt might be entertained by
competent authorities, it appears to have been felt by those
immediately responsible for action, in different civilised nations
where science is encouraged, that so rare a phenomenon as a transit of
Venus could not be allowed to pass without every exertion being made to
utilise it.'
Sir George Airy, very naturally, attaches more value to the result of
the British expeditions, or at least of that part of the operations for
which he was responsible, than others are disposed to do. In an address
to the Astronomical Society, he expressed the opinion that 'the results
now presented are well worthy of very great confidence.... Considering
that the number of observers was eighteen, and that they made
fifty-four observations, and considering also the degree of training
they had, and their zeal, and the extreme care that was taken in the
choice of stations, I think,' he said, 'that there will not be anything
to compete with the value which has been deduced.' This is, as I have
said, very naturally his opinion; and although ordinarily it is rather
for the employers than for the employed to estimate the value of the
results sent in, yet at least we cannot object to his just and generous
praise of those who have worked under his orders.
Nevertheless, it must not be forgotten that on a former occasion
when equal satisfaction was expressed with the result of a rather
less costly but still a laborious and difficult experiment, the
scientific world did not accept (and has since definitely rejected)
the conclusion thus confidently advanced. I refer to the famous
Harton Colliery experiment for determining the mass of the earth.
The case is so closely analogous to that we are dealing with, that
it will be instructive briefly to describe its leading features.
Maskelyne, formerly the chief Government astronomer of this country,
from observations of the effect of the mass of Mount Schehallien in
deflecting a plumb-line, had inferred that the density of the earth
is five times that of water. Bouguer from observations in Chimborazo,
and Colonel James from observations on Arthur's Seat, had deduced
very similar results. From pendulum observations on high mountains,
Carlini and Plana made the earth's density very nearly the same.
Cavendish, Reich, and our own Francis Baily, weighed the earth against
two great globes of lead, by a method commonly known as the Cavendish
experiment, but really invented by Michell. These experiments agreed
closely together, making the earth's density about 5-1/2 times that of
water, or giving to the earth a mass equivalent to that which would
be contained in 6,000 millions of millions of millions of tons. Now,
from the Harton Colliery experiments, in 1854, in which the earth's
weight was estimated by comparing the vibrations of a pendulum at the
mouth of the mine with those of a similar pendulum at a depth of about
1,260 feet, it appeared that the earth's density is rather more than
6-1/2 times that of water, corresponding to an increase in our estimate
of the earth's mass by nearly 1,100 millions of millions of millions
of tons, or by more than a sixth of the entire mass resulting from
the most trustworthy former measurements. Sir G. Airy considered that
'this result will compete on at least equal terms with those obtained
by other methods;' but nearly a quarter of a century has passed during
which no competent astronomer has adopted this opinion, or even
suggested any modification of the former mean estimate of the earth's
mass on account of the unexpectedly large value deduced from the Harton
experiment.
It appears to me probable that a similar fortune will attend the latest
measurement of the sun's distance. But fortunately the matter will
not rest merely on measurements already made. Many fresh measurements
will be made during the next few years by methods already tried and
_not_ (like Delisle's transit method) found wanting. The recent
close approach of the planet Mars was not allowed to pass without a
series of observations specially directed to the determination of the
sun's distance; and we know that observations of Mars are among the
most advantageous means available for the solution of this difficult
problem. It was indeed from such observations that the first really
trustworthy measures of the sun's distance were obtained two centuries
ago. The small planets which travel in hundreds between the paths of
Mars and Jupiter have also been pressed into the service. And now so
many of these are known that scarcely a month passes without one or
other of them being favourably placed for the purpose of distance
measurements. For this too their star-like discs make these bodies
specially suitable.
The most probable inference respecting the results obtained by the
British expedition is that their chief value resides in the evidence
which they afford respecting the Delislean method of observation. They
seem to demonstrate what had before been only surmised (though with
considerable confidence by some astronomers), that this method cannot
be relied upon to correct our estimate of the sun's distance. In the
transit of 1882, which by the way will be visible in this country, we
may be certain that other and more satisfactory methods of observation
will be employed.
Before concluding, it may be well to make a few remarks upon some
misapprehensions which seem to exist as to the propriety in the first
place, and the desirability in the second, of comments upon the
arrangements adopted by Government astronomers to utilize particular
astronomical phenomena, and upon the value of the results which may
be obtained by means of such arrangements. Many seem to suppose that
astronomical matters are in some sense like military or naval (warlike)
manoeuvres, to be discussed effectively only by those who 'are under
authority, having (also) soldiers under them,' in other words by
Government astronomers. It would be very unfortunate for science
were this so, seeing that in that case those chiefly responsible for
the selection of methods and the supervision of operations would be
perfectly free from all possibility of criticism. No one under their
authority would be very likely to speak unfavourably of their plans.
And no one possessing higher general authority would be likely to have
any adequate knowledge of astronomy to form an opinion, either as to
the efficiency of the arrangements adopted in any case, or as to the
significance of the results obtained. In warlike matters, to some
degree, the wisdom of the strategy employed is tested by results which
all can appreciate, seeing that they affect directly the well-being of
the nation. Moreover, there are special reasons in these cases why in
the first place there should be a complete system of subordination,
and why in the second few should undertake the study of the science
unless they proposed to take their part in its practical application
and therefore to submit to its disciplinary system. But it is quite
otherwise with the science of astronomy. The nation requires, chiefly
for the regulation of its commerce, a certain number of trained
astronomers, to carry out systematically observations of a certain
class,--observations having in the main scarcely any closer relation
to the real living science of astronomy than land surveying has to
such geology as Lyell taught, or the bone-trade to the science of
anatomy. The stars by their diurnal motion form the most perfect
time-measurers, therefore they must be constantly timed by trained
observers. The sun and moon are the most effective time-indicators for
seamen, and therefore their movements must be most carefully noted.
Our _Nautical Almanac_ in fact embodies the kind of astronomical
materials which Government astronomers are employed to collect and
arrange. Such work may rather be called celestial surveying than
astronomy. But from the days of Flamsteed, the first of our Astronomers
Royal (as the chief Government astronomer is technically called) whose
contemporary, Newton, discovered the great law of the universe, to
those of Maskelyne and Sir G. Airy, whose contemporaries, the elder and
the younger Herschel, disclosed the structure of the universe, there
have always been astronomers outside the ranks of official astronomy,
in no way desirous of entering those ranks, and in fact so taking
their course from the beginning of their study of the science as to
preclude themselves from all possibility of undertaking any official
duties in astronomy. 'Non sua se voluntas,' necessarily, 'sed suæ vitæ
rationes, hoc aditu laudis, qui semper optimo cuique maxime patuit,
prohibuerunt:' though, indeed, it may not untruly be said that to one
who apprehends the true sublimity of astronomy as a science the routine
of official astronomy is by no means inviting, and probably personal
tastes have had very much to do with the choice, by such men, of the
more attractive departments of astronomy. Be this as it may, it is
certain that the astronomers who thus keep outside the official ranks
are not only free, and may not only be fully competent, to express an
opinion on the arrangements made by Government astronomers, or on the
results obtained by them, but as the only members of the community
who are at once free and able so to do, their right to speak may
often involve, in some degree, the duty of speaking. If through some
mistake wrong arrangements were proposed for instance,--and all men,
even officials (Herbert Spencer says, _especially_ officials), are
apt to make mistakes,--then, unless non-official astronomers, who had
carefully examined the subject, expressed their doubts, it is certain
that there would be no means whatever of correcting the error, or even
of detecting its consequence, until many years had elapsed. The leading
official astronomers would in such a case be apt, in fact they are apt
enough as it is, to stand by each other,--a chief in one department
commending the zeal and energy of the chief in another department,
this chief in turn commending the industry and ability of the other,
and so forth,--while subordinates of all ranks might be apt either to
maintain a judicious silence, or else at least to avoid any utterance
which would endanger their position. It may, on the one hand, be to
some degree questioned whether it would be fitting that discipline
should be so far neglected in such a case that a subordinate should
have eyes to see, or ears to hear, or thoughts to note, any error on
the part of his superior in office. And on the other hand, those who
know little or nothing of astronomy can of course form no opinion on
astronomical matters, however high they may be in authority outside
matters scientific. To assert, then, that it is either improper or
undesirable for unofficial astronomers to comment on the plans or
results of astronomers employed and paid by the nation is practically
equivalent to asserting that it is improper or undesirable for the
work of these paid astronomers to be examined at all,--a conclusion
manifestly absurd.[7]
FOOTNOTES:
[Footnote 7: The following lines are from a letter of mine, which
appeared in the _Times_ of April 13, some time after the present
article was written:--
'A few months ago I said in these columns that the determination of
the sun's distance, then recently communicated to Parliament--namely,
93,375,000 miles--was probably some 800,000 miles too great; and I
spoke of the method on which the determination was based as to some
degree discredited by the wide range of difference both between that
result and the mean of the best former measurements, and between the
several results of which that one was itself the mean. Captain Tupman,
as straightforward as he is skilful and zealous, announces as the
result of a re-examination of the British observations a distance
about 600,000 miles less than the above, or, more exactly, about
92,790,000 miles, as the sun's mean distance. But while he obtains from
the ingress observations a mean distance of only 92,300,000 miles,
he obtains from the egress observations a mean distance of about
93,040,000 miles; and the value, 92,790,000 miles, is only obtained as
the mean of these two values duly weighted, the egress observations
being more satisfactory than the ingress observations. 'It appears to
me that the doubts which I formerly expressed as to the trustworthiness
of the method employed, are to some degree justified.
'To the general public it will be more interesting to inquire what
probably is the true mean distance of the sun. To this it may be
replied that in all probability the sun's mean distance does not lie so
much as 600,000 miles on either side of the value 92,300,000 miles' (it
should be 92,400,000).]
_THE PAST HISTORY OF OUR MOON._
The moon, commonly regarded as a mere satellite of the earth, is
in truth a planet, the least member of that family of five bodies
circling within the asteroidal zone, to which astronomers have given
the name of the terrestrial planets. There can be no question that
this is the true position of the moon in the solar system. In fact,
the fashion of regarding her as a mere attendant of our earth may be
looked upon as the last relic of the old astronomy in which our earth
figured as the fixed centre of the universe, and the body for whose
sake all the celestial orbs were fashioned. In this aspect, also, the
moon is a far more interesting object of research than when viewed as
belonging to another and an inferior order. We are able to recognise,
in her, appearances probably resulting from the relative smallness of
her dimensions, and hence to derive probable information as to the
condition of other orbs in the solar system which fall below the earth
in point of size. Precisely as the study of the giant planets, Jupiter
and Saturn, has led astronomers to infer that certain peculiarities
must result from vastness of dimensions, so the study of the dwarf
planets, Mars, our moon, and Mercury, may indicate the relations we are
to associate with inferiority of size.
This thought immediately introduces us to another conception, which
causes us to regard with even greater interest the evidence afforded
by the moon's present condition. It can scarcely be questioned that
the size of any member of the solar system, or rather the quantity
of matter in its orb, assigns, so to speak, the duration of that
orb's existence, or rather of the various stages of that existence.
The smaller body must cool more rapidly than the larger, and hence
the various periods during which the former is fit for this or that
purpose of planetary life (I speak with purposed vagueness here) are
shorter than the corresponding periods in the life of the latter.
Thus the sun, viewed in this way, is the youngest member of the solar
system, while the tiniest members of the asteroid family, if not the
oldest in reality, are the oldest to which the telescope has introduced
us. Jupiter and Saturn come next to the sun in youth; they are still
passing through the earliest stages of planetary existence, even if we
ought not rather to adopt that theory of their condition which regards
them as subordinate suns, helping the central sun to support life on
the satellites which circle around them. Uranus and Neptune are in
a later stage, and perchance when telescopes have been constructed
large enough to study these planets with advantage, we may learn
something of that stage, interesting as being intermediate to the
stages through which our earth and Venus on the one hand, and the giant
brothers Jupiter and Saturn on the other, are at present passing. After
our earth and Venus, which are probably at about the same stage of
planetary development (though owing to the difference in their position
they may not be equally adapted for the support of life), we come to
Mars and Mercury, both of which must be regarded as in all probability
much more advanced and in a sense more aged than the earth on which
we live. In a similar sense,--even as an ephemeron is more aged after
a few hours of existence than a man after as many years,--the small
planet which we call 'our moon' may be described as in the very
decrepitude of planetary existence, nay (some prefer to think), as even
absolutely dead, though its lifeless body still continues to advance
upon its accustomed orbit, and to obey the law of universal attraction.
Considerations such as these give singular interest to the discussion
of the past history of our moon, though they add to the difficulty of
interpreting the problems she presents to us. For we have manifestly to
differentiate between the effects due to the moon's relative smallness
on the one hand, and those due to her great age on the other. If
we could believe the moon to be an orb which simply represents the
condition to which our earth will one day attain, we could study her
peculiarities of appearance with some hope of understanding how they
had been brought about, as well as of learning from such study the
future history of our own earth. But clearly the moon has had another
history than our earth. Her relative smallness has led to relations
such as the earth never has presented and never will present. If our
earth is, as astronomers and physicists believe, to grow dead and cold,
all life perishing from her surface, it is tolerably clear, from what
we already know of her history, that the appearance she will present
in her decrepitude will be utterly unlike that presented by the moon.
Grant that after the lapse of enormous time-intervals the oceans now
existing on the earth will be withdrawn beneath her solid crust,
and even (which seems incredible) that at a more distant future the
atmosphere now surrounding her will have become greatly reduced in
quantity, either by similar withdrawal or in any other manner, yet the
surface of the earth would present few features of resemblance to that
of the moon. Viewed from the distance at which we view the moon, there
would be few crateriform mountains indeed compared with those on the
moon; those visible would be small by comparison with lunar craters
even of medium dimensions; and the radiated regions seen on the moon's
surface would have no discernible counterpart on the surface of the
earth. The only features of resemblance, under the imagined conditions,
would be probably the partially flat sea bottoms (though these would
bear a different proportion to the more elevated regions) and the
mountain ranges, the only terrestrial features of volcanic disturbance
which would be relatively more important than their lunar counterparts.
I do not purpose, however, to discuss the probable future of the
earth, having only indicated the differences just touched upon in order
to remind the reader at the outset that we have not in 'the moon' a
representation of the earth at any stage of her history. Other and
different relations are presented for our consideration, although it
may well be that by carefully discussing them we may learn somewhat
respecting our earth, as also respecting the past history and future
development of the solar system.