rather the surveying, and mapping out, and bringing into cultivation, of
lands already discovered. Probably Herschel may be justly regarded as
the next true pioneer. We shall not, however, properly appreciate the
stages through which astronomy has passed, nor shall we be prepared
adequately to welcome the discoveries of modern times unless we pay some
attention to the intervening age. Moreover, during this era several
facts of great moment gradually came into recognition; and the
importance of the discovery we have now to speak of can hardly be
over-estimated.
Our whole direct knowledge of the planetary and stellar universe, from
the early observations of the ancients down to the magnificent
discoveries of a Herschel, depends entirely upon our happening to
possess a sense of sight. To no other of our senses do any other worlds
than our own in the slightest degree appeal. We touch them or hear them
never. Consequently, if the human race had happened to be blind, no
other world but the one it groped its way upon could ever have been
known or imagined by it. The outside universe would have existed, but
man would have been entirely and hopelessly ignorant of it. The bare
idea of an outside universe beyond the world would have been
inconceivable, and might have been scouted as absurd. We do possess the
sense of sight; but is it to be supposed that we possess every sense
that can be possessed by finite beings? There is not the least ground
for such an assumption. It is easy to imagine a deaf race or a blind
race: it is not so easy to imagine a race more highly endowed with
senses than our own; and yet the sense of smell in animals may give us
some aid in thinking of powers of perception which transcend our own in
particular directions. If there were a race with higher or other senses
than our own, or if the human race should ever in the process of
development acquire such extra sense-organs, a whole universe of
existent fact might become for the first time perceived by us, and we
should look back upon our past state as upon a blind chrysalid form of
existence in which we had been unconscious of all this new wealth of
perception.
It cannot be too clearly and strongly insisted on and brought home to
every mind, that the mode in which the universe strikes us, our view of
the universe, our whole idea of matter, and force, and other worlds, and
even of consciousness, depends upon the particular set of sense-organs
with which we, as men, happen to be endowed. The senses of force, of
motion, of sound, of light, of touch, of heat, of taste, and of
smell--these we have, and these are the things we primarily know. All
else is inference founded upon these sensations. So the world appears to
us. But given other sense-organs, and it might appear quite otherwise.
What it is actually and truly like, therefore, is quite and for ever
beyond us--so long as we are finite beings.
Without eyes, astronomy would be non-existent. Light it is which conveys
all the information we possess, or, as it would seem, ever can possess,
concerning the outer and greater universe in which this small world
forms a speck. Light is the channel, the messenger of information; our
eyes, aided by telescopes, spectroscopes, and many other "scopes" that
may yet be invented, are the means by which we read the information that
light brings.
Light travels from the stars to our eyes: does it come instantaneously?
or does it loiter by the way? for if it lingers it is not bringing us
information properly up to date--it is only telling us what the state of
affairs was when it started on its long journey.
Now, it is evidently a matter of interest to us whether we see the sun
as he is now, or only as he was some three hundred years ago. If the
information came by express train it would be three hundred years behind
date, and the sun might have gone out in the reign of Queen Anne without
our being as yet any the wiser. The question, therefore, "At what rate
does our messenger travel?" is evidently one of great interest for
astronomers, and many have been the attempts made to solve it. Very
likely the ancient Greeks pondered over this question, but the earliest
writer known to me who seriously discussed the question is Galileo. He
suggests a rough experimental means of attacking it. First of all, it
plainly comes quicker than sound. This can be perceived by merely
watching distant hammering, or by noticing that the flash of a pistol is
seen before its report is heard, or by listening to the noise of a
flash of lightning. Sound takes five seconds to travel a mile--it has
about the same speed as a rifle bullet; but light is much quicker than
that.
The rude experiment suggested by Galileo was to send two men with
lanterns and screens to two distant watch-towers or neighbouring
mountain tops, and to arrange that each was to watch alternate displays
and obscurations of the light made by the other, and to imitate them as
promptly as possible. Either man, therefore, on obscuring or showing his
own light would see the distant glimmer do the same, and would be able
to judge if there was any appreciable interval between his own action
and the response of the distant light. The experiment was actually tried
by the Florentine Academicians,[22] with the result that, as practice
improved, the interval became shorter and shorter, so that there was no
reason to suppose that there was any real interval at all. Light, in
fact, seemed to travel instantaneously.
Well might they have arrived at this result. Even if they had made far
more perfect arrangements--for instance, by arranging a looking-glass at
one of the stations in which a distant observer might see the reflection
of his own lantern, and watch the obscurations and flashings made by
himself, without having to depend on the response of human
mechanism--even then no interval whatever could have been detected.
If, by some impossibly perfect optical arrangement, a lighthouse here
were made visible to us after reflection in a mirror erected at New
York, so that the light would have to travel across the Atlantic and
back before it could be seen, even then the appearance of the light on
removing a shutter, or the eclipse on interposing it, would seem to
happen quite instantaneously. There would certainly be an interval: the
interval would be the fiftieth part of a second (the time a stone takes
to drop 1/13th of an inch), but that is too short to be securely
detected without mechanism. With mechanism the thing might be managed,
for a series of shutters might be arranged like the teeth of a large
wheel; so that, when the wheel rotates, eclipses follow one another very
rapidly; if then an eye looked through the same opening as that by which
the light goes on its way to the distant mirror, a tooth might have
moved sufficiently to cover up this space by the time the light
returned; in which case the whole would appear dark, for the light would
be stopped by a tooth, either at starting or at returning, continually.
At higher speeds of rotation some light would reappear, and at lower
speeds it would also reappear; by noticing, therefore, the precise speed
at which there was constant eclipse the velocity of light could be
determined.
[Illustration: FIG. 73.--Diagram of eye looking at a light reflected in
a distant mirror through the teeth of a revolving wheel.]
This experiment has now been made in a highly refined form by Fizeau,
and repeated by M. Cornu with prodigious care and accuracy. But with
these recent matters we have no concern at present. It may be
instructive to say, however, that if the light had to travel two miles
altogether, the wheel would have to possess 450 teeth and to spin 100
times a second (at the risk of flying to pieces) in order that the ray
starting through any one of the gaps might be stopped on returning by
the adjacent tooth.
Well might the velocity of light be called instantaneous by the early
observers. An ordinary experiment seemed (and was) hopeless, and light
was supposed to travel at an infinite speed. But a phenomenon was
noticed in the heavens by a quick-witted and ingenious Danish
astronomer, which was not susceptible of any ordinary explanation, and
which he perceived could at once be explained if light had a certain
rate of travel--great, indeed, but something short of infinite. This
phenomenon was connected with the satellites of Jupiter, and the
astronomer's name was Roemer. I will speak first of the observation and
then of the man.
[Illustration: FIG. 74.--Fizeau's wheel, shewing the appearance of
distant image seen through its teeth. 1st, when stationary, next when
revolving at a moderate speed, last when revolving at the high speed
just sufficient to cause eclipse.]
Jupiter's satellites are visible, precisely as our own moon is, by
reason of the shimmer of sunlight which they reflect. But as they
revolve round their great planet they plunge into his shadow at one part
of their course, and so become eclipsed from sunshine and invisible to
us. The moment of disappearance can be sharply observed.
Take the first satellite as an example. The interval between successive
eclipses ought to be its period of revolution round Jupiter. Observe
this period. It was not uniform. On the average it was 42 hours 47
minutes, but it seemed to depend on the time of year. When Roemer
observed in spring it was less, and in autumn it was more than usual.
This was evidently a puzzling fact: what on earth can our year have to
do with the motion of a moon of Jupiter's? It was probably, therefore,
only an apparent change, caused either by our greater or less distance
from Jupiter, or else by our greater or less speed of travelling to or
from him. Considering it thus, he was led to see that, when the time of
revolution seemed longest, we were receding fastest from Jupiter, and
when shortest, approaching fastest.
_If_, then, light took time on its journey, _if_ it travelled
progressively, the whole anomaly would be explained.
In a second the earth goes nineteen miles; therefore in 42-3/4 hours
(the time of revolution of Jupiter's first satellite) it goes 2.9
million (say three million) miles. The eclipse happens punctually, but
we do not see it till the light conveying the information has travelled
the extra three million miles and caught up the earth. Evidently,
therefore, by observing how much the apparent time of revolution is
lengthened in one part of the earth's orbit and shortened in another,
getting all the data accurately, and assuming the truth of our
hypothetical explanation, we can calculate the velocity of light. This
is what Roemer did.
Now the maximum amount of retardation is just about fifteen seconds.
Hence light takes this time to travel three million miles; therefore its
velocity is three million divided by fifteen, say 200,000, or, as we now
know more exactly, 186,000 miles every second. Note that the delay does
not depend on our _distance_, but on our _speed_. One can tell this by
common-sense as soon as we grasp the general idea of the explanation. A
velocity cannot possibly depend on a distance only.
[Illustration: FIG. 75.--Eclipses of one of Jupiter's satellites. A
diagram intended to illustrate the dependence of its apparent time of
revolution (from eclipse to eclipse) on the motion of the earth; but not
illustrating the matter at all well. TT' T'' are successive positions of
the earth, while JJ' J'' are corresponding positions of Jupiter.]
Roemer's explanation of the anomaly was not accepted by astronomers. It
excited some attention, and was discussed, but it was found not
obviously applicable to any of the satellites except the first, and not
very simply and satisfactorily even to that. I have, of course, given
you the theory in its most elementary and simple form. In actual fact a
host of disturbing and complicated considerations come in--not so
violently disturbing for the first satellite as for the others, because
it moves so quickly, but still complicated enough.
The fact is, the real motion of Jupiter's satellites is a most difficult
problem. The motion even of our own moon (the lunar theory) is difficult
enough: perturbed as its motion is by the sun. You know that Newton said
it cost him more labour than all the rest of the _Principia_. But the
motion of Jupiter's satellites is far worse. No one, in fact, has yet
worked their theory completely out. They are perturbed by the sun, of
course, but they also perturb each other, and Jupiter is far from
spherical. The shape of Jupiter, and their mutual attractions, combine
to make their motions most peculiar and distracting.
Hence an error in the time of revolution of a satellite was not
_certainly_ due to the cause Roemer suggested, unless one could be sure
that the inequality was not a real one, unless it could be shown that
the theory of gravitation was insufficient to account for it. This had
not then been done; so the half-made discovery was shelved, and properly
shelved, as a brilliant but unverified speculation. It remained on the
shelf for half a century, and was no doubt almost forgotten.
[Illustration: FIG. 76.--A Transit-instrument for the British
astronomical expedition, 1874. Shewing in its essential features the
simplest form of such an instrument.]
Now a word or two about the man. He was a Dane, educated at Copenhagen,
and learned in the mathematics. We first hear of him as appointed to
assist Picard, the eminent French geodetic surveyor (whose admirable
work in determining the length of a degree you remember in connection
with Newton), who had come over to Denmark with the object of fixing the
exact site of the old and extinct Tychonic observatory in the island of
Huen. For of course the knowledge of the exact latitude and longitude of
every place whence numerous observations have been taken must be an
essential to the full interpretation of those observations. The
measurements being finished, young Roemer accompanied Picard to Paris,
and here it was, a few years after, that he read his famous paper
concerning "An Inequality in the Motion of Jupiter's First Satellite,"
and its explanation by means of an hypothesis of "the successive
propagation of light."
The later years of his life he spent in Copenhagen as a professor in the
University and an enthusiastic observer of the heavens,--not a
descriptive observer like Herschel, but a measuring observer like Sir
George Airy or Tycho Brahe. He was, in fact, a worthy follower of Tycho,
and the main work of his life is the development and devising of new and
more accurate astronomical instruments. Many of the large and accurate
instruments with which a modern observatory is furnished are the
invention of this Dane. One of the finest observatories in the world is
the Russian one at Pulkowa, and a list of the instruments there reads
like an extended catalogue of Roemer's inventions.
He not only _invented_ the instruments, he had them made, being allowed
money for the purpose; and he used them vigorously, so that at his death
he left great piles of manuscript stored in the national observatory.
Unfortunately this observatory was in the heart of the city, and was
thus exposed to a danger from which such places ought to be as far as
possible exempt.
Some eighteen years after Roemer's death a great conflagration broke out
in Copenhagen, and ruined large portions of the city. The successor to
Roemer, Horrebow by name, fled from his house, with such valuables as he
possessed, to the observatory, and there went on with his work. But
before long the wind shifted, and to his horror he saw the flames
coming his way. He packed up his own and his predecessor's manuscript
observations in two cases, and prepared to escape with them, but the
neighbours had resorted to the observatory as a place of safety, and so
choked up the staircase with their property that he was barely able to
escape himself, let alone the luggage, and everything was lost.
[Illustration: FIG. 77.--Diagram of equatorially mounted telescope; CE
is the polar axis parallel to the axis of the earth; AB the declination
axis. The diurnal motion is compensated by motion about the polar axis
only, the other being clamped.]
Of all the observations, only three days' work remains, and these were
carefully discussed by Dr. Galle, of Berlin, in 1845, and their
nutriment extracted. These ancient observations are of great use for
purposes of comparison with the present state of the heavens, and throw
light upon possible changes that are going on. Of course nowadays such a
series of observations would be printed and distributed in many
libraries, and so made practically indestructible.
Sad as the disaster was to the posthumous fame of the great observer, a
considerable compensation was preparing. The very year that the fire
occurred in Denmark a quiet philosopher in England was speculating and
brooding on a remarkable observation that he had made concerning the
apparent motion of certain stars, and he was led thereby to a discovery
of the first magnitude concerning the speed of light--a discovery which
resuscitated the old theory of Roemer about Jupiter's satellites, and
made both it and him immortal.
James Bradley lived a quiet, uneventful, studious life, mainly at Oxford
but afterwards at the National Observatory at Greenwich, of which he was
third Astronomer-Royal, Flamsteed and Halley having preceded him in that
office. He had taken orders, and lectured at Oxford as Savilian
Professor. It is said that he pondered his great discovery while pacing
the Long Walk at Magdalen College--and a beautiful place it is to
meditate in.
Bradley was engaged in making observations to determine if possible the
parallax of some of the fixed stars. Parallax means the apparent
relative shift of bodies due to a change in the observer's position. It
is parallax which we observe when travelling by rail and looking out of
window at the distant landscape. Things at different distances are left
behind at different apparent rates, and accordingly they seem to move
relatively to each other. The most distant objects are least affected;
and anything enormously distant, like the moon, is not subject to this
effect, but would retain its position however far we travelled, unless
we had some extraordinarily precise means of observation.
So with the fixed stars: they were being observed from a moving
carriage--viz. the earth--and one moving at the rate of nineteen miles a
second. Unless they were infinitely distant, or unless they were all at
the same distance, they must show relative apparent motions among
themselves. Seen from one point of the earth's orbit, and then in six
months from an opposite point, nearly 184 million miles away, surely
they must show some difference of aspect.
Remember that the old Copernican difficulty had never been removed. If
the earth revolved round the sun, how came it that the fixed stars
showed no parallax? The fact still remained a surprise, and the question
a challenge. Picard, like other astronomers, supposed that it was only
because the methods of observation had not been delicate enough; but now
that, since the invention of the telescope and the founding of National
Observatories, accuracy hitherto undreamt of was possible, why not
attack the problem anew? This, then, he did, watching the stars with
great care to see if in six months they showed any change in absolute
position with reference to the pole of the heavens; any known secular
motion of the pole, such as precession, being allowed for. Already he
thought he detected a slight parallax for several stars near the pole,
and the subject was exciting much interest.
Bradley determined to attempt the same investigation. He was not
destined to succeed in it. Not till the present century was success in
that most difficult observation achieved; and even now it cannot be done
by the absolute methods then attempted; but, as so often happens,
Bradley, in attempting one thing, hit upon another, and, as it happened,
one of still greater brilliance and importance. Let us trace the stages
of his discovery.
Atmospheric refraction made horizon observations useless for the
delicacy of his purpose, so he chose stars near the zenith, particularly
one--[gamma] Draconis. This he observed very carefully at different
seasons of the year by means of an instrument specially adapted for
zenith observations, viz. a zenith sector. The observations were made in
conjunction with a friend of his, an amateur astronomer named Molyneux,
and they were made at Kew. Molyneux was shortly made First Lord of the
Admiralty, or something important of that sort, and gave up frivolous
pursuits. So Bradley observed alone. They observed the star accurately
early in the month of December, and then intended to wait six months.
But from curiosity Bradley observed it again only about a week later. To
his surprise, he found that it had already changed its position. He
recorded his observation on the back of an old envelope: it was his wont
thus to use up odd scraps of paper--he was not, I regret to say, a tidy
or methodical person--and this odd piece of paper turned up long
afterwards among his manuscripts. It has been photographed and preserved
as an historical relic.
Again and again he repeated the observation of the star, and continually
found it moving still a little further and further south, an excessively
small motion, but still an appreciable one--not to be set down to errors
of observation. So it went on till March. It then waited, and after a
bit longer began to return, until June. By September it was displaced as
much to the north as it had been to the south, and by December it had
got back to its original position. It had described, in fact, a small
oscillation in the course of the year. The motion affected neighbouring
stars in a similar way, and was called an "aberration," or wandering
from their true place.
For a long time Bradley pondered over this observation, and over others
like them which he also made. He found one group of stars describing
small circles, while others at a distance from them were oscillating in
straight lines, and all the others were describing ellipses. Unless this
state of things were cleared up, accurate astronomy was impossible. The
fixed stars!--they were not fixed a bit. To refined and accurate
observation, such as was now possible, they were all careering about in
little orbits having a reference to the earth's year, besides any proper
motion which they might really have of their own, though no such motion
was at present known. Not till Herschel was that discovered; not till
this extraordinary aberration was allowed for could it be discovered.
The effect observed by Bradley and Molyneux must manifestly be only an
apparent motion: it was absurd to suppose a real stellar motion
regulating itself according to the position of the earth. Parallax could
not do it, for that would displace stars relatively among each other--it
would not move similarly a set of neighbouring stars.
At length, four years after the observation, the explanation struck him,
while in a boat upon the Thames. He noticed the apparent direction of
the wind changed whenever the boat started. The wind veered when the
boat's motion changed. Of course the cause of this was obvious
enough--the speed of the wind and the speed of the boat were compounded,
and gave an apparent direction of the wind other than the true
direction. But this immediately suggested a cause for what he had
observed in the heavens. He had been observing an apparent direction of
the stars other than the true direction, because he was observing from a
moving vehicle. The real direction was doubtless fixed: the apparent
direction veered about with the motion of the earth. It must be that
light did not travel instantaneously, but gradually, as Roemer had
surmised fifty years ago; and that the motion of the light was
compounded with the motion of the earth.
Think of a stream of light or anything else falling on a moving
carriage. The carriage will run athwart the stream, the occupants of the
carriage will mistake its true direction. A rifle fired through the
windows of a railway carriage by a man at rest outside would make its
perforations not in the true line of fire unless the train is
stationary. If the train is moving, the line joining the holes will
point to a place in advance of where the rifle is really located.