JUPITER.
The Great Size of Jupiter--Comparison of his Diameter with that of
the Earth--Dimensions of the Planet and his Orbit--His
Rotation--Comparison of his Weight and Bulk with that of the
Earth--Relative Lightness of Jupiter--How Explained--Jupiter still
probably in a Heated Condition--The Belts on Jupiter--Spots on his
Surface--Time of Rotation of different Spots various--Storms on
Jupiter--Jupiter not Incandescent--The Satellites--Their
Discovery--Telescopic Appearance--Their Orbits--The Eclipses and
Occultations--A Satellite in Transit--The Velocity of Light
Discovered--How is this Velocity to be Measured
Experimentally?--Determination of the Sun's Distance by the
Eclipses of Jupiter's Satellites--Jupiter's Satellites
demonstrating the Copernican System.
In our exploration of the beautiful series of bodies which form the
solar system, we have proceeded step by step outwards from the sun. In
the pursuit of this method we have now come to the splendid planet
Jupiter, which wends its majestic way in a path immediately outside
those orbits of the minor planets which we have just been considering.
Great, indeed, is the contrast between these tiny globes and the
stupendous globe of Jupiter. Had we adopted a somewhat different method
of treatment--had we, for instance, discussed the various bodies of our
planetary system in the order of their magnitude--then the minor planets
would have been the last to be considered, while the leader of the host
would be Jupiter. To this position Jupiter is entitled without an
approach to rivalry. The next greatest on the list, the beautiful and
interesting Saturn, comes a long distance behind. Another great descent
in the scale of magnitude has to be made before we reach Uranus and
Neptune, while still another step downwards must be made before we
reach that lesser group of planets which includes our earth. So
conspicuously does Jupiter tower over the rest, that even if Saturn were
to be augmented by all the other globes of our system rolled into one,
the united mass would still not equal the great globe of Jupiter.
[Illustration: Fig. 56.--The Relative Dimensions of Jupiter and the
Earth.]
The adjoining picture (Fig. 56) shows the relative dimensions of Jupiter
and the earth, and it conveys to the eye a more vivid impression of the
enormous bulk of Jupiter than we can readily obtain by merely
considering the numerical statements by which his bulk is to be
accurately estimated. As, however, it will be necessary to place the
numerical facts before our readers, we do so at the outset of this
chapter.
Jupiter revolves in an elliptic orbit around the sun in the focus, at a
mean distance of 483,000,000 miles. The path of Jupiter is thus about
5.2 times as great in diameter as the path pursued by the earth. The
shape of Jupiter's orbit departs very appreciably from a circle, the
greatest distance from the sun being 5.45, while the least distance is
about 4.95, the earth's distance from the sun being taken as unity. In
the most favourable circumstances for seeing Jupiter at opposition, it
must still be about four times as far from the earth as the earth is
from the sun. This great globe will also illustrate the law that the
more distant a planet is, the slower is the velocity with which its
orbital motion is accomplished. While the earth passes over eighteen
miles each second, Jupiter only accomplishes eight miles. Thus for a
twofold reason the time occupied by an exterior planet in completing a
revolution is greater than the period of the earth. Not only has the
outer planet to complete a longer course than the earth, but the speed
is less; it thus happens that Jupiter requires 4,332.6 days, or about
fifty days less than twelve years, to make a circuit of the heavens.
The mean diameter of the great planet is about 87,000 miles. We say the
_mean_ diameter, because there is a conspicuous difference in the case
of Jupiter between his equatorial and his polar diameters. We have
already seen that there is a similar difference in the case of the
earth, where we find the polar diameter to be shorter than the
equatorial; but the inequality of these two dimensions is very much
larger in Jupiter than in the earth. The equatorial diameter of Jupiter
is 89,600 miles, while the polar is not more than 84,400 miles. The
ellipticity of Jupiter indicated by these figures is sufficiently marked
to be obvious without any refined measures. Around the shortest diameter
the planet spins with what must be considered an enormous velocity when
we reflect on the size of the globe. Each rotation is completed in about
9 hrs. 55 mins.
We may naturally contrast the period of rotation of Jupiter with the
much slower rotation of our earth in twenty-four hours. The difference
becomes much more striking if we consider the relative speeds at which
an object on the equator of the earth and on that of Jupiter actually
moves. As the diameter of Jupiter is nearly eleven times that of the
earth, it will follow that the speed of the equator on Jupiter must be
about twenty-seven times as great as that on the earth. It is no doubt
to this high velocity of rotation that we must ascribe the extraordinary
ellipticity of Jupiter; the rapid rotation causes a great centrifugal
force, and this bulges out the pliant materials of which he seems to be
formed.
Jupiter is not, so far as we can see, a solid body. This is an important
circumstance; and therefore it will be necessary to discuss the matter
at some little length, as we here perceive a wide contrast between this
great planet and the other planets which have previously occupied our
attention. From the measurements already given it is easy to calculate
the bulk or the volume of Jupiter. It will be found that this planet is
about 1,300 times as large as the earth; in other words, it would take
1,300 globes, each as large as our earth, all rolled into one, to form a
single globe as large as Jupiter.
If the materials of which Jupiter is composed were of a nature analogous
to the materials of the earth, we might expect that the weight of the
planet would exceed the weight of the earth in something like the
proportion of their volumes. This is the matter now proposed to be
brought to trial. Here we may at once be met with the query, as to how
we are to find the weight of Jupiter. It is not even an easy matter to
weigh the earth on which we stand. How, then, can we weigh a mighty
planet vastly larger than the earth, and distant from us by some
hundreds of millions of miles? Truly, this is a bold problem. Yet the
intellectual resources of man have proved sufficient to achieve this
feat of celestial engineering. They are not, it is true, actually able
to make the ponderous weighing scales in which the great planet is to be
cast, but they are able to divert to this purpose certain natural
phenomena which yield the information that is required.
Such investigations are based on the principle of universal gravitation.
The mass of Jupiter attracts other masses in the solar system. The
efficiency of that attraction is more particularly shown on the bodies
which are near the planet. In virtue of this attraction certain
movements are performed by those bodies. We can observe their character
with our telescopes, we can ascertain their amount, and from our
measurements we can calculate the mass of the body by which the
movements have been produced. This is the sole method which we possess
for the investigation of the masses of the planets; and though it may
be difficult in its application--not only from the observations which
are required, but also from the intricacy and the profundity of the
calculations to which those observations must be submitted--yet, in the
case of Jupiter at least, there is no uncertainty about the result.
The task is peculiarly simplified in the case of the greatest planet of
our system by the beautiful system of moons with which he is attended.
These little moons revolve under the guidance of Jupiter, and their
movements are not otherwise interfered with so as to prevent their use
for our present purpose. It is from the observations of the satellites
of Jupiter that we are enabled to measure his attractive power, and
thence to calculate the mass of the mighty planet.
To those not specially conversant with the principles of mechanics, it
may seem difficult to realise the degree of accuracy of which such a
method is capable. Yet there can be no doubt that his moons inform us of
the mass of Jupiter, and do not leave a margin of inaccuracy so great as
one hundredth part of the total amount. If other confirmation be needed,
then it is forthcoming in abundance. A minor planet occasionally draws
near the orbit of Jupiter and experiences his attraction; the planet is
forced to swerve from its path, and the amount of the deviation can be
measured. From that measurement the mass of Jupiter can be computed by a
calculation, of which it would be impossible to give an account in this
place. The mass of Jupiter, as determined by this method, agrees with
the mass obtained in a totally different manner from the satellites.
Nor have we yet exhausted the resources of astronomy in its bearing on
this question. We can discard the planetary system, and invite the
assistance of a comet which, flashing through the orbits of the planets,
occasionally experiences large and sometimes enormous disturbances. For
the present it suffices to remark, that on one or two occasions it has
happened that venturous comets have been near enough to Jupiter to be
much disturbed by his attraction, and then to proclaim in their altered
movements the magnitude of the mass which has affected them. The
satellites of Jupiter, the minor planets, and the comets, all tell the
weight of the giant orb; and, as they all concur in the result (at least
within extremely narrow limits), we cannot hesitate to conclude that the
mass of the greatest planet of our system has been determined with
accuracy.
The results of these measures must now be stated. They show, of course,
that Jupiter is vastly inferior to the sun--that, in fact, it would take
about 1,047 Jupiters, all rolled into one, to form a globe equal in
_weight_ to the sun. They also show us that it would take 316 globes as
heavy as our Earth to counterbalance the weight of Jupiter.
No doubt this proves Jupiter to be a body of magnificent proportions;
but the remarkable circumstance is not that Jupiter should be 316 times
as heavy as the earth, but that he is not a great deal more. Have we not
stated that Jupiter is 1,300 times as _large_ as the earth? How then
comes it that he is only 316 times as _heavy_? This points at once to
some fundamental contrast between the constitution of Jupiter and of the
earth. How are we to account for this difference? We can conceive of two
explanations. In the first place, it might be supposed that Jupiter is
constituted of materials partly or wholly unknown on the earth. There
is, however, an alternative supposition at once more philosophical and
more consistent with the evidence. It is true that we know little or
nothing of what the elementary substances on Jupiter may be, but one of
the great discoveries of modern astronomy has taught us something of the
elementary bodies present in other bodies of the universe, and has
demonstrated that to a large extent they are identical with the
elementary bodies on the earth. If Jupiter be composed of bodies
resembling those on the earth, there is one way, and only one, in which
we can account for the disparity between his size and his mass. Perhaps
the best way of stating the argument will be found in a glance at the
remote history of the earth itself, for it seems not impossible that the
present condition of Jupiter was itself foreshadowed by the condition of
our earth countless ages ago.
In a previous chapter we had occasion to point out how the earth seemed
to be cooling from an earlier and highly heated condition. The further
we look back, the hotter our globe seems to have been; and if we project
our glance back to an epoch sufficiently remote, we see that it must
once have been so hot that life on its surface would have been
impossible. Back still earlier, we find the heat to have been such that
water could not rest on the earth; and hence it seems likely that at
some incredibly remote epoch all the oceans now reposing in the deeps on
the surface, and perhaps a considerable portion of its now solid crust,
must have been in a state of vapour. Such a transformation of the globe
would not alter its _mass_, for the materials weigh the same whatever be
their condition as to temperature, but it would alter the _size_ of our
globe to a very considerable extent. If these oceans were transformed
into vapour, then the atmosphere, charged with mighty clouds, would have
a bulk some hundreds of times greater than that which it has at present.
Viewed from a distant planet, the cloud-laden atmosphere would indicate
the visible size of our globe, and its average density would accordingly
appear to be very much less than it is at present.
From these considerations it will be manifest that the discrepancy
between the size and the weight of Jupiter, as contrasted with our
earth, would be completely removed if we supposed that Jupiter was at
the present day a highly heated body in the condition of our earth
countless ages ago. Every circumstance of the case tends to justify this
argument. We have assigned the smallness of the moon as a reason why the
moon has cooled sufficiently to make its volcanoes silent and still. In
the same way the smallness of the earth, as compared with Jupiter,
accounts for the fact that Jupiter still retains a large part of its
original heat, while the smaller earth has dissipated most of its store.
This argument is illustrated and strengthened when we introduce other
planets into the comparison. As a general rule we find that the smaller
bodies, like the earth and Mars, have a high density, indicative of a
low temperature, while the giant planets, like Jupiter and Saturn, have
a low density, suggesting that they still retain a large part of their
original heat. We say "original heat" for the want, perhaps, of a more
correct expression; it will, however, indicate that we do not in the
least refer to the solar heat, of which, indeed, the great outer planets
receive much less than those nearer the sun. Where the original heat may
have come from is a matter still confined to the province of
speculation.
A complete justification of these views with regard to Jupiter is to be
found when we make a minute telescopic scrutiny of its surface; and it
fortunately happens that the size of the planet is so great that, even
at a distance of more millions of miles than there are days in the year,
we can still trace on its surface some significant features.
Plate XI. gives a series of four different views of Jupiter. They have
been taken from a series of admirable drawings of the great planet made
by Mr. Griffiths in 1897. The first picture shows the appearance of the
globe at 10h. 20m. Greenwich time on February 17th, 1897, through a
powerful refracting telescope. We at once notice in this drawing that
the outline of Jupiter is distinctly elliptical. The surface of the
planet usually shows the remarkable series of belts here represented.
They are nearly parallel to each other and to the planet's equator.
When Jupiter is observed for some hours, the appearance of the belts
undergoes certain changes. These are partly due to the regular rotation
of the planet on its axis, which, in a period of less than five hours,
will completely carry away the hemisphere we first saw, and replace it
by the hemisphere originally at the other side. But besides the changes
thus arising, the belts and other features on the planet are also very
variable. Sometimes new stripes or marks appear, and old ones disappear;
in fact, a thorough examination of Jupiter will demonstrate the
remarkable fact that there are no permanent features whatever to be
discerned. We are here immediately struck by the contrast between
Jupiter and Mars; on the smaller planet the main topographical outlines
are almost invariable, and it has been feasible to construct maps of the
surface with tolerably accurate detail; a map of Jupiter is, however, an
impossibility--the drawing of the planet which we make to-night will be
different from the drawing of the same hemisphere made a few weeks
hence.
It should, however, be noticed that objects occasionally appear on the
planet which seem of a rather more persistent character than the belts.
We may especially mention the object known as the great oblong Red Spot,
which has been a very remarkable feature upon the southern hemisphere of
Jupiter since 1878. This object, which has attracted a great deal of
attention from observers, is about 30,000 miles long by about 7,000 in
breadth. Professor Barnard remarks that the older the spots on Jupiter
are, the more ruddy do they tend to become.
The conclusion is irresistibly forced upon us that when we view the
surface of Jupiter we are not looking at any solid body. The want of
permanence in the features of the planet would be intelligible if what
we see be merely an atmosphere laden with clouds of impenetrable
density. The belts especially support this view; we are at once reminded
of the equatorial zones on our own earth, and it is not at all unlikely
that an observer sufficiently remote from the earth to obtain a just
view of its appearance would detect upon its surface more or less
perfect cloud-belts suggestive of those on Jupiter. A view of our earth
would be, as it were, intermediate between a view of Jupiter and of
Mars. In the latter case the appearance of the permanent features of the
planet is only to a trifling extent obscured by clouds floating over the
surface. Our earth would always be partly, and often perhaps very
largely, covered with cloud, while Jupiter seems at all times completely
enveloped.
From another class of observations we are also taught the important
truth that Jupiter is not, superficially at least, a solid body. The
period of rotation of the planet around its axis is derived from the
observation of certain marks, which present sufficient definiteness and
sufficient permanence to be suitable for the purpose. Suppose one of
these objects to lie at the centre of the planet's disc; its position is
carefully measured, and the time is noted. As the hours pass on, the
mark moves to the edge of the disc, then round the other side of the
planet, and back again to the visible disc. When it has returned to the
position originally occupied the time is again taken, and the interval
which has elapsed is called the period of rotation of the spot.
If Jupiter were a solid, and if these features were engraved upon its
surface, then it is perfectly clear that the time of rotation as found
by any one spot must coincide precisely with the time yielded by any
other spot; but this is not observed to be the case. In fact, it would
be nearer the truth to say that each spot gives a special period of its
own. Nor are the differences very minute. It has been found that the
time in which the red spot (the latitude of which is about 25 deg. south) is
carried round is five minutes longer than that required by some peculiar
white marks near the equator. The red spot has now been watched for
about twenty years, and during most of that time has had a tendency to
rotate more and more slowly, as may be seen from the following values of
its rotation period:--
In 1879, 9h. 55m. 33.9s.
In 1886, 9h. 55m. 40.6s.
In 1891, 9h. 55m. 41.7s.
Since 1891 this tendency seems to have ceased, while the spot has been
gradually fading away. Generally speaking, we may say that the
equatorial regions rotate in about 9h. 50m. 20s., and the temperate
zones in about 9h. 55m. 40s. Remarkable exceptions are occasionally met
with. Some small black spots in north latitude 22 deg., which broke out in
1880 and again in 1891, rotated in 9h. 48m. to 9h. 49-1/2m. It may,
therefore, be regarded as certain that the globe of Jupiter, so far as
we can see it, is not a solid body. It consists, on the exterior at all
events, of clouds and vaporous masses, which seem to be agitated by
storms of the utmost intensity, if we are to judge from the ceaseless
changes of the planet's surface.
[Illustration: PLATE XI.
Feb. 2nd. Feb. 4th.
Feb. 12th. Feb. 28th.
THE PLANET JUPITER.
1897.]
[Illustration: Fig. 57.--The Occultation of Jupiter (1).]
[Illustration: Fig. 58.--The Occultation of Jupiter (2).]
[Illustration: Fig. 59.--The Occultation of Jupiter (3).]
[Illustration: Fig. 60.--The Occultation of Jupiter (4).]
Various photographs of Jupiter have been obtained; those which have been
taken at the Lick Observatory being specially interesting and
instructive. Pictures of the planet obtained with the camera in
remarkable circumstances are represented in Figs. 57-60, which were
taken by Professor Wm. H. Pickering at Arequipa, Peru, on the 12th of
August, 1892.[21] The small object with the belts is the planet Jupiter.
The large advancing disc (of which only a small part can be shown) is
the moon. The phenomenon illustrated is called the "occultation" of the
planet. The planet is half-way behind the moon in Fig. 59, while in Fig.
60 half of the planet is still hidden by the dark limb of the moon.
It is well known that the tempests by which the atmosphere surrounding
the earth is convulsed are to be ultimately attributed to the heat of
the sun. It is the rays from the great luminary which, striking on the
vast continents, warm the air in contact therewith. This heated air
becomes lighter, and rises, while air to supply its place must flow in
along the surface. The currents so produced form a breeze or a wind;
while, under exceptional circumstances, we have the phenomena of
cyclones and hurricanes, all originated by the sun's heat. Need we add
that the rains, which so often accompany the storms, have also arisen
from the solar beams, which have distilled from the wide expanse of
ocean the moisture by which the earth is refreshed?
The storms on Jupiter seem to be vastly greater than those on the earth.
Yet the intensity of the sun's heat on Jupiter is only a mere
fraction--less, indeed, than the twenty-fifth part--of that received by
the earth. It is incredible that the motive power of the appalling
tempests on the great planet can be entirely, or even largely, due to
the feeble influence of solar heat. We are, therefore, led to seek for
some other source of such disturbances. What that source is to be will
appear obvious when we admit that Jupiter still retains a large
proportion of primitive internal heat. Just as the sun itself is
distracted by violent tempests as an accompaniment of its intense
internal fervour, so, in a lesser degree, do we observe the same
phenomena in Jupiter. It may also be noticed that the spots on the sun
usually lie in more or less regular zones, parallel to its equator, the
arrangement being in this respect not dissimilar to that of the belts on
Jupiter.
It being admitted that the mighty planet still retains some of its
internal heat, the question remains as to how much. It is, of course,
obvious that the heat of the planet is inconsiderable when compared
with the heat of the sun. The brilliance of Jupiter, which makes it an
object of such splendour in our midnight sky, is derived from the same
great source which illuminates the earth, the moon, or the other
planets. Jupiter, in fact, shines by reflected sunlight, and not in
virtue of any intrinsic light in his globe. A beautiful proof of this
truth is familiar to every user of a telescope. The little satellites of
the planet sometimes intrude between him and the sun, and cast a shadow
on Jupiter. The shadow is black, or, at all events, it seems black,
relatively to the brilliant surrounding surface of the planet. It must,
therefore, be obvious that Jupiter is indebted to the sun for its
brilliancy. The satellites supply another interesting proof of this
truth. One of these bodies sometimes enters the shadow of Jupiter and
lo! the little body vanishes. It does so because Jupiter has cut off the
supply of sunlight which previously rendered the satellite visible. But
the planet is not himself able to offer the satellite any light in
compensation for the sunlight which he has intercepted.[22]
Enough, however, has been demonstrated to enable us to pronounce on the
question as to whether the globe of Jupiter can be inhabited by living
creatures resembling those on this earth. Obviously this cannot be so.
The internal heat and the fearful tempests seem to preclude the
possibility of organic life on the great planet, even were there not
other arguments tending to the same conclusion. It may, however, be
contended, with perhaps some plausibility, that Jupiter has in the
distant future the prospect of a glorious career as the residence of
organic life. The time will assuredly come when the internal heat must
decline, when the clouds will gradually condense into oceans. On the
surface dry land may then appear, and Jupiter be rendered habitable.
From this sketch of the planet itself we now turn to the interesting and
beautiful system of five satellites by which Jupiter is attended. We
have, indeed, already found it necessary to allude more than once to
these little bodies, but not to such an extent as to interfere with the
more formal treatment which they are now to receive.
The discovery of the four chief satellites may be regarded as an
important epoch in the history of astronomy. They are objects situated
in a remarkable manner on the border-line which divides the objects
visible to the unaided eye from those which require telescopic aid. It
has been frequently asserted that these objects have been seen with the
unaided eye; but without entering into any controversy on the matter, it
is sufficient to recite the well-known fact that, although Jupiter had
been a familiar object for countless centuries, yet the sharpest eyes
under the clearest skies never discovered the satellites until Galileo
turned the newly invented telescope upon them. This tube was no doubt a
very feeble instrument, but very little power suffices to show objects
so dose to the limit of visibility.
[Illustration: Fig. 61.--Jupiter and his Four Satellites as seen in a
Telescope of Low Power.]
The view of the planet and its elaborate system of satellites as shown
in a telescope of moderate power, is represented in Fig. 61. We here see
the great globe, and nearly in a line parsing through its centre lie
four small objects, three on one side and one on the other. These little
bodies resemble stars, but they can be distinguished therefrom by their
ceaseless movements around the planet, which they never fail to
accompany during his entire circuit of the heavens. There is no more
pleasing spectacle for the student than to follow with his telescope the
movements of this beautiful system.
[Illustration: Fig. 62.--Disappearances of Jupiter's Satellites.]
In Fig. 62 we have represented some of the various phenomena which the
satellites present. The long black shadow is that produced by the
interposition of Jupiter in the path of the sun's rays. In consequence
of the great distance of the sun this shadow will extend, in the form of
a very elongated cone, to a distance far beyond the orbit of the outer
satellite. The second satellite is immersed in this shadow, and
consequently eclipsed. The eclipse of a satellite must not be attributed
to the intervention of the body of Jupiter between the satellite and the
earth. Such an occurrence is called an occultation, and the third
satellite is shown in this condition. The second and the third
satellites are thus alike invisible, but the cause of the invisibility
is quite different in the two cases. The eclipse is much the more
striking phenomenon of the two, because the satellite, at the moment it
plunges into the darkness, may be still at some apparent distance from
the edge of the planet, and is thus seen up to the moment of the
eclipse. In an occultation the satellite in passing behind the planet
is, at the time of disappearance, close to the planet's bright edge,
and the extinction of the light from the small body cannot be observed
with the same impressiveness as the occurrence of an eclipse.
A satellite also assumes another remarkable situation when in the course
of transit over the face of the planet. The satellite itself is not
always very easy to see in such circumstances, but the beautiful shadow
which it casts forms a sharp black spot on the brilliant orb: the
satellite will, indeed, frequently cast a visible shadow when it passes
between the planet and the sun, even though it be not actually at the
moment in front of the planet, as it is seen from the earth.
The periods in which the four principal moons of Jupiter revolve around
their primary are respectively, 1 day 18 hrs. 27 min. 34 secs. for the
first; 3 days 13 hrs. 13 min. 42 secs., for the second; 7 days 3 hrs. 42
min. 33 secs, for the third; and 16 days 16 hrs. 32 min. 11 secs. for
the fourth. We thus observe that the periods of Jupiter's satellites are
decidedly briefer than that of our moon. Even the satellite most distant
from the great planet requires for each revolution less than two-thirds
of an ordinary lunar month. The innermost of these bodies, revolving as
it does in less than two days, presents a striking series of ceaseless
and rapid changes, and it becomes eclipsed during every revolution. The
distance from the centre of Jupiter to the orbit of the innermost of
these four attendants is a quarter of a million miles, while the radius
of the outermost is a little more than a million miles. The second of
the satellites proceeding outwards from the planet is almost the same
size as our moon; the other three bodies are larger; the third being the
greatest of all (about 3,560 miles in diameter). Owing to the minuteness
of the satellites as seen from the earth, it is extremely difficult to
perceive any markings on their surfaces, but the few observations made
seem to indicate that the satellites (like our moon) always turn the
same face towards their primary. Professor Barnard has, with the great
Lick refractor, seen a white equatorial belt on the first satellite,
while its poles were very dark. Mr. Douglass, observing with Mr.
Lowell's great refractor, has also reported certain streaky markings on
the third satellite.
A very interesting astronomical discovery was that made by Professor
E.E. Barnard in 1892. He detected with the 36-inch Lick refractor an
extremely minute fifth satellite to Jupiter at a distance of 112,400
miles, and revolving in a period of 11 hrs. 57 min. 22.6 secs. It can
only be seen with the most powerful telescopes.
The eclipses of Jupiter's satellites had been observed for many years,
and the times of their occurrence had been recorded. At length it was
perceived that a certain order reigned among the eclipses of these
bodies, as among all other astronomical phenomena. When once the laws
according to which the eclipses recurred had been perceived, the usual
consequence followed. It became possible to foretell the time at which
the eclipses would occur in future. Predictions were accordingly made,
and it was found that they were approximately verified. Further
improvements in the calculations were then perfected, and it was sought
to predict the times with still greater accuracy. But when it came to
naming the actual minute at which the eclipse should occur, expectations
were not always realised. Sometimes the eclipse was five or ten minutes
too soon. Sometimes it was five or ten minutes too late. Discrepancies
of this kind always demand attention. It is, indeed, by the right use of
them that discoveries are often made, and one of the most interesting
examples is that now before us.
The irregularity in the occurrence of the eclipses was at length
perceived to observe certain rules. It was noticed that when the earth
was near to Jupiter the eclipse generally occurred before the predicted
time; while when the earth happened to be at the side of its orbit away
from Jupiter, the eclipse occurred after the predicted time. Once this
was proved, the great discovery was quickly made by Roemer, a Danish
astronomer, in 1675. When the satellite enters the shadow, its light
gradually decreases until it disappears. It is the last ray of light
from the eclipsed satellite that gives the time of the eclipse; but that
ray of light has to travel from the satellite to the earth, and enter
our telescope, before we can note the occurrence. It used to be thought
that light travelled instantaneously, so that the moment the eclipse
occurred was assumed to be the moment when the eclipse was seen in the
telescope. This was now perceived to be incorrect. It was found that
light took time to travel. When the earth was comparatively near Jupiter
the light had only a short journey, the intelligence of the eclipse
arrived quickly, and the eclipse was seen sooner than the calculations
indicated. When the earth occupied a position far from Jupiter, the
light had a longer journey, and took more than the average time, so that
the eclipse was later than the prediction. This simple explanation
removed the difficulty attending the predictions of the eclipses of the
satellites. But the discovery had a significance far more momentous. We
learned from it that light had a measurable velocity, which, according
to recent researches, amounts to 186,300 miles per second.
One of the most celebrated attempts to ascertain the distance of the sun
is derived from a combination of experiments on the velocity of light
with astronomical measurements. This is a method of considerable
refinement and interest, and although it does not so fulfil all the
necessary conditions as to make it perfectly satisfactory, yet it is
impossible to avoid some reference to it here. Notwithstanding that the
velocity of light is so stupendous, it has been found possible to
measure that velocity by actual trial. This is one of the most delicate
experimental researches that have ever been undertaken. If it be
difficult to measure the speed of a rifle bullet, what shall we say of
the speed of a ray of light, which is nearly a million times as great?
How shall we devise an apparatus subtle enough to determine the velocity
which would girdle the earth at the equator no less than seven times in
a single second of time? Ordinary contrivances for measurement are here
futile; we have to devise an instrument of a wholly different character.
In the attempt to discover the speed of a moving body we first mark out
a certain distance, and then measure the time which the body requires to
traverse that distance. We determine the velocity of a railway train by
the time it takes to pass from one mile-post to the next. We learn the
speed of a rifle bullet by an ingenious contrivance really founded on
the same principle. The greater the velocity, the more desirable is it
that the distance traversed during the experiment shall be as large as
possible. In dealing with the measurement of the velocity of light, we
therefore choose for our measured distance the greatest length that may
be convenient. It is, however, necessary that the two ends of the line
shall be visible from each other. A hill a mile or two away will form a
suitable site for the distant station, and the distance of the selected
point on the hill from the observer must be carefully measured.
The problem is now easily stated. A ray of light is to be sent from the
observer to the distant station, and the time occupied by that ray in
the journey is to be measured. We may suppose that the observer, by a
suitable contrivance, has arranged a lantern from which a thin ray of
light issues. Let us assume that this travels all the way to the distant
station, and there falls upon the surface of a reflecting mirror.
Instantly it will be diverted by reflection into a new direction
depending upon the inclination of the mirror. By suitable adjustment the
latter can be so placed that the light shall fall perpendicularly upon
it, in which case the ray will of course return along the direction in
which it came. Let the mirror be fixed in this position throughout the
course of the experiments. It follows that a ray of light starting from
the lantern will be returned to the lantern after it has made the
journey to the distant station and back again. Imagine, then, a little
shutter placed in front of the lantern. We open the shutter, the ray
streams forth to the remote reflector, and back again through the
opening. But now, after having allowed the ray to pass through the
shutter, suppose we try and close it before the ray has had time to get
back again. What fingers could be nimble enough to do this? Even if the
distant station were ten miles away, so that the light had a journey of
ten miles in going to the mirror and ten miles in coming back, yet the
whole course would be accomplished in about the nine-thousandth part of
a second--a period so short that even were it a thousand times as long
it would hardly enable manual dexterity to close the aperture. Yet a
shutter can be constructed which shall be sufficiently delicate for the
purpose.
[Illustration: Fig. 63.--Mode of Measuring the Velocity of Light.]
The principle of this beautiful method will be sufficiently obvious from
the diagram on this page (Fig. 63), which has been taken from Newcomb's
"Popular Astronomy." The figure exhibits the lantern and the observer,
and a large wheel with projecting teeth. Each tooth as it passes round
eclipses the beam of light emerging from the lantern, and also the eye,
which is of course directed to the mirror at the distant station. In the
position of the wheel here shown the ray from the lantern will pass to
the mirror and back so as to be visible to the eye; but if the wheel be
rotating, it may so happen that the beam after leaving the lantern will
not have time to return before the next tooth of the wheel comes in
front of the eye and screens it. If the wheel be urged still faster, the
next tooth may have passed the eye, so that the ray again becomes
visible. The speed at which the wheel is rotating can be measured. We
can thus determine the time taken by one of the teeth to pass in front
of the eye; we have accordingly a measure of the time occupied by the
ray of light in the double journey, and hence we have a measurement of
the velocity of light.
It thus appears that we can tell the velocity of light either by the
observations of Jupiter's satellites or by experimental enquiry. If we
take the latter method, then we are entitled to deduce remarkable
astronomical consequences. We can, in fact, employ this method for
solving that great problem so often referred to--the distance from the
earth to the sun--though it cannot compete in accuracy with some of the
other methods.
The dimensions of the solar system are so considerable that a sunbeam
requires an appreciable interval of time to span the abyss which
separates the earth from the sun. Eight minutes is approximately the
duration of the journey, so that at any moment we see the sun as it
appeared eight minutes earlier to an observer in its immediate
neighbourhood. In fact, if the sun were to be suddenly blotted out it
would still be seen shining brilliantly for eight minutes after it had
really disappeared. We can determine this period from the eclipses of
Jupiter's satellites.
So long as the satellite is shining it radiates a stream of light across
the vast space between Jupiter and the earth. When the eclipse has
commenced, the little orb is no longer luminous, but there is,
nevertheless, a long stream of light on its way, and until all this has
poured into our telescopes we still see the satellite shining as before.
If we could calculate the moment when the eclipse really took place, and
if we could observe the moment at which the eclipse is seen, the
difference between the two gives the time which the light occupies on
the journey. This can be found with some accuracy; and, as we already
know the velocity of light, we can ascertain the distance of Jupiter
from the earth; and hence deduce the scale of the solar system. It must,
however, be remarked that at both extremities of the process there are
characteristic sources of uncertainty. The occurrence of the eclipse is
not an instantaneous phenomenon. The satellite is large enough to
require an appreciable time in crossing the boundary which defines the
shadow, so that the observation of an eclipse cannot be sufficiently
precise to form the basis of an important and accurate measurement.[23]
Still greater difficulties accompany the attempt to define the true
moment of the occurrence of the eclipse as it would be seen by an
observer in the vicinity of the satellite. For this we should require a
far more perfect theory of the movements of Jupiter's satellites than is
at present attainable. This method of finding the sun's distance holds
out no prospect of a result accurate to the one-thousandth part of its
amount, and we may discard it, inasmuch as the other methods available
seem to admit of much higher accuracy.
The four chief satellites of Jupiter have special interest for the
mathematician, who finds in them a most striking instance of the
universality of the law of gravitation. These bodies are, of course,
mainly controlled in their movements by the attraction of the great
planet; but they also attract each other, and certain curious
consequences are the result.
The mean motion of the first satellite in each day about the centre of
Jupiter is 203 deg..4890. That of the second is 101 deg..3748, and that of the
third is 50 deg..3177. These quantities are so related that the following
law will be found to be observed:
The mean motion of the first satellite added to twice the mean motion of
the third is exactly equal to three times the mean motion of the second.
There is another law of an analogous character, which is thus expressed
(the mean longitude being the angle between a fixed line and the radius
to the mean place of the satellite): If to the mean longitude of the
first satellite we add twice the mean longitude of the third, and
subtract three times the mean longitude of the second, the difference is
always 180 deg..
It was from observation that these principles were first discovered.
Laplace, however, showed that if the satellites revolved nearly in this
way, then their mutual perturbations, in accordance with the law of
gravitation, would preserve them in this relative position for ever.
We shall conclude with the remark, that the discovery of Jupiter's
satellites afforded the great confirmation of the Copernican theory.
Copernicus had asked the world to believe that our sun was a great
globe, and that the earth and all the other planets were small bodies
revolving round the great one. This doctrine, so repugnant to the
theories previously held, and to the immediate evidence of our senses,
could only be established by a refined course of reasoning. The
discovery of Jupiter's satellites was very opportune. Here we had an
exquisite ocular demonstration of a system, though, of course, on a much
smaller scale, precisely identical with that which Copernicus had
proposed. The astronomer who had watched Jupiter's moons circling around
their primary, who had noticed their eclipses and all the interesting
phenomena attendant on them, saw before his eyes, in a manner wholly
unmistakable, that the great planet controlled these small bodies, and
forced them to revolve around him, and thus exhibited a miniature of the
great solar system itself. "As in the case of the spots on the sun,
Galileo's announcement of this discovery was received with incredulity
by those philosophers of the day who believed that everything in nature
was described in the writings of Aristotle. One eminent astronomer,
Clavius, said that to see the satellites one must have a telescope which
would produce them; but he changed his mind as soon as he saw them
himself. Another philosopher, more prudent, refused to put his eye to
the telescope lest he should see them and be convinced. He died shortly
afterwards. 'I hope,' said the caustic Galileo, 'that he saw them while
on his way to heaven'"[24]