The boundaries of the solar system had been much extended by the
discovery of Uranus, but they were destined to receive still further
enlargement by the detection of another vast planet, revolving far
outside Uranus, the orbit of which forms, according to our present
knowledge, the outline of the planetary system.
I have here to describe one of the greatest discoveries that have ever
been made. It is not the magnificence of the outermost planet itself
that I refer to, though, indeed, it is bigger than Uranus. I am rather
thinking of the _way_ in which the discovery was made. I do not mean
any disrespect to Herschel when I say that the discovery of Uranus was
chiefly a stroke of good fortune; but I may be permitted to describe
it in this manner by way of emphasizing as strongly as I can how
utterly different was the train of ideas which led to the discovery of
Neptune. Herschel merely looked at one star after another till suddenly
he dropped on the planet, having beforehand not the slightest notion
that any such planet was likely to exist. But Neptune was shown to
exist before it was ever seen, and, in fact, the man that first saw
the planet, and knew it to be a planet, was not the discoverer. This
is rather a difficult subject; and it would take you years of hard
study to be able to understand the train of reasoning by which Neptune
was found. I shall, however, make an attempt to explain this matter
sufficiently to give at least some idea of the kind of problem that had
to be solved.
You will remember that law of Kepler which tells us that every planet
moves round the sun in an ellipse. If the planet be uninterfered
with in any way and guided only by the attraction of the sun, it
will forever continue to describe precisely the same ellipse without
the slightest alteration. It was ascertained that the path which
Uranus followed was not always regular. The early observations of the
planet, when it was mistaken for a star, have here been of the utmost
service. They have indicated the ellipse which Uranus described the
last time it went round, and our modern observations have taught us
the path which the planet is at present describing. These two ellipses
are slightly different, and the consequence is that, supposing we
take the observations of Uranus made 100 years ago, and calculate
from them where Uranus ought to be now, we find that the planet is
a little astray. Astronomers are not accustomed to be wrong in such
calculations, and when discrepancies arise, the first thing to be
done is to see what has caused them. It is certain that the position
in which Uranus is found this very night, for example, is not what it
would have been had the sun alone been guiding the planet. Perhaps you
will think that it is impossible for reliable computations to be made
about such matters; but I assure you they can, and the very fact that
the motion of Uranus appeared to be irregular made it interesting to
try and find out the cause of the disturbance.
I have already explained, when speaking about Mars (p. 187), that
there is an attraction between every two bodies, but in the group of
planets to which the earth belongs the sun’s attraction is so much
stronger than any other force that all the movements are guided mainly
by it. Nevertheless it is true that not only does the sun pull our
earth and all the other planets as well, but all the planets, including
the earth, are pulling one another. In fact, there is an incessant
struggle going on in the family party. Fortunately the sun is so much
more powerful than any other member, that he keeps them all pretty
well in order; and unless you look very carefully you will not see the
effects of the little struggles that are going on between every pair
of the system. Our earth itself is pulled and swayed to and fro by the
actions of its brothers and sisters. It is dragged perhaps a thousand
or two thousand miles this way by Jupiter, or it gets a good tug in the
other direction by Venus. Mars and Saturn also do their little best
to force the earth away from its strict path. However, our earth does
not suffer much from these irregularities. It pursues its route fairly
enough, just as a coach from London to Brighton will get safely to its
destination notwithstanding the fact that it has to swerve a little
from its path whenever it meets other vehicles on the way, or when the
coachman wishes to avoid a piece of the road on which stones have been
freshly laid down.
The track followed by Uranus was found to be somewhat irregular,
like that of every other planet. Jupiter gave it a pull, and so did
Saturn, and at first it was thought that the irregularities which were
perceived could be explained by the action of these planets, so big and
so well known. Here is a question for calculation; it involves a very
long and a very hard piece of work, but it is possible to estimate how
far each of the other planets is capable of dragging Uranus from its
path. Is it not remarkable that by working out long calculations we
should be able to find what one planet hundreds of millions of miles
away was able to do to another planet still further off, and not only
for to-day or yesterday, but for past time extending over more than a
century? If, however, you will listen to me a little longer, I think I
shall give you a proof that these sums could be worked out correctly.
When the calculations had been made which showed how much the known
planets could disturb Uranus, it was found that there were still some
deviations of the planet that remained unexplained. They were not
large; they only amounted to showing that the body was just a little
astray from the spot where the calculations indicated it should be. The
rest of astronomy was so perfect, and the law of attraction prevailed
so universally, that it was thought the law of attraction must provide
some way of explaining the behavior of Uranus. He could not have left
his track of his own accord; therefore there must be some agency at
work upon him of which we did not know. What could this unknown source
of disturbance be? Every such trouble had hitherto been found to be a
consequence of the attraction of gravitation; therefore there must be
some unknown body pulling at Uranus which no one had ever seen. Where
could it be? How was it to be discovered? Such were the questions that
were asked, and they were answered in a most satisfactory manner.
First of all, what sort of body could it be that was pulling Uranus?
It is obvious that none of the stars would be competent to produce
so great an effect; they are all so far off that they have nothing
whatever to say to any of the domestic matters in our little solar
system, which is simply a group by itself. It would be more reasonable
to suppose that there must be yet another planet which nobody had ever
recognized, but which affected Uranus so as to account for his truant
behavior. To begin to search for this planet with telescopes without
some guidance would be futile; in fact, astronomers had been scanning
the heavens for planets for nearly fifty years, and though several
had been discovered, they all belonged to the zone of little planets,
and none of them were big enough to pull Uranus about appreciably. Of
course, if all the stars could be blotted out of the sky, so that
nothing but planets were left, then, by sweeping the telescope over
the heavens, every planet that exists might be speedily found. The
difficulty is that the planets, which are either small or very distant,
look so like the stars that it is impossible to recognize them among
the millions of glittering points in the sky. It was, however, hoped
that the unknown planet would be large enough to be visible in the
telescope, if only we knew exactly where to point it.
Two illustrious astronomers, Adams of Cambridge, and Leverrier of
Paris, both separately undertook an astonishing piece of calculation.
They tried to find out the position of the unknown planet from the
mere fact that it deranged Uranus in a particular way. I dare say many
of those who are reading this book have learned simple equations in
algebra, and they have worked such questions as to find the length
of a pole, half of which is in mud, a quarter in water, and ten
feet above the water. Those who know this much can perhaps realize
the problem that had to be solved in trying to discover the unknown
planet. So difficult a question as this had to be solved in a way that
your masters would hardly allow you to use when working your sums in
algebra. I do not think they would let you make a series of guesses.
Let us try 20 feet, for instance, as the length of the pole; that will
make 10 feet in the mud, 5 feet in the water, and 5 feet outside. This
will not do; it is not enough; we must try again; and after another
guess or two, we see that a pole 40 feet long will exactly answer. We
do not use this method of guessing in algebra, because solving the
simple equation is a much better method. Adams and Leverrier found
that to discover the unknown planet was a question so very difficult,
that they were obliged to use a sort of guessing, but very intelligent
guessing, I need hardly assure you. They proceeded in this way
(Fig. 71). They would draw a circle outside the path of Uranus, and
then suppose that a planet was revolving in that circle. Its effect
upon Uranus would then be calculated, and it would be found whether
the observed irregularities could be in this manner accounted for. The
first planet they tried was not the right one; then they began again
with another, until at last, after many trials and much very hard work,
they saw that there might be a planet in a particular path far outside
Uranus, such that if this planet were of the right weight and moving
with the right speed, then it would pull Uranus exactly in the way
that astronomers had observed it to be pulled. They found at last that
there could be little doubt about the matter; for this unknown body
would account for all the facts. Then, indeed, they had solved their
equation; they had found the unknown.
[Illustration: FIG. 71.--Orbits of Uranus and Neptune.]
The two great astronomers had thus discovered a planet, but as yet it
was only a planet on paper. Those who could judge of the subject had
no doubt that the planet was really in the sky; but just as you like
to prove that you have found the correct answer to your sum, so people
were naturally anxious to prove the truth of this wonderful sum that
Adams and Leverrier had worked out. This was to be done by actually
seeing the planet of which the astronomers had asserted the existence.
Leverrier calculated that the new planet in a certain night would be in
a particular position on the sky. Accordingly he wrote to Dr. Galle, of
the observatory at Berlin, requesting him on the evening in question
to point his telescope to the very spot indicated, and there he would
see a planet which human eyes had never before beheld. Of course, Dr.
Galle was only too delighted to undertake so marvellous a commission.
The evening was fine; the telescope was opened; it was directed towards
the heavens; and there, in the very spot which the calculations
of Leverrier had indicated, shone the beautiful little planet. At
Cambridge arrangements had also been made to search for the new member
of the solar system, in accordance with Professor Adams’ calculations.
There also the planet that had given all this trouble to Uranus was
brought to light. At first it looked like a star, as all such planets
do; but that it was not a star was speedily proved, by the two tests
which are sure indications of a planet. First the body was so moving
that its position with respect to the adjacent stars was constantly
changing. Then, when a strong magnifying power was placed on the
telescope, the little object was seen, not to be a mere starlike point,
but to expand into the little disk which shows us we are not looking at
a distant sun, but at a world like our own.
Was not this truly a great discovery? Have we not shown you how
entitled the calculations of astronomers are to our respect, when we
find that they actually discovered the existence of a majestic planet
before the telescope had revealed it? See also the greatly increased
interest that belongs to Herschel’s discovery of Uranus. We can hardly
imagine anything that would have given more gratification to this old
astronomer than to think that his Uranus should have given rise to a
discovery even more splendid than his own. He died, however, more than
twenty years before this achievement.
The authorities who decide on such matters christened the new planet
Neptune; and this body wanders round on the outskirts of our solar
system, requiring for each journey a period of no less than 165 years.
The circle thus described has a radius thirty times as great as that of
the earth’s track.
Neptune is altogether invisible to the unaided eye, but it is
sufficiently bright to have been occasionally recorded as a star.
Indeed, nearly fifty years before it was actually discovered to be a
planet it had been included by the astronomer Lalande in a list of
stars he was observing. A curious circumstance was afterwards brought
to light. When reference was made to the books in which Lalande’s
observations were written, it was found that he had observed this
object twice, namely, on May 8 and May 10, 1785. Of course, if the
object had indeed been a star its position on the two days would have
been the same, but being a planet it had moved. When Lalande, on
looking over his papers, saw that the places of this supposed star
were different on the two nights, he concluded that he must have made
a mistake on the first night, and accordingly treated the object as
if the place on the 10th was the right one. Just think how narrowly
Lalande missed making a discovery! Unhappily for his renown, he took it
for granted that one or both of his observations were erroneous, and so
they must have been if the object had been a star. But they were both
right; it was the planet which had moved in the interval.
As Neptune is half as far again from the earth as Uranus, we can hardly
expect to learn much about the actual nature of the planet. We do know
that it has four times the diameter of the earth, so that it exceeds
the earth in the same proportion that the earth is larger than the moon.
Like the other great planets, Neptune is also enveloped with copious
clouds; in fact, it only weighs one-fifth part as much as it would do
if it were made of materials as substantial as are those of the earth.
Like our earth, Neptune is attended by one moon, which revolves round
the planet in a little more than six days.
The orbit of this great planet marks the boundary of our known system
of planets. We have seen how the five great planets of antiquity have
been increased in these modern days by the addition of two more, Uranus
and Neptune, while the discovery of a multitude of small planets has
given a further increase to the number of the sun’s family. We have
still some other objects in our solar system to describe; some of them
are excessively big; these are the comets. Some of them are exceedingly
small; they are the shooting stars. We shall talk about comets and
shooting stars in our next lecture.