[ Pobierz całość w formacie PDF ]
historical fact.
The Nebular Theory gives a physical account of the origin of the solar system, consisting of
the sun in the centre, with the planets and their attendant satellites. Laplace perceived the
significance of the fact that all the planets revolved in the same direction around the sun; he
noticed also that the movements of rotation of the planets on their axes were performed in
the same direction as that in which a planet revolves around the sun; he saw that the orbits of
the satellites, so far at least as he knew them, revolved around their primaries also in the
file:///D|/Great Astronomers (Ball, R. S.)/greatastronomers2.htm (93 of 156) [03.04.2007 19:19:46]
Great Astronomers
same direction. Nor did it escape his attention that the sun itself rotated on its axis in the
same sense. His philosophical mind was led to reflect that such a remarkable unanimity in the
direction of the movements in the solar system demanded some special explanation. It would
have been in the highest degree improbable that there should have been this unanimity unless
there had been some physical reason to account for it. To appreciate the argument let us first
concentrate our attention on three particular bodies, namely the earth, the sun, and the
moon. First the earth revolves around the sun in a certain direction, and the earth also rotates
on its axis. The direction in which the earth turns in accordance with this latter movement
might have been that in which it revolves around the sun, or it might of course have been
opposite thereto. As a matter of fact the two agree. The moon in its monthly revolution
around the earth follows also the same direction, and our satellite rotates on its axis in the
same period as its monthly revolution, but in doing so is again observing this same law. We
have therefore in the earth and moon four movements, all taking place in the same direction,
and this is also identical with that in which the sun rotates once every twenty-five days. Such
a coincidence would be very unlikely unless there were some physical reason for it. Just as
unlikely would it be that in tossing a coin five heads or five tails should follow each other
consecutively. If we toss a coin five times the chances that it will turn up all heads or all tails is
but a small one. The probability of such an event is only one-sixteenth.
There are, however, in the solar system many other bodies besides the three just mentioned
which are animated by this common movement. Among them are, of course, the great
planets, Jupiter, Saturn, Mars, Venus, and Mercury, and the satellites which attend on these
planets. All these planets rotate on their axes in the same direction as they revolve around the
sun, and all their satellites revolve also in the same way. Confining our attention merely to the
earth, the sun, and the five great planets with which Laplace was acquainted, we have no
fewer than six motions of revolution and seven motions of rotation, for in the latter we include
the rotation of the sun. We have also sixteen satellites of the planets mentioned whose
revolutions round their primaries are in the same direction. The rotation of the moon on its
axis may also be reckoned, but as to the rotations of the satellites of the other planets we
cannot speak with any confidence, as they are too far off to be observed with the necessary
accuracy. We have thus thirty circular movements in the solar system connected with the sun
and moon and those great planets than which no others were known in the days of Laplace.
The significant fact is that all these thirty movements take place in the same direction. That
this should be the case without some physical reason would be just as unlikely as that in
tossing a coin thirty times it should turn up all heads or all tails every time without exception.
We can express the argument numerically. Calculation proves that such an event would not
generally happen oftener than once out of five hundred millions of trials. To a philosopher of
Laplace's penetration, who had made a special study of the theory of probabilities, it seemed
well-nigh inconceivable that there should have been such unanimity in the celestial
movements, unless there had been some adequate reason to account for it. We might, indeed,
add that if we were to include all the objects which are now known to belong to the solar
system, the argument from probability might be enormously increased in strength. To Laplace
file:///D|/Great Astronomers (Ball, R. S.)/greatastronomers2.htm (94 of 156) [03.04.2007 19:19:46]
Great Astronomers
the argument appeared so conclusive that he sought for some physical cause of the
remarkable phenomenon which the solar system presented. Thus it was that the famous
Nebular Hypothesis took its rise. Laplace devised a scheme for the origin of the sun and the
planetary system, in which it would be a necessary consequence that all the movements
should take place in the same direction as they are actually observed to do.
Let us suppose that in the beginning there was a gigantic mass of nebulous material, so highly
heated that the iron and other substances which now enter into the composition of the earth
and planets were then suspended in a state of vapour. There is nothing unreasonable in such
a supposition indeed, we know as a matter of fact that there are thousands of such nebulae to
be discerned at present through our telescopes. It would be extremely unlikely that any object
could exist without possessing some motion of rotation; we may in fact assert that for rotation
to be entirety absent from the great primeval nebula would be almost infinitely improbable. As
ages rolled on, the nebula gradually dispersed away by radiation its original stores of heat,
and, in accordance with well-known physical principles, the materials of which it was formed
would tend to coalesce. The greater part of those materials would become concentrated in a
mighty mass surrounded by outlying uncondensed vapours. There would, however, also be
regions throughout the extent of the nebula, in which subsidiary centres of condensation
would be found. In its long course of cooling, the nebula would, therefore, tend ultimately to
form a mighty central body with a number of smaller bodies disposed around it. As the nebula
was initially endowed with a movement of rotation, the central mass into which it had chiefly
condensed would also revolve, and the subsidiary bodies would be animated by movements of
revolution around the central body. These movements would be all pursued in one common
direction, and it follows, from well-known mechanical principles, that each of the subsidiary
masses, besides participating in the general revolution around the central body, would also
possess a rotation around its axis, which must likewise be performed in the same direction.
Around the subsidiary bodies other objects still smaller would be formed, just as they
themselves were formed relatively to the great central mass.
As the ages sped by, and the heat of these bodies became gradually dissipated, the various
objects would coalesce, first into molten liquid masses, and thence, at a further stage of
cooling, they would assume the appearance of solid masses, thus producing the planetary
bodies such as we now know them. The great central mass, on account of its preponderating
dimensions, would still retain, for further uncounted ages, a large quantity of its primeval heat, [ Pobierz całość w formacie PDF ]
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historical fact.
The Nebular Theory gives a physical account of the origin of the solar system, consisting of
the sun in the centre, with the planets and their attendant satellites. Laplace perceived the
significance of the fact that all the planets revolved in the same direction around the sun; he
noticed also that the movements of rotation of the planets on their axes were performed in
the same direction as that in which a planet revolves around the sun; he saw that the orbits of
the satellites, so far at least as he knew them, revolved around their primaries also in the
file:///D|/Great Astronomers (Ball, R. S.)/greatastronomers2.htm (93 of 156) [03.04.2007 19:19:46]
Great Astronomers
same direction. Nor did it escape his attention that the sun itself rotated on its axis in the
same sense. His philosophical mind was led to reflect that such a remarkable unanimity in the
direction of the movements in the solar system demanded some special explanation. It would
have been in the highest degree improbable that there should have been this unanimity unless
there had been some physical reason to account for it. To appreciate the argument let us first
concentrate our attention on three particular bodies, namely the earth, the sun, and the
moon. First the earth revolves around the sun in a certain direction, and the earth also rotates
on its axis. The direction in which the earth turns in accordance with this latter movement
might have been that in which it revolves around the sun, or it might of course have been
opposite thereto. As a matter of fact the two agree. The moon in its monthly revolution
around the earth follows also the same direction, and our satellite rotates on its axis in the
same period as its monthly revolution, but in doing so is again observing this same law. We
have therefore in the earth and moon four movements, all taking place in the same direction,
and this is also identical with that in which the sun rotates once every twenty-five days. Such
a coincidence would be very unlikely unless there were some physical reason for it. Just as
unlikely would it be that in tossing a coin five heads or five tails should follow each other
consecutively. If we toss a coin five times the chances that it will turn up all heads or all tails is
but a small one. The probability of such an event is only one-sixteenth.
There are, however, in the solar system many other bodies besides the three just mentioned
which are animated by this common movement. Among them are, of course, the great
planets, Jupiter, Saturn, Mars, Venus, and Mercury, and the satellites which attend on these
planets. All these planets rotate on their axes in the same direction as they revolve around the
sun, and all their satellites revolve also in the same way. Confining our attention merely to the
earth, the sun, and the five great planets with which Laplace was acquainted, we have no
fewer than six motions of revolution and seven motions of rotation, for in the latter we include
the rotation of the sun. We have also sixteen satellites of the planets mentioned whose
revolutions round their primaries are in the same direction. The rotation of the moon on its
axis may also be reckoned, but as to the rotations of the satellites of the other planets we
cannot speak with any confidence, as they are too far off to be observed with the necessary
accuracy. We have thus thirty circular movements in the solar system connected with the sun
and moon and those great planets than which no others were known in the days of Laplace.
The significant fact is that all these thirty movements take place in the same direction. That
this should be the case without some physical reason would be just as unlikely as that in
tossing a coin thirty times it should turn up all heads or all tails every time without exception.
We can express the argument numerically. Calculation proves that such an event would not
generally happen oftener than once out of five hundred millions of trials. To a philosopher of
Laplace's penetration, who had made a special study of the theory of probabilities, it seemed
well-nigh inconceivable that there should have been such unanimity in the celestial
movements, unless there had been some adequate reason to account for it. We might, indeed,
add that if we were to include all the objects which are now known to belong to the solar
system, the argument from probability might be enormously increased in strength. To Laplace
file:///D|/Great Astronomers (Ball, R. S.)/greatastronomers2.htm (94 of 156) [03.04.2007 19:19:46]
Great Astronomers
the argument appeared so conclusive that he sought for some physical cause of the
remarkable phenomenon which the solar system presented. Thus it was that the famous
Nebular Hypothesis took its rise. Laplace devised a scheme for the origin of the sun and the
planetary system, in which it would be a necessary consequence that all the movements
should take place in the same direction as they are actually observed to do.
Let us suppose that in the beginning there was a gigantic mass of nebulous material, so highly
heated that the iron and other substances which now enter into the composition of the earth
and planets were then suspended in a state of vapour. There is nothing unreasonable in such
a supposition indeed, we know as a matter of fact that there are thousands of such nebulae to
be discerned at present through our telescopes. It would be extremely unlikely that any object
could exist without possessing some motion of rotation; we may in fact assert that for rotation
to be entirety absent from the great primeval nebula would be almost infinitely improbable. As
ages rolled on, the nebula gradually dispersed away by radiation its original stores of heat,
and, in accordance with well-known physical principles, the materials of which it was formed
would tend to coalesce. The greater part of those materials would become concentrated in a
mighty mass surrounded by outlying uncondensed vapours. There would, however, also be
regions throughout the extent of the nebula, in which subsidiary centres of condensation
would be found. In its long course of cooling, the nebula would, therefore, tend ultimately to
form a mighty central body with a number of smaller bodies disposed around it. As the nebula
was initially endowed with a movement of rotation, the central mass into which it had chiefly
condensed would also revolve, and the subsidiary bodies would be animated by movements of
revolution around the central body. These movements would be all pursued in one common
direction, and it follows, from well-known mechanical principles, that each of the subsidiary
masses, besides participating in the general revolution around the central body, would also
possess a rotation around its axis, which must likewise be performed in the same direction.
Around the subsidiary bodies other objects still smaller would be formed, just as they
themselves were formed relatively to the great central mass.
As the ages sped by, and the heat of these bodies became gradually dissipated, the various
objects would coalesce, first into molten liquid masses, and thence, at a further stage of
cooling, they would assume the appearance of solid masses, thus producing the planetary
bodies such as we now know them. The great central mass, on account of its preponderating
dimensions, would still retain, for further uncounted ages, a large quantity of its primeval heat, [ Pobierz całość w formacie PDF ]