Rotation (astronomy)

“ Until rotation was fully instituted, the areolar law could have no power over the multitudinous movements going on in the nebula, but from that time it would begin to act, and condensation would increase it at the region where it began; and as all increase had to be propagated from there, inwards, outwards, and in all directions, the differences in velocity of rotation throughout the sun must endure as long as he continues to contract. In this we find an immense field for producing heat in the sun, from the eternal churning which must be going on in the interior. ”

“ As it journeys about the sun, it keeps turning round and round continually as if pivoted upon a mighty knitting needle transfixing it from North Pole to South Pole. In reality, however, there is no such material axis to regulate the constant direction of the rotation, just as there are no actual supports to uphold the earth itself in space. The causes which keep the celestial spheres poised, and which control their motions, are far more wonderful than any mechanical device. ”

“ In observational astronomy the earth's rotation serves as a timekeeper, and thus a retardation of terrestrial rotation will appear astronomically as an acceleration of the motion of the heavenly bodies. It is only in the case of the moon's motion that such an apparent acceleration can be possibly detected. Now, as Laplace first pointed out, there must be a slow variation in the moon's mean motion arising from the secular changes in the eccentricity of the earth's orbit around the sun. ”

“ Outlines of Astronomy, Herschel.
Art. 92. Cause of Rotation of the Earth on its Axis.--If there is one fact true in relation to the earth as a planet, it is that the earth rotates on its axis every 24 hours. Day in and day out, for centuries past, this revolution has taken place as the earth journeys in its annual path round the sun.
Not only does the earth rotate on its axis, but every other planet rotates on its axis in varying times, as the following table shows--
Further, the sun also rotates on its axis in a period of 26 days. ”

“ All the bodies which circle around a planet having been thus formed from rings which its atmosphere successively abandoned as rotation became more and more rapid, this rotation should take place in less time than is required for the orbital revolution of any of the bodies which have been cast off, and this holds true for the sun as compared with the planets. ”

“ The rotation of the earth about its axis will speedily carry the telescope away from the star, but if the driving clock be started, its effect is to turn the telescope toward the west just as fast as the earth's rotation carries it toward the east, and by these compensating motions to keep it directed toward the star. ”

“ But, while turning upon itself, the Earth circulates upon its orbit, and at the end of a diurnal rotation it is still obliged to turn during three minutes, fifty-six seconds in order to present exactly the same meridian to the fixed Sun which, in consequence of the rotary period of our planet, is a little behind. The solar day is thus one of twenty-four hours. There are 366 rotations in the year.
And now let us come back to the consequences of the Earth's motion. In the first place our planet does not turn vertically nor on its side, but is tipped or inclined a certain quantity: 23° 27′. ”

“ The problem of determining the changes is, however, simpler than others in perturbations. The method is again that of the variation of elements. The position and velocity being given in all three co-ordinates, a certain osculating plane is determined for each instant in which the planet is moving at that instant. This plane remains invariable so long as no third body acts; when it does act the position of the plane changes very slowly, continually rotating round the radius vector of the planet as an instantaneous axis of rotation. ”

“ For our sun has rotated once in 25 days at its equator, and it has been calculated that it would need to reach a rotatory speed of once in one hour before its centrifugal force would be great enough to divide it, to break it up. And because of that eon-long slowness of a star's natural increase of rotatory speed, there seemed, indeed, no slightest peril of our own sun dividing or breaking up thus, because before it could reach that speed of rotation required, unthinkable ages must elapse. ”

“ Now, it is manifest that this rotation cannot be that of the medium as a whole about an axis, for the magnetic field may be of any breadth, and there is no evidence of any motion the velocity of which increases with the distance from a single fixed line in the field. If there is any motion of rotation, it must be a rotation of very small portions of the medium each about its own axis, so that the medium must be broken up into a number of molecular vortices. ”

“ At last, when the planet had become consolidated, and when consequently the mutual distances of the several 332particles constituting the planet had been reduced to but a fraction of what those distances were originally, the speed of the planet’s rotation had become enormously increased. In this manner we learn how, from the very slow rotation which the nebulous material had at first, a solid planet may be made to rotate on its axis as rapidly as the planets in the solar system do to-day. ”

“ It can scarcely be doubted that the flattened form of the system is due to rapid rotation, and indeed there is direct evidence of strong rotational velocity; but it is one of the unexplained mysteries of evolution that nearly all celestial bodies have come to be endowed with fast rotation. ”

“ With each passing day the sun's rate of rotation is growing greater by the same amount, with each passing day it is lessening its rotatory period by 4 hours. And that steady increase of rotation of the sun, if it continues, spells destruction for the sun as we know it! All know that the sun in rotating generates in its own mass a certain amount of centrifugal force, force which tends to break up its mass. That force is not large enough, however, in our own sun to affect its great mass, since our sun's speed of rotation is not great. ”

“ But this circumstance must greatly affect the rotation of 269 the resulting planet. In a decidedly hoop-shaped nebulous ring, the differences of velocity between the inner and outer surfaces will be very small; and such a ring, aggregating into a mass whose greatest diameter is at right angles to the plane of the orbit, will almost certainly give to this mass a predominant tendency to rotate in a direction at right angles to the plane of the orbit. ”

“ Now, in the case of a planet rotating rapidly on its axis, it is clear that if the observer is situated in, or nearly in, the plane of the planet's equator, one edge of its disk will be approaching his eye while the opposite edge is retreating, and the lines in the spectrum of a beam of light from the advancing edge will be shifted toward the blue, while those in the spectrum of the light coming from the retreating edge will be shifted toward the red. And, by carefully noting the amount of the shifting, the velocity of the planet's rotation can be computed. ”

“ If the planet, in a more primitive state, existed in the form of a ring revolving round the Sun, having its present orbit for that of the centre of gravity of the ring, the momentum of rotation must, by virtue of the principle of conservation of movement, have existed in some form in the ring. It is easy to perceive that this momentum is precisely the amount which must be distributed among the particles of the ring, in order to preserve to all the condition of dynamical equilibrium, while those of each generating surface of the ring were wheeling round with the same angular velocity. ”

“ Taking into account the increasing oblateness of the original spheroid in the successive stages of its concentration, and the different proportions of the detached rings, it seems to us that the respective rotatory motions are not at variance with the hypothesis.
Not only the directions, but also the velocities of rotation are thus explicable. It might naturally be supposed that the large planets would revolve on their axes more slowly than the small ones: our terrestrial experiences incline us to expect this. ”

“ Its revolution with the earth will tend to make it change its angular position, and therefore it places itself parallel to the earth's axis; when in this position the daily rotation no longer causes any change in its direction in space, so it continues to point to the pole star (Fig. 48) . It would be an interesting experiment to measure with a delicate chemical balance the force with which the axis raises itself, and in this way weigh the rotational motion of the earth. ”

“ For the selection of the centre of rotation in a system which includes several bodies is arbitrary. The Moon turns all her faces successively to the Sun, and therefore with regard to the Sun, she does rotate on her axis; and yet she revolves round the Sun as truly as she revolves round the Earth. And the only really simple and consistent mode of speaking of rotation, is to refer the motion not to any relative centre, but to absolute space. ”

“ Indeed, as the denser parts of the planet necessarily lie in its interior, and have therefore neither the velocity nor the radius of the more superficial portions, it seems necessary to admit that the moments of momentum of the planets will be proportional to some lower power of the radius than the fifth. The total moment of momentum of the planets by rotation, when multiplied by a constant factor, and added to the revolutional moment of momentum, will remain absolutely constant. ”

“ This case affords an example of stable motion, but I have adduced it principally with the object of illustrating another point not immediately connected with stability, but important to a proper comprehension of the theory of stability.
The motion of a satellite about its planet is one of revolution or rotation. When the satellite moves in an ellipse of any given degree of eccentricity, there is a certain amount of rotation in the system, technically called rotational momentum, and it is always the same at every part of the orbit. ”

“ I had calculated that while directing through the main bar a current of apergy sufficient to keep the Astronaut at a fixed elevation, I could easily spare for the eastward conductor sufficient force to create in the space of one hour the impulse required, but that in the course of that hour the gradually increasing apergic force would drive me 500 miles westward. Now in six hours the Earth's rotation would carry an object close to its surface through an angle of 90° ”

“ This last supposition implies that, calling the two bodies A and B for brevity, the motion of the centre of inertia of B relatively to A must, when the distance between them was great, have been directed with great exactness to pass through the centre of inertia of A; such great exactness that the rotational momentum or moment of momentum after collision was no more than to let the sun have his present slow rotation when shrunk to his present dimensions. ”

“ To speak of the planet’s axis of rotation is, in one sense, another assumption, because available observations can scarcely be said to enable us to demonstrate that the planet does rotate on its axis, yet we can have no moral doubt about it. ”

“ If a star is rotating, one edge or ‘limb’ is coming towards us and the other going away from us. We can measure speeds towards us or away from us by means of the Doppler effect on the spectrum, obtaining a definite result in miles per second. Thus we can and do measure the equatorial speed of rotation of the sun by observing first the east limb then the west limb and taking the difference of velocity shown. That is all very well on the sun, where you can cover up the disk except the special part that you want to observe ”

“ Here, then, are certain phenomena in connection with the solar system, for which up to the present no explanation as to the physical cause of rotation has ever been offered. Surely there is some physical cause, to account for such a rotation, and if there be a physical cause, then the problem to be solved is--find the physical cause to account for the continuous and ever-recurring rotation of all the planets and the sun on their axes, which shall be so effective and continual that, year in and year out, the rotation of all the planets may be continued as observed. ”