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The last Of Galileo's great astronomical discoveries related to the libration of the moon. I think that the detection of this phenomenon shows his acuteness of observation more remarkably than does any one of his other achievements with the telescope. It is well known that the moon constantly keeps the same face turned towards the earth.
From the observation at Bologna in 1666-67 of some very faint spots, Domenico Cassini concluded a rotation or libration of Venus he was not sure which in about twenty-three hours. By Bianchini in 1726 the period was augmented to twenty-four days eight hours.
There were besides two other numerical coincidences quite as extraordinary; an identity of direction, relative to the stars, of the equator and orbit of the moon; exactly the same precessional movements of these two planes. This group of singular phenomena, discovered by J.D. Cassini, constituted the mathematical code of what is called the Libration of the Moon.
From all these experiments it appears, that the spectra in the eye are not owing to the mechanical impulse of light impressed on the retina, nor to its chemical combination with that organ, nor to the absorption and emission of light, as is observed in many bodies; for in all these cases the spectra must either remain uniformly, or gradually diminish; and neither their alternate pretence and evanescence as in the first experiment, nor the perpetual changes of their colours as in the second, nor the flash of light or colours in the pressed eye as in the third, nor the rotation or libration of the spectra as in the fourth, could exist.
Dr. Klein has frequently seen this rill with great distinctness, and at other times sought for it in vain; though on each occasion the conditions of illumination, libration, and definition were practically similar. I have sometimes found this cleft an easy object with a 4 inch achromatic.
It has lofty walls and a central mountain. HECATAEUS. An immense walled-plain, 115 miles in length, on the S.W. of Vendelinus, with a very irregular rampart and a conspicuous central mountain. It is flanked E. and W. by other large enclosures, which can only be seen to advantage when libration is favourable.
The first, that the perfection of government lies upon such a libration in the frame of it, that no man or men in or under it can have the interest, or, having the interest, can have the power to disturb it with sedition. The second, that monarchy, reaching the perfection of the kind, reaches not to the perfection of government, but must have some dangerous flaw in it.
This swell or libration of the sea is so prodigiously long, and the sensible effect of its height, of course, so much diminished, that it is not often attended to; the gradual slope engrossing almost the whole horizon when the eye is not very much elevated above its surface: but persons who have sailed in those parts may recollect that, even when the sea is apparently the most still and level, a boat or other object at a distance from the ship will be hidden from the sight of one looking towards it from the lower deck for the space of minutes together.
The inclination of her axis to the plane of her orbit, and her consequent libration in latitude. Her varying angular velocity, and consequent libration in longitude. The mechanism consists of a train-arm, T, which turns upon the vertical pivot, P, fixed in the stand.
In April, 1634, Galileo's afflictions were increased by the death of a favorite, intelligent, and attached daughter. He consoled his solitude, and lightened the hours of sickness, by continuing the observations which he was now forbidden to publish to the world; and the last of his long train of discoveries was the phenomenon known by the name of the moon's libration.
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