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Updated: May 4, 2025
This produces the phenomenon that is called libration, the result of which is that, along the border between the day and night hemispheres there is a narrow strip where the sun rises and sets once in each of her years, which are about two hundred and twenty-five of our days in length.
Venus's orbit being so nearly circular, and her orbital motion so nearly invariable, she has but a very slight libration with reference to the sun, and the east and west lunes on her surface, where day and night would alternate once in her year of 225 days, would be so narrow as to be practically negligible.
And they go away delighted with the comparison. Thus, then, the moon always presents the same face to the earth; still, to be quite exact, it should be added that in consequence of certain fluctuations from north to south and from west to east, called libration, she shows rather more than the half of her disc, about 0.57.
The reason for taking the images with so long an interval between is, that although each one represents the same object, each must be taken at a different angle; and for an object so distant as the moon, the difference caused by the libration would, it is believed, be sufficient for the desired result.
Galileo loses his favourite Daughter He falls into a state of melancholy and ill health Is allowed to go to Florence for its recovery in 1638 But is prevented from leaving his House or receiving his Friends His friend Castelli permitted to visit him in the presence of an Officer of the Inquisition He composes his celebrated Dialogues on Local Motion Discovers the Moon's Libration Loses the sight of one Eye The other Eye attacked by the same Disease Is struck blind Negociates with the Dutch Government respecting his Method of finding the Longitude He is allowed free intercourse with his Friends His Illness and Death in 1642 His Epitaph His Social, Moral, and Scientific Character.
The percolations from above are heated to steam by stones rich in 'grough brimstone. Here it was that Humboldt saw apparent lateral shiftings and perpendicular oscillations of fixed stars; and our Admiralty, not wishing to be behind him, directed Professor P. Smyth's attention to 'scintillations in general. Only the youngest of travellers would use such a place as an observatory; and only the youngest of observers would have considered this libration of the stars an extraordinary phenomenon.
Although Galileo had for a long time abandoned his astronomical studies, yet his attention was directed, about the year 1636, to a curious appearance in the lunar disc, which is known by the name of the moon's libration. When we examine with a telescope the outline of the moon, we observe that certain parts of her disc, which are seen at one time, are invisible at another.
Notwithstanding the libration of the stars,* which we had observed towards the east, we could not attribute the slowness of the rising to an extraordinary refraction of the rays occasioned by the horizon of the sea; for it is precisely at the rising of the sun, as Le Gentil daily observed at Pondicherry, and as I have several times remarked at Cumana, that the horizon sinks, on account of the elevation of temperature in the stratum of the air which lies immediately over the surface of the ocean.
It is noteworthy that from the outset De la Rue recognised the value of stereoscopic vision, which is now known to be of supreme accuracy. In 1853 he combined pairs of photographs of the moon in the same phase, but under different conditions regarding libration, showing the moon from slightly different points of view.
He amuses his old age with the studies of his youth and manhood, and writes dialogues on Motion, and even discovers the phenomena of the moon's libration; and by means of the pendulum he gives additional importance to astronomical science. But he is not allowed to leave his retirement, not even to visit his friends in Florence.
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