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MODERN astronomers, enjoying the advantage of excellent instruments, have not only improved all the ancient theories of the celestial motions, but have established several others of the highest importance, some of them absolutely new, and the rest merely conjectural before. Among these are to be distinguished the libration of the Moon, the aberrations of the fixed stars, the nutation of the Earth's axis, the catalogues of the fixed stars, the figure of the Earth, and the general laws of the motion of comets.
As soon as men began to contemplate the Moon, it was perceived, that it constantly presented the same face to the Earth, for it's spots were always the same, and arranged in the same manner. The ancient astronomers stopped at this general observation; but an attentive examination of the lunar spots showed Galileo, that the Moon had a libratory motion on it's centre, in consequence of which certain spots toward the edges disappeared for a time, then reappeared, and thus continued to disappear and reappear alter nately. This motion is what is called the libration of the Moon. Galileo explained it generally by a rotatory motion about an axis, which he ascribed to the Moon while at the same time it was revolving round the Earth; but he determined neither the exact posi
tion of this axis, nor the precise quantities of the libratory motions it ought to produce, both in latitude and longitude.
As the primary planets, which revolve on their axes, present to us different spots, and these appearances are even the marks, by which their rotatory motions have been discovered; des Cartes, seeing nothing similar in the Moon, maintained, that it has no rotatory motion. To account for the constant appearance of the same spots, he supposed the lunar globe to consist of two hemispheres of unequal gravity, separated by the circle perpendicular to a line drawn from the Earth to the centre of the Moon; and he concluded, that, both these hemispheres being subjected to the action of the centrifugal force, which is produced by the revolution of the Moon round the Earth, the heaviest, or most massive, having the greatest centrifugal force, must constantly keep itself farther from us. As to the libratory motion, this, according to the same author, is produced by a slight libration of the circle, that forms the base of the two hemispheres. It is unnecessary to remark how hypothetical all this explanation is. The pretended inequality in the gravity or mass of the two hemispheres is destitute of all probability; and besides, it gives only a vague and insufficient reason for the phenomena of the libration.
The celebrated Dominic Cassini, and his son James, worthy of such a father, were the first who gave a complete and accurate explanation of this motion of the Moon, which is conformable to observation, and has consequently been adopted by all astronomers.
It is given by James Cassini in the Memoirs of the Academy of Sciences at Paris for the year 1721, and in his Elements of Astronomy, published in 1740. According to him the libration of the Moon is produced by the combination of two motions, one of which is the revolution of this planet round the Earth, the other is a rotatory motion of the Moon round an axis, subject to the following conditions:
1st, The axis of rotation of the Moon is inclined to the plane of the ecliptic in an angle of 87 degrees and half, and to the plane of the lunar orbit in an angle of 82 degrees and half: so that the plane of the Moon's equator makes an angle of two degrees and a half with the plane of the ecliptic, and an angle of seven degrees and a half with the plane of the lunar orbit.
2dly, The poles of the Moon are in the circumference of the great circle, which is formed by cutting it's globe at every instant by a plane parallel to that great circle in the heavens, which passes through the poles of the ecliptic, and those of the lunar orbit. This circle may be called the colure of the Moon, for the same reason as the great circle, which passes through the poles of the ecliptic and those of the equinoctial line, is called the colure of the solstices.
3dly, The globe of the Moon turns round it's axis according to the order of the signs, or from west to east, in the space of 27 days 5 hours; a period equal to that of the Moon's return to the node of it's orbit with the ecliptic. This motion is analogous to the revolution which the Earth makes round it's axis in
the order of the signs, returning to the same colure in the space of 23 hours 56 minutes.
The general result of these suppositions is, that, if we prolong in imagination the axis of the globe of the Moon into the heavens, the extremities of this axis will appear to us, to describe round the poles of the ecliptic, from which they are two degrees and half distant, two polar circles, from east to west, in 18 years and 7 months, or in the same time and direction as the nodes of the Moon. This motion, we see, is similar to that, by which the poles of the Earth make their revolution round the poles of the ecliptic from east to west, in two circles distant from them 23 degrees and a half, and in a period of about 25000 years: which produces the apparent motion of the stars from west to east in the same time, and in consequence the precession of the equinoxes. A full explanation of the phenomena of the Moon's libration does not belong to this work, but must be sought in treatises on astronomy.
A discovery still greater, both with respect to it's difficulty, and to it's influence on all parts of astronomy, is that of the causes, which produce the apparent aberration of the fixed stars. For this we are indebted to Bradley, the english Hipparchus.
Among the reasons, that were alleged against the system of Copernicus, at the time when it was first brought forward, was this, as we have already ob served, that, if the Earth really revolved round the Sun, it ought to occasion a parallax in the stars, which was termed the parallax of the great orb, when
passes from one point of it's orbit to that diametri
cally opposite. This was a solid objection, to which Copernicus and Galileo could answer only by conjectures, that did not obtain a complete assent. Succeeding astronomers, persuaded of the existence of this parallax, employed every means of detecting it's quantity. Some imagined they had determined it; and ventured to say, that it was four or five seconds. A greater number, supported by more accurate observations, found it altogether imperceptible; and this opinion prevailed. But it did not subvert the copernican system: it only led to the conclusion, that the distance from the Earth to the stars was so immensely great, that it must be considered as infinite with regard to the diameter of the terrestrial orbit. Still certain perceptible motions observed in the stars, contrary for the most part to those, which the parallax of the great orb and the precession of the equinoxes should have produced, remained to be accounted for. These irregular motions were designated under the general denomination of the apparent aberrations of the fixed stars. Not knowing to what to ascribe them, astronomers took every precaution to avoid the errours, which they might have occasioned in determining the motions of the planets with regard to the fixed
Mr. Molyneux, an astronomer of Ireland, undertook in 1725, to determine these motions of aberration. He observed them at Kew, near London, with an excellent sector made by Graham, but he was unable to reduce them to general laws.