Other Phenomena.

Mercury will be in his superior conjunction on the 17th of this month, at half past 6 in the morning. Mars will be in conjunction with the star marked a in Leo on the 6th, at which time the star will be 40' south of the planet. The Moon will be in conjunction with a in Libra, at 30 m. past 2 in the morning of the 13th. Venus will be in conjunction with a in Leo on the 17th, when the star will be 67 south of the planet. On the 29th, Venus will be in superior conjunction, and 19′ north of the Sun.

On the SATELLITES of the PLANETS.

Reflecting upon the progress of science, and the means by which discoveries have been effected, the mind is forcibly struck with the circumstance that the most important consequences have frequently resulted from the most trivial incidents. To these, in conjunction with what Lord Bacon has denominated the fortunes or adventures of experiment,' we are indebted for a great part of the natural knowledge at present possessed by man; because, as the same great philosopher has justly observed, the wonders of Nature commonly lie out of the high road and beaten paths.'

The discovery of the satellites is traced to an incident of the simplest kind. The children of a Dutch spectacle maker, being at play with some spectacleglasses, made use of two of them together, the one convex and the other concave, in looking at the weathercock of a church, and observed that it appeared much nearer and larger than usual. The celebrated Galileo having heard of this incident, conceived the idea of applying it to astronomical purposes; and was therefore led, if not to the actual invention of the telescope, to such an improvement of it as constituted the value of the invention in a practical sense. By the aid of this instrument, the satellites were discovered; the truth of the Copernican system practically con

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firmed; and the velocity and aberration of light made known, with all the inferences and consequences resulting from these four remarkable facts.

The satellites are planets of the second order, which revolve about their primary planets as these do about the Sun, and also accompany them in their revolutions about that central luminary. The Moon is therefore a satellite of the Earth; but her vicinity and comparative importance to the inhabitants of this globe are such as to cause her movements and phenomena to merit separate consideration, several particulars of which have already been submitted to our readers. The satellites of Jupiter were the easiest to be observed, and, therefore, were the first discovered. Galileo first saw them on the 7th of January 1610, to whom they appeared as minute stars at small distances from the planet, and ranged nearly in the same right line. But he soon perceived that they not only accompanied Jupiter in his revolution about the Sun, but that they changed their positions with respect to each other. Further observations showed that their distance from the planet never exceeded certain limits, and that the maxima of these distances were always the same for the same satellite, by which it is distinguished from the others.

To arrive at a knowledge of the laws by which the movements of this class of the heavenly bodies are regulated, it is necessary to consider the phenomena they present, and compare them with those exhibited by the other branches of the solar system. On directing the telescope to the planet, the satellites are seen sometimes on the right of it, and sometimes on the left. When seen on the right, for instance, they are observed to approach the planet, then disappear behind it, and afterwards reappear on the left; on which side their greatest digression is observed to be nearly the same as it had been on the right. Then they return again to the right of the planet, and are

sometimes seen to pass over its disc. Their motion is from west to east, like that of their primaries; and ascitis apparently much slower towards the limits than in other parts of their progress, it naturally suggests the idea of its being performed in a circle, the plane of which passes nearly through the eye of the observer. When the satellites of Jupiter are in superior conjunction with respect to the Sun, they ought to pass through his shadow, and this is what is frequently observed to take place, in proceeding from their western to their eastern digression. Sometimes they pass above or below the shadow, but never over the disc of the planet ; and in returning to their former positions, those which passed through the shadow traverse the disc; and those which passed above it return below, and the contrary. This is also applicable to the satellites of Saturn, while those of Uranus are never seen to pass either through his shadow, or over his disc. Observations also show that the time between an inferior and a superior conjunction of a satellite is nearly equal to the time of its semi-revolution.

Hence it is inferred that the satellites move round their primaries in curves that return into themselves, with motions that are nearly uniform; and that the planes of their orbits are inclined to the planes of the orbits of their primaries. Astronomers contented themselves for some time with observing only the configurations of the satellites with respect to each other. They perceived that eclipses of these satellites must take place, but their telescopes were not sufficiently powerful to observe them; and the times of their revolutions were therefore determined by the period which elapsed between two of their greatest digressions on the same side of the planet, either both east or both west. But the improvement in telescopes has now rendered observations of the eclipses of the satellites the most accurate means of determining the periodic times of their revolutions.

The middle of the eclipse is sensibly the time of the heliocentric conjunction; the time elapsed between the' middles of two consecutive eclipses is also, without sensible error, the period of a synodic revolution, which is always 360° of relative motion: if to this motion, therefore, there be added the motion of the planet in the interval, the whole motion of the satellite will be obtained. By employing the interval embracing a great number of revolutions, the mean will give the time of each with greater accuracy.

If, in the greatest digressions, the distances of the satellites from the centres of their primaries be measured, and these distances compared with the times of their sidereal revolutions (which are easily foundfrom the synodic), it will be seen that these small bodies follow the law of Kepler respecting the revolutions of the planets; and when this truth is sufficiently established for the other satellites of Jupiter, it may be employed to determine the revolution of the first, which is too near the body of the planet to admit of this being accurately accomplished by means of its eclipses.

Each planet and its satellites have therefore a great analogy to the Sun, and his system of planets. Hence each presents a small world, in which the same phenomena are observed as in the great world; and as the times of the revolutions are shorter, the changes are more rapid.

Jupiter is accompanied by four satellites, Saturn by seven, and Uranus by six. The following table contains the mean distances of Jupiter's satellites from the centre of that planet, in the estimation of which the diameter of the planet is taken for unity; and the times of their revolutions according to the researches of M. Delambre.

From the calculations of the same astronomer it also results that half the mean duration of the eclipses of these satellites which take place in the nodes are, as seen through common acromatic telescopes, for the

The masses of these satellites in terms of that of Jupiter, according to the same author, are as follow:

the last of which is very nearly equal to that of the Moon, which, in terms of the mass of Jupiter, is 0.000047258.

The inclination of the orbits of three of these satellites to the equator of the planet he states as follows:-That of the first satellite, from its nearness to the planet, is not susceptible of being ascertained with sufficient accuracy.

The first three satellites, which are nearest to each other, are connected together by remarkable cir