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and, following the proportion just stated in the case
The Sun's rising and setting at intervals this month
Equation of Time. The following table will enable the reader to adjust his clock or watch to true or equal time, by subtracting a certain number of minutes and seconds from apparent time :
m. $. Saturday, - - 1st, from the time on the dial SUBTRACT 10 11 Thursday, - - 6th, - • - • • • • - • • 11 43 Tuesday, - - 11th, - - • - - . . . 13 5 Sunday, - - 16th, - • - • • • • • • • - 14 14 Friday, - - 21st, - - - - • • .- - • - - 15 10 Wednesday, 26th, • • • • • • • • • • - 15 50 Monday - - 31st, - - - - - - - - . - - 16 12,
The Sun enters the sign Scorpio at a minute after I in the morning of the 24th.
The Moon enters its last quarter at 58 m. past 11 in the forenoon of the 6th. The new Moon, or change, will be at 51 m. past 10 in the forenoon of the 13th : it enters its first quarter at 49 m. past 8 in the morning of the 21st, and is full at 16'm. past midnight of the 28th. The time of the Moon's rising, the first six days after the full in September, will be as follows :
Sept. 30th, - ....... - 48 m. past 6
......... 35 - - - 7
--- 34 - - - 9 ."
On the 1st day of the month, the Moon will eclipse the star u Ceti: the immersion will be 40 m. past 10 in the evening, and the emersion at 41 m. past 11; in the former case, the star will be near 5 m. south of the Moon's centre, and in the latter 81 on the same side. On the 30th, there will be also an eclipse of the 188, of which the immersion will be at 15 m. past 1 in the morning, and the emersion 21 m. past 2: at the immersion, the star will be 4'} south of the Moon's centre, and at the emersion it will be 9'} on the same side. .. On the 20th, at 33 m. past 5 o'clock in the morning, there will be an eclipse of Jupiter's second satellite.
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VIEW OF THE SOLAR SYSTEM.
Of Jupiter. Jupiter is, with the exception of Venus, the most brilliant of all the planets : he is the largest body in the whole solar system, excepting the Sun, around which they all revolve, his diameter being eleven times greater than that of the earth, or nearly 90,000 English miles in length. He moves from the west to the east in a period of 4332 days, or in 11 years 318 days. Before he comes into opposition, and, when distant from the Sun, in the course of its revolution, about 115°, his motion becomes retrograde, and increases in swiftness till he comes in opposition. The motion then becomes gradually slower, and is direct when the planet advances within 1150 of the Sun. The direction of the retrograde motion is about 121 days, and the arc of retrogradation described is about 10°. With respect to all the superior planets, the farther they are from the Sun, the less is the arc of retrogradation, but the longer time is taken in describing it; for, were the distance of the planets indefinitely great, the arc of retrogradation would be extremely small.
Jupiter has the same general appearance with Mars, only that the belts on his surface are much larger and more permanent. They are not to be seen but by a good telescope, and their appearances differ very much at different times, and even at the same time in telescopes of different powers. Their number is very variable, as sometimes only one, and at others there are as many as eight. They are generally parallel to one another, but not always so; and their breadth is likewise yariable, one belt having been observed to grow narrow, while another in its neighbourhood has increased in breadth. They seem almost always to be of an uniform tint; but in very favourable weather, they sometimes seem to consist of a number of curved lines, something like strokes made with the graver. The time of their continuance is very uncertain, sometimes remaining unchanged for three months ; at others, new belts have been found, in an hour or two. In some of these belts, large black spots have occasionally appeared, which were observed to move swiftly over the disk from east to west, and to return in a short time to the same place. The belts of Jupiter were first observed at Naples by Zuppi and Bartoli, two Jesuits; and about the year 1660, they were observed by Campani, with refracting telescopes of his own construction, and not much inferior in dis, tinctness to those of the present day, the great modern improvement in refracting telescopes consisting rather in the reduction of their size than in the increase of their magnifying power.
The figure of Jupiter is that of an oblate spheroid, the longest diameter being to the shortest in the proportion as 13 to 12 ; his rotation being from west to east, like that of the Sun, and the plane of his equątor being very nearly coincident with that of his orbit, 60 that there can be scarcely any difference of seasons in that planet, His rotation has been observed to be somewhat quicker in his aphelign than his perihelion. The axis of rotation is nearly perpen
dicular to the plane of the ecliptic, and the planet makes one diurnal revolution in 9 h. 55 m. 37 s.' The changes in the appearance of his spots, and the difference in the time of their rotation, make it probable that they do not adhere to Jupiter, but are clouds transported by the winds with different velocities in an atmosphere subject to violent agitations. .
The apparent diameter of this planet differs from 46'' to 31"; but his mean apparent diameter is equal to 36".
Jupiter is accompanied by four satellites, which were first discovered by Galileo on the 8th of January 1610. The relative situation of these small bodies changes at every instant: they oscillate, as it were, on each side the planet, and it is by the extent of these oscillations that the rank of the satellites is determined, that being called the first satellite whose oscillation is the least. They are sometimes seen to pass over the disk of the planet, and project a shadow in the form of a well defined black spot, when they describe a chord over his disk. Hence it is inferred that Jupiter and his satellites are opaque bodies enlightened by the Sun; and when the latter interpose between the Sun and Jupiter, they produce real solar eclipses precisely similar to those which the Moon occasions on the earth. This phenomenon leads to the explanation of another which the satellites present. They are often observed to disappear, though, at some distance from the disk of the planet, the third and fourth reappear sometimes on the same side of the disk.
The shadow which Jupiter projects behind it is the only, cause that can explain these disappearances, which are perfectly similar to eclipses of the Moon. The circumstances which accompany them leave no doubt of the reality of the cause. The satellites are always observed to disappear on the side of the disk opposite to the Sun, and consequently on the same side to which the conical shadow is projected. They are eclipsed nearest the disk when the planet is nearest
to its opposition. Lastly, the duration of these eclipses answers to the time which should elapse while they traverse the shadow of Jupiter. .
Thus, it is evident that these satellites move from west to east. Their mean sidereal and synoidical revoJutions, as seen from the centre of Jupiter, are very accurately determined by comparing eclipses at long intervals from each other, and observed near the opposition of the planet. It is thus discovered that the motion of Jupiter's satellites is almost circular and uniform, because this hypothesis corresponds very nearly with those eclipses which happen when we see the planet in the same position relatively to the Sun. Therefore, the positions of the satellites at every instant, as seen from the centre of Jupiter, may be determined. From henee results a simple and exact method of comparing with each other the distances of Jupiter and of the Sun from the earth, a method which antient astronomers could not possess. They only judged of its distance by the time of its revolution; as they estimated those planets to be farthest from us whose period of revolution was the longest. ;
It may further be observed, that, when Jupiter is in quadrature with the Sun, the earth is farthest out of the line that passes through the centres of the Sun and Jupiter, and therefore the shadow of the planet is then most exposed to our view : but, even then, the body of the planet will hide from us one side of that part of the shadow which is nearest to it, through which the first satellite passes ; which is the reason that, though we see the entrance of that satellite into the shadow, or its coming out from that thenee, as the earth is situated on the east or west side thereof, we cannot see them both; whereas the other satellites going through the shadow at a greater distance from Jupiter, 'their ingress and egress are both visible..“