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The Sun appears to go round the earth in twentyfour hours, and the fixed stars in nearly four minutes less, or in 23 h.56 m. 4 s.; so that any given fixed star is found to gain 3 m. 56 s. upon the Sun every day, which amounts to one diurnal revolution in a year; and therefore in 365 days, as measured by the returns of the Sun to the meridian, there are 366 days, as measured by the return of any given fixed star to the meridian; the former of which are called solar days, and the latter sidereal. The difference between the solar and sidereal days is occasioned by the immense distance of the fixed stars; for the earth's orbit, though 190 millions of miles in diameter, is but as a point compared with that distance; and, therefore, any meridian of the earth will revolve from a fixed star, to that star again, in exactly the same time as if the earth had only a diurnal motion, and was to remain for ever in the same part of its orbit. This, however, is not the case with respect to the Sun; for, as the earth advances almost a degree, eastward in its orbit in the same time that it turns eastward round its axis, or completes its diurnal revolution, whatever star passes over the meridian any day with the Sun, will pass over the same meridian the next day, when the Sun is 3 m. 56" short of it. If the year contained exactly 360 days, as the ecliptic does 360 degrees, the Sun's apparent place would change a degree every day; and, in this case, the sidereal days would be just 4 minutes shorter than the solar ones.
The daily revolution of the earth, which is known to be uniform, is always completed, when any particular meridian is exactly parallel to the situation which it had at a certain time of the preceding day. For the same meridian can never be brought round from the Sun, to the Sun again, by one entire révolution of the earth upon its axis, but it will require as much more of another revolution, as is equivalent to the space that the earth has advanced in its orbit during that time; which is, at a medium, the 365th part of
a circle. So that, in 365 days, the earth will have turned 366 times round its axis ; and therefore, as one complete rotation makes a sidereal day, in a year there will be a sidereal day more than there are solar days, be that number what it may. .
It may be observed, that the regular return of the fixed stars to the meridian affords an easy method of determining whether our chronometers keep true time. For if, through a small hole in a window shutter, or other fixed object, it be observed at what time a given star disappears behind any building at a small distance; then if the same star disappears, the next night, 3 m. 56 s. sooner by the clock or, watch, than it did the night before ; and on the second night 7 m. 52 s. sooner, and so on, it is a proof that the instrument goes right; but if it does not observe this. rule, it is evidently not accurate, and requires to be regulated. As the disappearing of a star is instanta-. neous, this rule may be depended on to half a second.
The following are the times of Sun-rising and setting at London for this month :
1st. Sun rises 27 m. past 7 Sun sets 33 m. past.4. 9th -.• - 13 ..-7 --- 47 -•-• 4. . 18th -... 56 ....6 .....4.... 5. Equation of Time.-[See the month of January.] The following table will show what is to be added to the time pointed out on the dial, to obtain true or equal time for every 5th day of February :
m. s. . Tuesday, Feb. 1, to the time on the dial ADD 13 56 Sunday, 6, - • • .
. : 14 26 to obtain Friday, • . 11, -............ - 14 35 ( true, time Wednesday, 16,............. - 14 26 ( by the Monday, - 21,-..-.-.-•...•- 13 59 clock. Saturday, - 26, ......... ..-- 13 16 ..
The Sun enters Pisces on the 19th, at 20 m. past 5 o'clock in the morning. Venus appears. stationary on the 18th.
The Moon is full on the 4th, at 46 m. past 6 in the afternoon: it enters its last quarter on the 12th, at
45 m. past 4 in the morning: the preceding new Moon is at 11 m. past 7 in the morning of the 20th, and it enters its first quarter at 26 in. past 10 in the morning of the 27th. The time of the Moon's rising for the first five days after she is full, will be as fol. lows, viz. on the
5th of February, 34 m. past 5 in the afternoon.
9th, -•-• 36 10 ditto.. On the 1st day of this month the Moon will eclipse the star marked v D in astronomical catalogues, The immersion will take place at 13 m. past 11 in the evening, when the star will be 7 minutes south of the Moon's centre; and the emersion will be at 13 m. past 12, the sťar being 8 m. south of the Moon's centre.
On the 12th day, the Moon will eclipse the starys. The immersion will happen at 18 m. past 2 in the morning, when the star is 7 m. north of the Moon's centre; and the emersion will take place at 16 m, past 3 in the morning, the star being 10 m. north of the Moon's centre.
At 20 m. past 5 in the morning of the 18th, Mers cury will be in conjunction with the Sun. On the 24th, at 9 in the morning, Jupiter will be in opposition to that luminary.
The eclipses of Jupiter's first satellite for February, that are visible in London and its vicinity, are as follow, viz. the immersions take place on the
3d day, at 42 m. past 2 o'clock in the morning. i 4th, • • • 11 -•.- 9...----- evening. 10th, - - - 36 -.-- 4 -•.•••• morning. 11th, -... .... 11 ...
-.- evening. 18th, - • - $8 - - - - 12 -•••••• evening. 20th,
••••• evening, 26thy
5 •-•• • morning. 27th, , ,
VIEW OF THE SOLAR SYSTEM. · In our last month's observations on the Solar System, we have described some particulars relating to the Sun; we shall now continue the subject, acknowledging ourselves indebted chiefly to the discoveries of the indefatigable Dr. Herschel · The Sun agrees with the fixed stars in many particulars; as in the property of emitting light continually, and in retaining constantly its relative situation with but little variation; and hence the Sun is considered as a fixed star comparatively near us, and the stars as suns at immense distances from the earth. The Sun, like many other stars, is supposed to have a progressive motion directed towards the constellation Hercules; and it revolves on its axis in 25 d. 10 b., this axis being directed towards a point about half way between the pole star and Lyra, the plane of rotation being inclined something more than 7° to that in which the earth revolves. The direction of this motion is from west to east. All the rotations of the different bodies which compose the solar system, as far as they have been ascertained, are in the same direction, and likewise all their revolutions about the central bodies, excepting, perhaps, those of some of the comets, and those of one or two of the satellites of the Georgian planet. The time and direction of the Sun's rotation are ascertained by the change of the situation of the spots, which, we have already observed, are very frequently visible on his disk, and which some astronomers suppose to be elevations, and others excavations in the luminous matter covering the Sun's surface. M. Lalande supposes them to be parts of the solid body of the Sun, which, by certain agitations of the luminous ocean, with which he conceives the Sun to be surrounded, are left nearly or entirely bare. Dr. Herschel considers this ocean as consisting rather of a flame than of a liquid substance, and he attributes the spots to the emission of an æriform fluid,
not yet in combustion, which displaces the general luminous atmosphere, and which is afterwards to serve as fuel for supporting the process; hence he supposes the appearance of copious spots to be indicative of the approach of warm seasons on the surface of the earth, a theory which he has attempted, not very successfully, to maintain by historical evidence. The spots are usually surrounded by margins less dark than themselves, which are called shallows, and which are considered as parts of an inferior stratum, consisting of opaque clouds, capable of protecting the immediate surface of the Sun from the excessive heat produced by combustion in the superior stratum, and perhaps rendering it habitable to animated beings. To this theory of the illustrious Herschel, Dr. Young replies, if we inquire into the intensity of the heat which must necessarily exist wherever this combustion is performed, we shall soon be convinced that no clouds, however dense, could impede its rapid transmission to the parts below; besides, he adds, the diameter of the Sun is 111 times as great as that of the earth; and at its surface a heavy body would fall through no less than 450 feet in a single second; so that if every other circumstance permitted human beings to reside on it, their own weight would present an insuperable difficulty, since it would become thirty times as great as upon the surface of the earth, and a man of moderate size would weigh more than two tons.
But whatever becomes of these and other very ingenious and highly interesting speculations, it is evident upon all theories that the Sun is an eminent, large, and lucid planet, the first and only primary one belonging to our system. Its similarity to the other globes of the solar system, with regard to its solidity; its atmosphere; its surface diversified with mountains and valleys; its rotation on its axis, lead to the conclusion that it must be inhabited, like the other planets, by beings whose organs are adapted to the pecu-.