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While high-church homines in ease et luxury vivunt;
In FEBRUARY 1817.
THE Sun enters Pisces on the 18th, at 38 m. past 10 at night. On the 9th, at 3 in the morning, Mercury will be in his inferior conjunction; and on the 21st he will appear stationary. On the 10th, Saturn will be in conjunction with the star marked 2 in Aquarius, the star being 47' south. On the 18th, also, Venus will be in conjunction with the star e in X, the star being 19' north of the planet. Saturn will be in conjunction on the 15th, at 15 m. past 1 in the afternoon. And Jupiter will be in quadrature on the 28th, at 1 in the afternoon.
Of the Sun's Rising and Setting for every fifth
Saturday, Feb. 1, Sun rises 27 m. after 7. Sets 33 m. after 4
In order to find the true time by a good sun-dial, the numbers in the following Table must be added to the apparent time, as shewn by that instrument, for every 5th day of the month.
February 1, to the time on the dial add 13 59
The following example will explain the use of this Table to such of our readers as have not previously attended to the subject; and for this purpose we insert it.-Ex. Suppose that, on the 16th of January, the time on a good sun-dial was observed to be 20 m. past 2; and it was required to find the mean or true time answering to that instant; this is obtained by adding the equation of time for the given day to the time observed on the dial; hence 2 h. 20 m.+10 m. 10 s. 2 h. 30 m. 10 s.; the time required..
If the given day is not one of those stated in the preceding Table, the quantity to be added to the observed time must be found by proportion. Thus, take the difference corresponding to the two days in the Table between which the given day falls, and also the first of these two days from that for which the time is required; then say, as 5 is to this last difference, so is the first difference to a fourth number; which added to the number answering to the first of the days that was taken from the Table, gives the quantity to be added to the observed time. Suppose it were required to find the mean time corresponding to 3 h. in the afternoon of the 24th of Jan. observed on a good sun-dial; then the difference of the numbers answering to the two nearest days in the Table (21 and 26) is 12 m. 58 s.- -11 m. 44 s.— 1 m. 14s., or 74 s.; and 24-21-3; therefore, 5:3:74 s.: 44 s.; which being added to the number answering to the 21st day, gives 11 m. 44 s. + 44 s. 12 m. 28 s.; and consequently 3 h. 12 m. 28s. is the mean time required.
The Moon will be full at 15 m. past 2 on the morning of the 2d. She will enter her last quarter at 47 m. past 7 on the evening of the 8th. There will be a new Moon on the 16th, at 19 m. after 4 in the morning; and her first quarter will commence at 27 m. after 8 on the morning of the 24th.
The Moon will pass the first meridian at a conve
nient time for observation, on the following days of this month, viz.
1st Satellite, Feb. 13, 2d Satellite,
The eclipses of Jupiter's first and second satellite for this month, as visible at Greenwich and its neighbourhood, will be as follow:
6 in the evening.
57 past 5 in the morning.
There will also be an eclipse of the third satellite on the 22d. The immersion will take place at past 3 in the morning, and the emersion at 40 m. past 5, A.M.
On the Obliquity of the Ecliptic.
THE values assigned to the obliquity of the ecliptic, by astronomers of different ages, are different; and regularly diminish, from the most distant periods at which observations have been recorded to the present time. Nor can these differences be entirely attributed either to the imperfection of instruments or observations; for this cause would sometimes have given the results too great, and at others too little; and there is almost an infinity of chances to one that they should all agree in indicating this progressive diminution if it were not real. The theory of universal gravitation also completely confirms the same result. By this it is proved that the different attractions of the planetary bodies of which the solar system is composed, ought necessarily to cause a change in this obliquity; and according to the actual disposition of this system, the inclination of the plane of the ecliptic to that of the
equator ought to diminish by a quantity nearly equal to fifty seconds in a century, or about half a second a year. M. Laplace has published, in the Connaissance des Tems, for 1811, a Table of the observed obliquity at distant intervals, compared with the calculated obliquity for the same epochs; and of which the following is the substance:
Date of the Observed Obli- Calculated Ob-
BEFORE THE CHRISTIAN ERA.
Excess or defect
23°.900553 23°.866128 +0°.034425 23.822217 23 .768613 +0.053604 23 .760828 23 .755275 +0.005553 |23 .760828|23 .734278] +0.026550
SINCE THE CHRISTIAN ERA.
23°.692491| 23°.704722 -0°.012231 23 .647878 23 .664735—0 .016415 23 .667804 23 .638050] +0.029754 23.594715 23 .586939 +0.007776 23.573889 23 .580567 -0.006678 23 .534001 23 .539581 -0.005580 1437 23 .529996 23 .518053 +0.011943
The whole of these observations fully establish the successive diminution of the obliquity of the ecliptic. Their near coincidence with the numbers deduced from theory, from which they deviate sometimes in excess and sometimes in defect, also shows that this diminution is occasioned solely by the attraction of the planets (particularly of Venus and Jupiter) upon each other, and upon the Earth.
The practical method of ascertaining this diminution is by comparing the positions of the same stars, with respect to the ecliptic, at very distant epochs of time. This difference is the most remarkable in the stars near the summer and winter solstices. Those which were antiently on the north of the ecliptic at the summer solstice, are now removed further
from its plane; and, on the contrary, those stars which, according to the testimony of antient astronomers, were situated on the south of the ecliptic in the vicinity of the same solstice, have approached its plane, and some of them even passed to the north of it. Analogous changes have also taken place with regard to those near the winter solstice. All the stars have likewise participated in this apparent motion; but less in proportion as they are situated nearer the line of the equinoxes, about which they appear to be revolving, as about an axis. It is, therefore, extremely natural to conclude, from these phenomena, that the plane of the ecliptic has really varied its position with respect to the heavens, and produced, in a contrary direction, those appearances which have been observed in the stars; for to suppose that these changes had really taken place in the positions of the stars, would be to attribute an inconceivable agreement to these heavenly bodies.
The young astronomer may find the obliquity of the ecliptic in the following manner :-Near the time of the summer solstice, observe the meridian altitude of the sun's centre for several days together, with the utmost care; and from the greatest of these observed altitudes subtract the height of the equator, and the remaining arc will be the Sun's greatest declination, which being an arc of a great circle, and 90° from each of the equinoctial points, will also be the obliquity of the ecliptic. Or this obliquity may be found by observing the meridian altitude of the Sun's centre on the days of the summer and winter solstice; and the difference of these altitudes will be the distance of the tropics, half of which measures the obliquity of the ecliptic.
But the most important circumstance which theory proves is, that the diminution of the ecliptic will not always be progressive. The time will arrive when the quantity of this diminution will decrease, until it cease entirely, and the obliquity become