Obrazy na stronie
PDF
ePub

111.

turning it round its axis. Consequently, when the plate Moon is in those signs which rise or set with the smallest angles, she rises or sets with the least difference of time; and with the greatest difference in Fig. III. those signs which rise or set with the greatest angles.

But, because all who read this treatise may not be provided with globes, though in this case it is requisite to know how to use them, we shall substitute the figure of a globe; in which FUP is the axis, os TR the tropic of Cancer, Lt vs the tropic of Capricorn, s EU is the ecliptic touching both the tropics, which are 47 degrees from each other, and AB the horizon. The equator being in the middle between the tropics, is cut by the ecliptic in two opposite points, which are the beginnings of op Aries and a Libra; K is the hour-circle with its index, F the north pole of the globe elevated to a , considerable latitude, suppose 40 degrees above the horizon; and P the south pole depressed as much Fig. III. below it. Because of the oblique position of the sphere in this latitude, the ecliptic has the high ele. vation N o above the horizon, making the angle The differNUS of 73, degrees with it when s Cancer is on ent angles the meridian, at which time = Libra rises in the the eclip. east. But let the globe be turned half round its axis, tic and ho

rizon. till vs Capricorn comes to the meridian and op Aries rises in the east, and then the ecliptic will have the low elevation NL above the horizon, making only an angle NUL of 26} degrees with it; which is 47 degrees less than the former angle, equal to the distance between the tropics.

276. In northern latitudes, the smallest angle Least and made by the ecliptic and horizon is when Aries rises, greatest, at which time Libra sets; the greatest when Libra rises, at which time Aries sets. From the rising of Aries to the rising of Libra (which is twelve* side

* The ecliptic, together with the fixed stars, make 366 apparent diurnal revolutions about the Earth in a year; the Sun only 365. Therefore the stars gain 3 minutes 56 sc

Days.
Signs.
Degrees.

50 43

2

50 100 1410

261

4

231
61

28

1610 150 1510

दा

ral hours) the angle increases ; and from the rising of Libra to the rising of Aries, it decreases in the same proportion. By this article and the preceding it appears that the ecliptic rises fastest about Aries,

and slowest about Libra. Result of 277. On the parallel of London, as much of the the quan: ecliptic rises about Pisces tity of this and Aries in two hours as

Rising Setting angle at

Diff. Diff. London. the Moon goes through in six days: and therefore

H. M. I. M. while the Moon is in these 131 signs, she differs but two hours in rising for six days 38 101

37 1710

32 together; that is, about 20

mg minutes later every day or

191 night than on the preced

211 ing, at a mean rate. But

1510 in fourteen days afterward,

150 the Moon comes to Virgou

10!m 1211 1510 22

1410 and Libra, which are the 121 opposite signs to Pisces 13 21/1 1010 47 and Aries; and then she 14118 41

56 differs almost four times as

1710 much in rising; namely, 17

40/1

3511 12 one hour and about fifteen 18

30/1 minutes later every day or

191 100

16 night than the former, while 20

710 17/1 16 she is in these signs. The 2018

171 15 annexed table shews the 238 2011

11 daily mean difference of 24.

1610

2411 the Moon's rising and set

3011 ting on the parallel of Lon- 260 130

2610 561 7 don, for 28 days; in which

28|ரூ 91 0000 time the moon finishes her,

24 20 18 17

8 9

151 28/1

25/1

8/1

30 39

130

15 16m

410 461

5

[blocks in formation]
[blocks in formation]

conds upon the Sun every day; so that a sideral day contains only 23 hours 56 minutes of mean solar time ; and a natural or solar day 24 hours. Hence 12 sideral hours are one minute 58 seconds shorter than 12 solar hours.

III.

period round the ecliptic, and gets 9 degrees into PLATE the same sign from the beginning of which she set out. Thus it appears by the table, that when the Moon is in me and she rises an hour and a quarter later every day than she rose on the former; and differs only 28, 24, 20, 18 or 17 minutes in setting. But, when she comes to * and op, she is only 20 or 17 minutes later in rising; and an hour and a quarter later in setting

278. All these things will be made plain by putting small patches on the ecliptic of a globe, as far from one another as the Moon moves from any point of the celestial ecliptic in 24 hours, which at a mean rate is* 13 degrees; and then, in turning the globe round, observe the rising and setting of the patches in the horizon, as the index points out the different times on the hour-circle. A few of these patches are represented by dots at () 1 2 3, &c. on the ecliptic, Fig. III. which has the position LUI when Aries rises in the east; and by the dots 0123, &c. when Libra rises in the east, at which time the ecliptic has the position EU18: making an angle of 62 degrees with the horizon in the latter case, and an angle of no more than 15 degrees with it in the former; supposing the globe rectified to the latitude of London.

279. Having rectified the globe, turn it until the patch at 0, about the beginning of * Pisces in the half LUI of the ecliptic, comes to the eastern side of the horizon; and then, keeping the ball steady, set the hour-index to XII, because that hour may perhaps be more easily remembered than any other. . Then turn the globe round westward, and in that time, suppose the patch 0 to have moved thence

* The Sun advances almost a degree in the ecliptic in 24 hours, the same way that the Moon moves; and therefore the Moon by advancing 132 degrees in that time, goes little more than 12 degrees farther from the Sun than she was on the day before.

to 1, 13% degrees, while the Earth turns once round its axis, and you will see that 1 rises only about 20 minutes later' than 0 did on the day before. Turn the globe round again, and in that time suppose the same patch to have moved from 1 to 2; and it will rise only 20 minutes later by the hour-index than it did at 1 on the day or turn before. At the end of the next turn suppose the patch to have gone from 2 to 3 at U, and it will rise 20 minutes later than it did at 2, and so on for six turns, in which time there will scarce be two hours difference; nor would there have been so much, if the 6 degrees of the Sun's motion in that time had been allowed for. At the first turn the patch rises south of the east, at the middle turn due east, and at the last turn north of the east. But these patches will be 9 hours in setting on the western side of the horizon, which shews that the Moon's setting will be so much retarded in that week in which she moves through these two signs. The cause of this difference is evident ; for Pisces and Aries make only an angle of 15 degrees with the horizon when they rise; but they make an angle of 62 degrees with it when they set. As the signs Taurus, Gemini, Cancer, Leo, Virgo, and Libra, rise successively, the angle increases gradually which they make with the horizon, and decreases in the same proportion as they set. And for that reason, the Moon differs gradually more in the time of her rising every day while she is in these signs, and less in her setting: after which, through the other six signs, viz. Scorpio, Sagittary, Capricorn, Aquarius, Pisces, and Aries, the rising difference becomes less every day, until it be at the least of all, namely, in Pisces and Aries.

280. The Moon goes round the ecliptic in 27 days 8 hours: but not from change to change in less than 29 days 12 hours: so that she is in Pisces and Aries at least once in every lunation, and in some lunations twice.

ferent

1

281. If the Earth had no annual motion, the Why the Sun would never appear to shift his place in the Moon is ecliptic. And then every new Moon would fall in full in difthe samesign and degree of the ecliptic, and every

signs. full Moon in the opposite : for the Moon would go precisely round the ecliptic from change to change. So that if the Moon were once full in Pisces or Aries, she would always be full when she came round to the same sign and degree again. And as the full Moon rises at sun-set (because when any point of the ecliptic sets, the opposite point rises) she would constantly rise within two hours of sun-set, on the parallel of London, during the week in which she was full. But in the time that the Moon goes round the ecliptic from any conjunction or opposition, the Earth goes almost a sign forward : and therefore the Sun will seem to go as far forward in that time, namely, 27 degrees; so that the Moon must go 27 degrees more than round, and as much farther as the Sun advances in that interval, which is 21s degrees, before she can be in conjunction with, or opposite to the Sun again. Hence it is evident that there can be but one conjunction or opp s'ion of the Sun and Moon in a year in any Her periparticular part of the ecliptic. This may be fami-odical and liarly exemplified by the hour and minute-hands of synodical a watch, which are never in conjunction or oppo. exemplifisition in that part of the dial-plate where they were

ed. so last before.

And indeed if we compare the twelve hours on the dial-plate to the twelve signs of the ecliptic, the hour- hand to the Sun, and the minute-hand to the Moon, we shall have a tolerable near resemblance in miniature to the motions of our great celestial luminaries. The only difference is, that while the Sun goes once round the ecliptic, the Moon makes 124 conjunctions with him: but, while the huur-hand goes round the dial-plate, the minute- . hand makes only 11 conjunctions with it; because the minute-hand moves slower in respect to the hour.

Hh

revolution

« PoprzedniaDalej »