« AnteriorContinuar »
he ceases to shine on the one side of it, and begins
to shine upon the other. Fig. III.
Let S be the Sun, ABCDEFGH Saturn's orbit, and IKLM.NO the Earth's orbit. Both Saturn and the Earth move according to the order of the letters: when Saturn is at A his ring is turned edgewise to the Sun S, and he is then seen from the Earth as if he had lost his ring, let the Earth be in any part of its orbit whatever, except between N and 0; for while it describes that space, Saturn is apparently so near the Sun as to be hid in his beams. As Saturn goes from A to C, his ring appears more and more open to the Earth: at Cthe ring appears most open of all; and seems to grow narrower and narrower, as Saturn goes
from C'to E, and when he comes to E, the ring is again turned edgewise both to the Sun and Earth; and as neither of its sides are illuminated, it is invisible to us, because its edge is too thin to be perceptible; and Saturn appears again as if he had lost his ring. But as he goes from E to G, his ring opens more and more to our view on the under side; and seems just as open at G as it was at C; and may be seen in the night time from the Earth in any part of its orbit, except about M, when the Sun hicles the planet from our view. As Saturn goes from G to A, his ring turns more and more edgewise to us, and therefore it seems to grow narrower and narrower; and at A, it disappears as before. Hence, while Saturn goes from A to E, the Sun shines on the upper side of his ring, and the under side is dark; and while he goes from E to A, the Sun shines on the under side of his ring, and the upper side is dark.
It may perhaps be imugi.ed that this article might hare been placed inore properly alter | 81,
than hre; but when the candid reader considers Fig. I. and that all the various phenomena of Saturn's ring
depend upon a cause similar to that of our Earth's
seasons, he will readily allow that they are best ex. Plate VI. plained together; and that the two figures serve to illustrate each other. 205. The Earth's orbit being elliptical, and the The Earth
nearer the Sun keeping constantly in its lower focus, which is
Sun in 1,377,000 miles from the middle point of the longer winter axis, the Earth comes twice so much, or 2,754,000 than min miles, nearer the Sun at one time of the year than at another : for the Sun appearing to us under a larger angle in winter than in summer, proves that the Earth is nearest the Sun in winter (see the Note on Article 185). But here this natural ques. Why the
weather is tion will arise: Why have we not the hottest weather when the Earth is nearest the Sun? In answer it when the must be observed, that the eccentricity of the Earth's Earth is orbit, or 1,377,000 miles, bears no greater propor- the Sun. tion to the Earth's mean distance from the Sun, than 17 does to 1000; and therefore this small differ. ence of distance cannot occasion any sensible differ. ence of heat or cold. But the principal cause of this difference is, that in winter the Sun's rays fall so obliquely upon us, that any given number of them is spread over a much greater portion of the Earth's surface where we live, and therefore each point must then have fewer rays than in summer. Moreover, there comes a greater degree of cold in the long winter nights, than there can return of heat in so short days; and on both these accounts the cold must increase. But in summer the Sun's rays
fall more perpendicularly upon us, and therefore come with greater force, and in greater numbers on the same place; and by their long continuance, a much greater degree of heat is imparted by day than can fly off by night.
206. That a greater number of rays fall on the same place, when they come perpendicularly, than when they come obliquely on it, will appear by the figure. For, let AB be a certain number of the Fig. IL Sun's rays falling on CD (which let us suppose to
be London) on the 21st of June : but, on the 22d of December, the line CD, or London, has the ob. lique position CD to the same rays; and therefore scarce a third part of them falls upon it, or only those between A and e; all the rest, c B, being expended on the space d P, which is more than double the length of CD or Cd. Besides, those parts which are once heated, retain the heat for some time; which, with the additional heat daily imparted, makes it continue to increase, though the Sun declines toward the south; and this is the reason why July is hotter than June, although the Sun has withdrawn from the summer tropic; as we find it is generally hotter at three in the afternoon, when the Sun has gone toward the west, than at noon when he is on the meridian. Likewise, those places which are well cooled require time to be heated again; for the Sun's rays do not heat even the surface of any body till they have been some time upon it. And therefore we find January, for the most part, colder than December, although the Sun has withdrawn from the winter tropic, and begins to dart his beams more perpendicularly upon us, when we have the position CF. An iron bar is not heated immediately upon being put into the fire, nor grows cold till some time after it has been taken out.
The Method of finding the Longitude by the Eclips
es of Jupiter's Satellites: the amazing Velocity of Light demonstrated by these Eclipses.
EOGRAPHERS arbitrarily choose to First me. 207. G
call the meridian of some remarkable and lon. place the first meridian. There they begin their places, reckoning; and just so many degrees and minutes
as any other place is to the eastward or westward of that meridian, so much east or west longitude they say it has. A degree is the 360th part of a circle,
be it great or small, and a minute the 60th part of a Piat: V. degree. The English geographers reckon the longi. tude from the meridian of the Royal Observatory at Greenwich, and the French from the meridian of Paris.
208. If we imagine two great circles, one of Fig. II. which is the meridian of any given place, to inter- Hour cirsect each other in the two poles of the Earth, and to cles. cut the equator Æ at every 15th degree, they will be divided by the poles into 24 semi-circles, which divide the equator into 24 equal parts; and as the Earth turns on its axis, the planes of these semicircles come successively one after another every hour to the Sun. As in an hour of time there is a revo- An hour lution of fifteen degrees of the equator, in a minute of time of time there will be a revolution of 15 minutes of 15 de. the equator, and in a second of time a revolution of grees of 15 seconds. There are two tables annexed to this
motion. chapter, for reducing mean solar time into degrees and minutes of the terrestrial equator; and also for converting degrees and parts of the equator into mean solar time.
209. Because the Sun enlightens only one half of the Earth at once, as it turns round its axis, he rises to some places at the same moment of absolute time that he sets at to others; and when it is mid-day to some places, it is mid-night to others. The XII on the middle of the Earth's enlightened side, next the Sun, stands for mid-day; and the opposite XII, on the middle of the dark side for midnight. If we suppose this circle of hours to be fixed in the plane of the equinoctial, and the Earth to turn round within it, any particular meridian will come to the different hours so as to shew the true time of the day or night at all places on that meridian. Therefore,
210. To every place 15 degrees eastward from any given meridian, it is noon an hour sooner than on that meridian; because their meridian comes
to 15 de
to the Sun an hour sooner; and to all places 15 degrees westward, it is noon an hour later, \ 208, because their meridian comes an hour later to the Sun;
and so on; every 15 degrees of motion causing an And con. hour's difference of time. Therefore they who have sequently noon an hour later than we, have their meridian, grees of
that is their longitude, 15 degrees westward from us; longitude. and they who have noon an hour sooner than we,
have their meridian 15 degrees eastward from ours;
and so for every hour's difference of time, 15 deLunar e. grees difference of longitude. Consequently, if the useful in beginning or ending of a lunar eclipse be observed, finding the suppose at London, to be exactly at midnight, and longitude. in some other place at 11 at night, that place is 15
degrees westward from the meridian of London ; if the same eclipse be observed at one in the morning at another place, that place is 15 degrees eastward
from the said meridian. Eclipses 211. But as it is not easy to determine the exact
moment either of the beginning or ending of a lunar lites much eclipse, because the Earth's shadow through which better for the Moon passes is faint and ill-defined about the pose.
edges, we have recourse to the eclipses of Jupiter's satellites, which disappear much more quickly as they enter into Jupiter's shadow, and emerge more suddenly out of it. The first or nearest satellite to Ju. piter is the most advantageous for this purpose, because its motion is quicker than the motion of any of the rest, and therefore its immersions and emersions are more frequent and more sudden than those of the others are.
212 The English astronomers have calculated tables for shewing the times of the eclipses of Jupiter's satellites to great precision, for the meridian of Greenwich. Now, let an observer, who has these tables, with a good telescope and a well-regulated clock, at any other place of the Earth, observe the