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and project a shadow in the form of a well-defined revolution in about 30,686 mean solar days, or in black spot, which then describes a chord of this about 84 Julian years. disc.

Jupiter and his satellites, therefore, 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.

These phenomena lead to the explanation of another which the satellites present. They are often observed to disappear, though at some distance from the disc of the planet: the third and fourth reappear sometimes on the same side of the disc.

Its distance from the sun is upwards of nineteen hundred millions of miles; and its apparent diameter is scarcely 4",0.

Six satellites accompany this planet, which move in orbits nearly perpendicular to the plane of the ecliptic.

The sizes and relative situations of the sun, earth, and moon, will now engage our attention. It will hardly be necessary to state that the first of these great bodies forms the centre of our system, round which the planets and satellites revolve, or rather The shadow which Jupiter projects behind it (rela-round the common centre of gravity of the entire setively to the sun) is the only cause that can explain ries. these disappearances, which are perfectly similar to The first thing that strikes the mind when coneclipses of the moon. The circumstances which templating this glorious orb, is its astonishing magaccompany them leave no doubt of the reality of nitude. This vast globe is found to be about 882,000 this cause. The satellites are always observed to miles in diameter, and, consequently, contains a disappear on the side of the disc opposite to the mass of matter equal to more than thirteen hundred sun, and consequently on the same side to which thousand spheres the size of our earth. the conical shadow is projected. They are eclipsed nearest the disc, when the planet is nearest to its opposition.

Finally, the duration of these eclipses answers to the time which should elapse while they transverse the shadow of Jupiter.

Thus it appears that these satellites move from west to east in returning orbits round the planet. Observations of their eclipses are the most exact means of determining their motions. Their mean sidereal and synodical revolutions, 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.

Saturn can hardly be seen by the naked eye. When examined by a telescope, it exhibits a very remarkable appearance. It is surrounded by a thin, flat, broad, luminous ring, which encompasses the body of the planet, but does not touch it. This ring casts a strong shadow upon the planet, and is divided into two, by a distinct line in the middle of its breadth. The rings are circular, but appear elliptical from being viewed obliquely.

According to Dr. Herschel, the dimensions of the rings, and the space between, are as follow:

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Miles.

190,248
204,883

The only motion which an ordinary observer can trace in the sun is one of rotation, on its axis in the space of rather more than twenty-five days. This motion has been ascertained by means of a variety of dark spots, which are discovered by the telescope on the sun's disc. They first appear on his eastern limb, and after a period of about thirteen days, disappear on his western. These spots vary both in number, magnitude, and shape; sometimes 40 or 50, and at other times only one or two are visible. Most of them have a very dark nucleus, or central part, surrounded by an umbra, or faint shadow. Some of the spots are as large as would cover the whole continent of Europe; and one was seen in the year 1779, which was computed to be more than fifty thousand miles in diameter. With regard to the nature of this globe, it appears highly probable, from the observations of Sir William Herschel, that the sun is a solid and opaque body, surrounded with luminous clouds which float in the solar atmosphere, and that the dark nucleus of the spots is the opaque body of the sun appearing through occasional openings in its atmosphere.

The following are Sir Isaac Newton's observations on the sun; 1. That its heat is seven times greater in Mercury than with us, and that water there would 146,345 immediately disappear in the state of vapour. 2. That 184,393 the quantity of matter in the sun is to that of Jupiter as 1100 to 1; and that the distance of Jupiter from the sun is in the same ratio to the sun's diameter; 20,000 consequently the centre of the sun and Jupiter is 7,200 nearly in the superfices of the former. 3. That the 2,839 quantity of matter in the sun is to that of Saturn as 2360 to 1; and the distance of Saturn from the sun is in a ratio little less than that of the sun's semidiameter, so that the common centre of gravity of Saturn and the sun is a little within the latter. Hence the common centre of gravity of all the planets cannot be more than the length of the solar diameter from the centre of the sun.

Besides this ring, Saturn has seven moons of different sizes, and its body is surrounded also by belts, like those of Jupiter.

The mean distance of this planet from the sun is 9.5387861; that of the earth being considered as unity. This makes its mean distance about 890 millions of miles. His mean sidereal revolution is performed in 29.456 Julian years. The rotation on his axis is performed in about ten hours and a half. His true diameter is about 76068 miles.

Uranus, or the Georgium Sidus, is the last planet known in our system. It was discovered by Sir William Herschel, March 13th, 1781, who gave it the name which it now bears. It performs its sidereal

4.

From other observations, which we cannot detail without far exceeding our prescribed limits, Sir W. Herschel was induced to suppose, that the appearance of copious spots indicated the approach of warm seasons on the surface of the earth; and he endeavoured to maintain this opinion by historical evidence; connecting the varying temperature of our atmosphere with the appearance and disappearance

of the solar spots. The spots or shallows, which our | drops of falling rain to be spherical. This fact has author considers as parts of an inferior stratum, con- been beautifully illustrated by a modern poet

sisting of opaque clouds, are capable of protecting the immediate surface of the sun from the excessive heat produced by combustion in the superior stratum, and perhaps of rendering it habitable to animated beings. But if stars are suns, and suns are habitable, a very extensive field of examination is thus opened to our view.

"That self-same law which moulds a tear,

And bids it trickle from its source;
That law preserves the earth a sphere,
And guides the planets in their course."

The mutual attraction between the planets and the sun keeps them from flying off from their orbits by the centrifugal force, which is generated by their Some distinguished philosophers, among whom we revolving in a curve; and the latter force again keeps may reckon Sir Isaac Newton, consider the sun's rays them from falling into the sun, which would be the as composed of small particles, which move with uni- case, if it were not for the motion impressed upon form velocities in uniform mediums, but with varia- them. Thus these two powers balance each other, ble velocities in mediums of variable densities. These and preserve order in the entire system. particles, they say, act upon the minute constituent This doctrine, which is parts of bodies, not by impact, but at some indefi- founded on the demonnitely small distance; they attract and are attracted; strations of Sir Isaac Newand on being reflected or refracted, they excite a vi-ton, may be thus illusbratory motion in the component particles. Others, trated. If a planet, as our represent fire as a substance sui generis, unalterable in earth at a, gravitates, or its nature, and incapable of being produced or de- is attracted towards the stroyed; natually existing in equal quantities in all sun, y, so as to fall from places, imperceptible to our senses, and only discover- a to p, in the time that able by its effects, when it is collected in a less space the projectile force would

than that which, from its tendency to an universal | have carried it from a to x, it will describe the curve and equable diffusion, it would otherwise occupy. Ita u by the combined action of these two forces, in has also been argued, that the matter of the sun's rays is not derived from the sun in any shape, but that the rays, whether direct or reflected, are of use only as they impel the particles of fire in parallel directions: that parallelism being destroyed, by intercepting the solar rays, the fire instantly assumes its natural state of uniform diffusion.

M. de Luc, in his Lettres Physiques, is of opinion, that the solar rays are the principal cause of heat; but that they only heat such bodies as do not allow them a free passage. In this respect he agrees with Newton; but then he differs totally from him concerning the nature of the rays of the sun. He does not admit the emanations of any luminous corpuscles from the sun, or rather self-shining substances, but supposes all space to be filled with an ether of great elasticity and small density, and that light consists in the vibrations of this ether, as sound consists in the vibrations of the air. Upon Newton's supposition, says an excellent writer, the causes by which particles of light, and the corpuscles constituting other bodies, are mutually attracted and repelled, is uncertain. The cause of the uniform diffusion of fire, by a series of vibrations, as already stated, is equally inexplicable; and we add to the other difficulties attending the supposition of an universal ether, by the want of a first mover to make the sun vibrate. See LIGHT.

the same time that the projectile force, singly, would have carried it from a to x, or the gravitating power, singly, have caused it to descend from a top; and these two forces being duly proportioned, the planet obeying them both, will move in the circle a u b c. But, if whilst the projectile force would carry the planet from a to ", the sun's attraction should bring it down from a to f, the gravitating power would then be too strong for the projectile force, and would cause the planet to describe the curve a c. When the planet comes to c, the gravitating power (which always increases as the square of the distance from the sun, y, diminishes) will be yet stronger, for the projectile force, and by conspiring, in some degree, therewith, will accelerate the planet's motion all the way from c to g, causing it to describe the arcs a a deƒ, &c., all in equal times. Having its motion thus accelerated, it thereby acquires so much centrifugal force, or tendency to fly off at g, in the line 9 h, as overcomes the sun's attraction; and the centrifugal force being too great to allow the planet to be brought nearer to the sun, or even to move round him in the circle k l m, &c. it goes off, and ascends in the curve g jo q, &c., its motion decreasing as gradually from g to a as it increased from a to g, because the sun's attraction now acts against the planet's projectile motion just as much as it acted with it before. When the planet has got round to a, its projectile We do not in the present case propose, mathema- force is as much diminished from its mean state as it tically, investigating the doctrine of gravitation, or was augmented at g; and thus the sun's attraction the motions of the heavenly bodies, but it will be ne- being more than sufficient to keep the planet from cessary to premise, that there are two forces in con-going off at a, it describes the same orbit over again. tinued operation, the one termed the centripetal, the other the centrifugal, the first tending to draw all bodies towards each other, while the second acts in precisely the opposite direction. This species of universal attraction, which was first examined by our own immortal Newton, and to which his attention was accidentally drawn by the fall of an apple from a tree, is but a branch of the same system of forces which holds all the bodies on the earth's surface together. Iron owes its tenacity to the attraction of cohesion, and it is a similar force which causes the

by the virtue of the same forces or powers. A double projectile force will always balance a quadruple power of gravity. Let the planet at a have twice as great an impulse from thence towards x as it had before; that is, the same length of time that it was projected from a to v, as in the last example; let it now be projected from a to w, and it will require four times as much gravity to retain it in its orbit; that is, it must fall as far from a to r in the time that the pro jectile force would carry it from a to c, otherwise it would not describe the curve a t, as is evident from

the figure. But in as much as the planet moves from | tronomer; and in most ephemerides the computations a to c, in the higher part of its orbit, it moves from ƒ are made in apparent time. to g, or from g to j, in the lower part thereof; be- The astronomical year is divided into four parts, cause from the joint action of these two forces, it determined by the two equinoxes and the two solmust always describe equal areas in equal times stices. The interval between the vernal and authroughout its annual course. These areas are repre-tumnal equinoxes is (on account of the eccentricity sented by the triangles a y c, &c., whose contents 'are equal to one another from the properties of the ellipse.

of the earth's orbit, and its unequal velocity therein) nearly eight days longer than the interval between the autumnal and vernal equinoxes. These intervals were, in 1801, nearly as follow:

D. H. M.

From the vernal equinox
to the summer solstice=92 21 50
From the summer sol-
stice to the autumnal
equinox.

From the autumnal e-
quinox to the winter
solstice

The Earth we inhabit must now be examined; its motions with reference to the occurrence of day and night, the phenomena of the seasons, &c., will readily be understood. Its mean distance from the sun is 23984 times its own semi-diameter; so that it is nearly 95 millions of miles distant from that luminary. It performs its mean sidereal revolution in 365.2563612 mean solar days, or 365 days, 6 hours, 9 minutes, 9 seconds, 6: but the time employed in going from one equinox to the same again, or from one tropic to the same again (whence called the tro-From the winter solstice pical revolution), is only 365.2422414 mean solar days, or 365 days, 5 hours, 48 minutes, 49 seconds, 7. The axis of the earth is inclined to the pole of the ecliptic, at an angle which, at the commencement of the present century, was 23° 27′ 56′′ 5: which angle is called the obliquity of the ecliptic. It is observed to decrease at the rate of 0", 4755 in a year. But this variation is confined within certain limits, and cannot exceed 2o 42'.

This angle is also subject to a periodical change, called the nutation; depending principally on the place of the moon's node: whereby the axis of the earth appears to describe a small ellipse in the

heavens.

is

The intersection of the equator with the ecliptic not always in the same point; but is constantly retrograding or receding contrary to the order of the signs. Consequently the equinoctial points appear to move forward on the ecliptic; and whence this phenomenon is called the precession of the equinoxes. The quantity of this annual change, caused by the action of the sun and moon, and which is called the luni-solar precession, is 50′′.41; from which we must deduct the direct motion caused by the planets, equal to 0.31: and the difference, or 50′′.10 is the general precession in longitude. It is subject to a small secular variation. A complete revolution of the equinoxes is performed in 25.868 years.

A mean solar day, as adopted by the public in this country, is the time employed by the earth in revolving on its axis, as compared with the sun, supposed to move at a mean rate in its orbit, and to make 365.2425 revolutions in a mean Gregorian year. But the mean solar day, adopted by astronomers, is founded on the assumption that the sun makes only 365.2422414 revolutions in a mean Gregorian year. It is divided into 24 mean solar hours; and these are again subdivided into mean solar minutes and se

conds.

.93 13 44

=89 16 44 to the vernal equinox =89 1 33

D. H. M.

186 11 34

=178 18 17

7 17 17

The figure of the Earth is that of an oblate spheriod; the axis of the poles being to the diameter of the equator as 304 to 305.

The mean diameter of the Earth is about 7916 miles: its equatorial diameter is 7924 miles, and its polar diameter 7898 miles.

As a necessary consequence from this circumstance, the degrees of latitude increase in length as we recede from the equator to the poles. But different meridians under the same latitude present different results; the general fact however is well ascertained.

The peculiar phenomena of the seasons, to which we may now call the reader's attention, are occasioned by the annual motion of the earth in its orbit. To understand this, we must bear in mind that the axis of the earth is inclined to the plane of its orbit, and that it always keeps parallel to itself, or is directed constantly to the same point of the heavens.

Let the accompanying figure represent the earth in different parts of its elliptic orbit. In the spring, the circle which separates the light from the dark side of the globe, called the terminator, passes through the poles, as appears in the position a. The apparent day is the time employed by the The earth, then, in its diurnal rotation about its earth in revolving on its axis, as compared with the axis, has every part of its surface as long in light as apparent place of the sun. This day is also divided in shade; therefore the days are equal to the nights into 24 apparent hours; which are again subdivided all over the world; the sun being at that time verinto apparent minutes and seconds. This mode of tical to the equatorial parts of the earth. As the reckoning is still used by the public in many parts earth proceeds in its orbit, and comes into the posion the continent; and is frequently referred to by the tion b, the sun becomes vertical to those parts of practical astronomer on various occasions. In fact, the earth under the tropic, and the inhabitants of the apparent culmination of the sun is the commence- the northern hemisphere will enjoy summer on acment of the astronomical day to every practical as-count of the solar rays falling more perpendicularly

upon them; they will also have their days longer than their nights, in proportion as they are more distant from the equator; and those within the polar circle will have constant daylight. At the same time the inhabitants of the southern hemisphere have winter, their days being shorter than their nights, in proportion as they are farther from the equator; and the inhabitants of the polar regions will have constant night. The earth then continues its course to the position c, when the terminator again passes through the poles, and the days and nights are equal. After this the earth advances to the position d, at which time the inhabitants of the northern hemisphere have winter, and their days are shorter than their nights.

In summer, when the earth is at b, the sun is farther from it than in winter, and in fact, the disc of the sun appears longer in the winter than in the summer. The difference of heat is not owing to the sun's being nearer to us, or more remote, but to the degree of obliquity with which its rays strike any part of the earth. (See HEAT and SUN.)

The moon is the constant attendant of the earth, and revolves around it in 27 days, 7 hours; but the period from one new or full moon to another, is about 29 days 12 hours. She is the nearest of all the heavenly bodies; being only about two hundred and forty thousand miles distant from the earth. She is much smaller than the earth; being only about 2160 miles in diameter.

scores of these circular plains, most of which are considerably below the level of the surrounding country, may be perceived with a good telescope, on every region of the lunar surface.

The preceding view of the moon's disc is taken from Russel's great map, and will furnish a tolerable notion of its rugged and unequal surface as seen through a telescope.

By the observations made by Dr. Herschel, in November, 1779, and the four following months, we learn that the altitude of the lunar mountains has been very much exaggerated. His observations were made with great caution, by means of a Newtonian reflector, 6 feet 8 inches long, and with a magnifying power of 222 times, determined by experiment; and the method which he made use of to ascertain the altitude of those mountains, which during that time he had an opportunity of examining, seems liable to no objection. The rock situated near Lacus Niger, was found to be about one mile in height, but none of the other mountains, which he measured, proved to be more than half of that altitude; and Dr. Herschel concludes that, with a very few exceptions, the generality of the lunar mountains do not exceed half a mile in their perpendicular elevation.

To Dr. Herschel we are also indebted for an account of several burning volcanoes, which he saw at different times in the moon. In the 77th vol. of the Phil. Trans. he says, "I perceive three volcanoes in different places of the dark part of the new moon. Two of them are nearly extinct; or, otherwise in a state of going to break out. The third showed an actual eruption of fire, or of luminous matter." On the next night, Dr. Herschel saw the volcano burn with greater violence than on the preceding evening. He considered the eruption as resembling a small piece of burning charcoal when it is covered by a thin coat of white ashes, which frequently adhere to it, when it has been some time ignited, and it had a degree of brightness about as strong as that with which such a coal would be seen to glow in faint daylight.

[graphic]

The surface of the moon when viewed with a telescope, presents an interesting and variegated aspect; The phases of the moon, as they appear at eight being diversified with mountains, valleys, rocks, and different points of her orbit, are represented in the plains, in every variety of form and position. Some accompanying figure, where s represents the sun, the of these mountains form long and elevated ridges, re-earth being in the centre, and a b c, &c., the moon's sembling the chains of the Alps and the Andes; while orbit. When the moon is at k, in conjunction with a variety of others, of a conical form, rise to a great the sun, her dark side being entirely towards the height, from the middle of level plains, somewhat resembling the Peak of Teneriff. But the most singular feature of the moon is, those circular ridges and cavities which diversify every portion of her surface. A range of mountains of a circular form, rising three or four miles above the level of the adjacent districts, surrounds, like a mighty rampart, an extensive plain; and, in the middle of this plain or cavity, an insulated conical hill rises to a considerable elevation. Several RTS & SCIENCES.-VOL. I.

earth, she will be invisible, as at a, and is then called
the new moon. When she comes to her first octant
at i, a quarter of her enlightened hemisphere will be
turned towards the earth, and she will then appear
When she has run through the
horned, as at h.
quarter of her orbit, and arrived at q, she shows us
the half of her enlightened hemisphere, as at g, when
it is said she is one half full. At p she is in her se-
cond octant, and by showing us more of her enlight-

K

flux will be diminished; these are called neap tides. The sun being farther from our hemisphere in March and September, than in February and October, is the cause why the greatest tides happen a little before the vernal, and a little after the autumnal equinox.

ened hemisphere than at g, she appears gibbous. I moon's first and third quarters, then the flux and reAt her opposition at o, her whole enlightened side is turned towards the earth, when she appears round as at e, and she is said to be full; having increased all the way round. On the other side she decreases again all the way from e to a; thus, in her third octant, part of her dark side being turned towards the earth, she again appears gibbous. At m When the moon is in the equator, the tides are she appears still farther decreased, showing again ex-equally high in both parts of the lunar day; but as actly one-half of her illuminated side. But when she she declines towards either pole, the tides are altercomes to her fourth octant, she presents only a quar-nately higher or lower in northern or southern latiter of her enlightened hemisphere, and again appears horned. And at a, having now completed her course, she again disappears, and becomes a new moon again, as at first.

account of the narrowness of the inlets by which they communicate with the ocean.

tudes. The tides are so retarded in their passage through channels, and so affected by capes and headlands, as to happen variously at different places. The tide raised in the German Ocean, when the moon is The Tides form an exceedingly interesting part of three hours past the meridian, takes three hours to the subject of physical astronomy. The ocean covers arrive at London Bridge. Lakes have no tides, bemore than half the globe; and this large body of cause every part is attracted alike. The Mediterwater is in continual motion, ebbing and flowing al-ranean and Baltic seas have but small elevations on ternately, that is, if the tide be supposed at high water mark, it will, after a short period, subside, and flow back for about six hours, when it will be at low water mark. The time of high water, however, is not always the same, but is about three-quarters of an hour later each succeeding day, for near thirty days, when it begins as before. Thus we may suppose at a certain place, it is high water at three o'clock in the afternoon on the day of new moon; the next day it will be high water at three-quarters of an hour after three, the day following at half-past four, and so on till the next new moon, when it will be again high water at three. This answers to the motion of the moon; for she rises every day about three-quarters of an hour later than the preceding one; and thus completes her revolution round the earth in about thirty days.

According to the Newtonian principle of attraction, these phenomena are thus explained. The waters at d, on the side of the earth next the moon e, are more

attracted than the central parts a c, and these again move more than the waters on the opposite side at b, therefore the distance between the earth's centre and the waters on its surface under and opposite the moon will be increased. To explain this more fully, it should be borne in mind, that though the earth's diameter bears a considerable proportion to its distance from the moon, yet this diameter is nothing when compared to the earth's distance from the sun, consequently the difference of the sun's attraction on the sides of the earth opposite to him will be far less than the difference of the moon's attraction on the sides opposite to her; therefore the moon must raise the tides higher than they could be by the sun. Sir Isaac Newton has determined that the influence of the sun in this case is three times less than that of the moon. The tides, then, are properly the joint production of the sun and moon; or, in fact, there are two tides, a solar and a lunar, whose effects are joint or opposite, according to the situation of the bodies by which they are affected. When the sun and moon act together, as at new and full moon, the flux and reflux become considerable; and are called spring tides. But when one tends to elevate the waters, and the other to depress them, as at the

Having examined the phenomena of the tides in connection with the attractive influence of the moon, it will now be advisable to observe the effects of that body when its orbit or path brings it between our planet and the sun, or vice versa, in which case an eclipse is produced.

Eclipses, especially of the sun, have always been considered as events of the most portentous kind. Isaiah, and others of the sacred writers, speak of them as indicative of the wrath of the Almighty. Homer, Pindar, Pliny, and many others of the ancients, also make mention of them in a similar way; and it is used to be noticed, more particularly by the superstitious, that an eclipse was often accompanied by a national calamity, or an occurrence of a striking nature, the malevolent effects of which were to continue, for the sun, as many years as the eclipse lasted hours, and for the moon as many months. Dionysius of Halicarnassus remarks, that both at the birth and death of Romulus there was a total eclipse of the sun, during which the darkness was as great as at midnight. It is also said that there was a solar eclipse on the day the foundation of Rome was laid.

An eclipse of the moon is mentioned by Ptolemy to have been observed by the Chaldeans at Babylon 720 years before the birth of our Saviour; the middle of the eclipse reducing the time to the meridian of Paris, was 6 hours, 48 minutes, March 19th. From this eclipse it is determined that the mean revolution of the moon is 27 days, 7 hours, 43′ 5′′. This is considered the first eclipse of the moon on record.

Thales rendered himself famous by foretelling an eclipse of the sun; he, however, only predicted the year in which it would happen, and this he was probably enabled to do by the Chaldean Saros, a period of 223 lunations. This eclipse is rendered remarkable by its happening just as the armies under Alyattes, king of Lydia, and Cyaxeres the Mede, were engaged; and being regarded by each party as an evil omen, inclined both to make peace: it has been clearly proved that this eclipse occurred 610 years before Christ, September 30. Xenophon observes, that the king of the Persians laid siege to the city of Larissa at the time the empire was taken from the Medes, but was not able by any means to make himself master of it; finally, a cloud coming over the sun made it disappear, so that the hearts of the inhabitants failed, and the city was taken. This cloud

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