ASTRONOMY.

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us, in form of a spot. An eye in Mars will sec Venus at aboutthe same distance from the Sun as Mercury appears to us, and theEarth about the same distance from the Sun that Venus appearsto us; and when the Earth is found in conjunction with ami verynear the Sun, the eye in Mars will see the Earth horned or falcated,and its attendant, the Moon , of the same figure, and at its utmostdistance from the Earth not above iifteen minutes of a degree : andas this planet’s di-tanee.from the Sun is to the distance of the Earth from the Sun as 152309 to 100000 , a spectator in Mars would see theSun ’s diameter less by about one-third than it appears to us, andconsequently the degree of light which Mars receives from theSun, is lets by five-ninths than that received by the Earth ; litisproportion will, however, admit of a sensible variation, on accountof the great eccentricity of the orbit of Mars . Though the periodor year of this planet, as ha; been already observed, is nearly twiceas long as ours, and his natural day, or the time in which tile Sun appears above itis horizon (setting aside the consideration of twi-light) is almost every where equal to itis night; yet it appearsthat in one and the same place, on Itis surface, there will be butvery little variety of seasons, or scarcely any difference of summerand winter: and the reason is, that the axis of his diurnal rotation isnearly at right angles with the plane of Itis orbit. It will be found,notwitlistanding, that places situated in dillereut latitudes, that is,at different distances from his equator, will have very diftirrent de-grees of heat, on account of the different inclination of the Sun ’srays to the horizon, as it is with us when the Sun is in the equi-noxes. From this consideration Dr. Gregory endeavoured to ac-count for the appearance of the fascia? in Stars , which are certainswaths or fillets seen in this planet, and posited parallel to Itis equa-tor; for as among us, the same climate has at different seasonsvery unequal degrees of heat, but as in Mars it is otherwise, thesame parallel having always a pretty equable degree of heat, itfollows, that these spots may probably be foruie<(| in Mars , or inhis atmosphere, as snow and clouds are in ours, viz. bv the con-stant different intensities of iieat and cold in the different parallels,and so come to be extended in circles or belts parallel to his equa-tor, or the circle of itis diurnal motion. And this same principlemay, perhaps, solve the phenomenon of Jupiter ’s belts, that planet,like Mars , having a perpetual equinox.—The above opinion ofDr. Gregory, and the supposition upon which it was founded, hasbeen entirely superseded by the accurate observations of the ce-lebrated Dr. Ilerschel upon this planet. In 1781, this great astro-nomer has observed his spots very minutely, from the motion ofwhich he has found his rotation upon his axis to be performed in£4h. 39m. 21 |s. He says that there cannot be more than twoseconds of uncertainty in this result, and he has also given repre-sentations of the spots of this planet in the Philosophical Transac-tions for 1781, page 134. He afterwards determined the inclina-tion of his equator to be 30° 18' upon the ecliptic, and 28° 42' uponhis orbit, which is only 5° 14'more than the obliquity of our eclip-tic; consequently the different seasons will take place on thisplanet very much like they are known to do upon our Earth , withthis difference, that the seasons there will be almost as long againas with ns, on account of the time that he takes in moving roundthe Sun being nearly twice as long as our year.

(166.) Jupiter has the same general appearance with Mars , onlythat the belts on his surface are much larger and more permanent.Their number is variable ; sometimes only one, at others no fewerthan eigiit may he perceived: their breadth is also variable, insome of these belts large black spot; have appeared, by which it isdetermined, with great accuracy, that this planet, which is thelargest in tire system, revolves on his axis in about 9 hours 56 mi-nutes. The figure of Jupiter is evidently an oblate spheroid, thelongest diameter being to the shortest as 74 to 69. His rotation isfrom west to east, and has been observed to be somewhat quickerin his aphelion than in his perihelion ; the inclination of his equatorto his orbit is very small, so that lie lias scarcely any difference ofseasons. Jupiter has four moons, which constantly revolve roundhim as lie proceeds in his course round the Sun ; their orbits arenearly circular, and make a small angle with the ecliptic, and withthe plane of Jupiter ’s orbit. It is evident, from this account of thesystem of Jupiter and his satellites, that occultations of them mustfrequently happen by their going behind their primary, or by com-ing in betwixt us and it. The former takes place when they pro-ceed towards tiie middle of their upper semicircle ; the latter wiienthey pass through the same part ol their inferior semicircle. . Oc-voi. i.— no. 20.

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cultations of tiie former kind happen to the 1st and 2nd satellite;at every revolution, the third very rarely escapes an occupation,but the fourth more frequently by reason of its greater distance.It is seldom that a satellite can lie dhcovered upon the disk of Ju-piter , even by the best telescopes, excepting at its first entrance,when, by reason of its being move directly illuminated by the raysot the Sun than the [danet itself, it appears like a lucid spot uponit. Sometimes, however, a satellite, in passing over the disk, ap-pears like a dark spol, and is easily to be distinguished. This issupposed to be owing to spots on the body of these secondary pla-nets : and it is remarkable, tiiat the same'satellite lias been knownto pass over tiie disk at one time as a dark spot, and at another soluminous that it could not be distinguished from Jupiter himself,except at Its coming on and going off'.

(167.) When these moons pa.s through their inferior semicir-cles, they ca-t a shadow upon Jupiter , and thus cause an eclipse oftiie Sun to his inhabitants; and in some situations this shadow mayhe observed going before or following the satellite. On the otherhand, in passing through their superior semicircles, the satellitesmay be eclipsed, in tiie same manner as our Moon is to us, by pas-sing through the shadow of Jupiter ; and this is actually the casewith tiie first, second, and third of these bodies; but the fourth,by reason of .the largeness of its orbit, passes sometimes above orbelow tiie shadow, as is the ease with our Moon . The beginningsand endings of these eclipses are easily seen by a telescope whentlie Earth is in a proper situation with regard to Jupiter and theSun ; but when tins or any other planet is in conjunction w ith theSun , the superior brightness of that luminary renders both it andtiie satellites invisible. From tiie time of his first appearing aftera conjunction until near tiie opposition, only the immersions oftiie satellites into his shadow, or the beginnings of the eclipse;, arevisible; at the opposition, only the occultations of the satellites,by going behind or coming before their primary, are observable;and from the opposition to the conjunction, only the immersions,or end of tiie eclipses, are to be seen. This is exactly true in thefirst satellite, of which we can never see an immersion" with its im-mediately subsequent emersion: and it is but rarely that they canbe both seen in the second ; as in order to their being so, that sa-tellite must he near one of its limits, at the same time that theplanet is near his perihelion and quadrature with the Sun . Withregard to the third, when Jupiter is more than 46° from conjunc-tion with, or opposition to, tiie Sun, both its immersions and im-mediately subsequent emersions are visible ; as they likewise arein the fourth, when the distance of Jupiter from conjunction oropposition is 24°. When Jupiter is in quadrature with the Sun ,the Earth is farthest out of the line that passes through the centresof tiie Sun and Jupiter , and therefore the shadow of the planet isthen most-exposed to our view ; but even then the body of theplanet will hide from us one side of that part of the shadow whichis neare.-t to it, through which the first satellite passes; which is (hereason that, though we see the entrance of- that satellite into theshadow, or its coming out from thence, as the Earth is situated ontiie east or west side thereof, we cannot see them Doth ; whereastiie other satellites going through the shadow at a greater distancefrom Jupiter , their ingress and egress are botli visible. Dr. ller-schel has lately discovered, from the increase and decrease of thelight of the satellites of Jupiter , that they turn round their axes ;and from the circumstance of their having an equal quantity oflustre in similar parts of their orbits, he infers that the period of ro-tation, in all the satellites, is equal to their sidereal revolution re-spectively.

(168.) Saturn , when viewed through a good telescope, makes amore remarkable appearance than any of the other planets. Galileofirst discovered his uncommon shape, and from the discoveriesmade by him and other astronomers, it appears that this planet issurrounded by a broad thin ring, the edge of which reflects littleor none of the Sun ’s light to us, but the planes of the ring reflecttiie light in the same manner that tiie planet itself does ; and if wesuppose the diameter of Saturn to be divided into 3 equal parts,tlie'diameter of the ring is about 7 of these parts. The ring is de-tached from the body of Saturn in such a manner, that tiie dis-tance between the innermost part of tiie ring and tiie body isequal to its breadth. If we had a view of the planet and his ringwith our eyes, perpendicular to one of the plane; of the lalivr, weshould see'them as in Plate XXII. fig. 9; but our eye is never somuch elevated above either plane as to have the visual ray stand4 K »t