lefs, appearing falcated or horned; after paffing the inferior conjunction, the planet is again feen horned, but the illuminated part then increases, and at the greatest elongation, half its difc is again feen enlightened. In the fuperior part of its orbit, as its elongation decreases, its face becomes more full and round, till the fuperior conjunction, after which time it is again diminished by the fame gradation as its increase was in the former cafe accomplished. There is no difficulty in accounting for this variety of phases, it being occafioned by the different pofitions of Venus with refpect to the Sun and Earth: for as the enlightened face of Venus muft of course be always oppofite to or facing the Sun, it will be more or less vifible to us according to our fituation at various times.

The furface of Venus is diverfified with spots like E our Moon, by the motion of which it is determined, that it revolves on its axis from west to eat in the fpace of twenty-three hours. When the air is in a good ftate for this kind of obfervations, mountains like thofe in the Moon may be discerned, with a very powerful telescope. Late obfervations of Herschel have however rendered thefe accounts uncertain *.

The face of the planet Mars is always round and F full, as its fuperior fituation requires, excepting at the time of the quadrature, or elongation of 90 degrees, when a small part of the unenlightened hemi

VOL. I.

* Philofophical Transactions, 1793.

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sphere

I

G'

sphere being turned towards us, its difc appears like the Moon about three days after the full.

By the fpots on Mars, its diurnal revolution is afcertained in the direction from weft to east. From the ruddy and obfcure appearance of this planet, as well as from other appearances, it is concluded, that its atmosphere is nearly of the fame denfity as that of the Earth. Mr. Herfchel has obferved, that two circles furrounding the poles of this planet are very white and luminous, probably from fnow lying there.

H. We have already had occafion to speak of the fatellites of Jupiter and Saturn. The annual parallax of these planets is not confiderable enough to bring any fenfible part of their dark hemifpheres towards us in any pofition of elongation; confequently their faces are always round and full.

The telescopic appearance of Jupiter affords a vaft field for the curious enquirer. It is in general encircled with one or more obfcure belts or bands parallel to the plane of its orbit, and confequently to each other. These are not regular or constant in their appearance. They have been feen to the number of five, and during the time of observation two have gradually disappeared. Sometimes but one is feen; and fometimes, when the number is more confiderable, one or more dark fpots are formed between the belts, which increase till the whole is united in one large dufky band. The spots of Jupiter are the brighter parts of its furface, and are not permanent, though more fo

than

than the belts; yet it is found that they re-appear after certain unequal intervals of time. The remarkable spot, by whofe motion the rotation of Jupiter on its axis was determined, disappeared in 1694, and was not feen again tilì 1708, when it re-appeared exactly in the fame place on its furface, and has been occafionally feen ever fince.

It has been conjectured, that these belts are k feas, and that the variations obferved both in them and the spots are occafioned by tides, which are differently affected, according to the positions of his moons. It is probable, however, that they are in its atmosphere. If an obferver, poffeffed of skill and patience equal to the task, would delineate the phases of Jupiter for the space of a periodical revolution, noting at the fame time the positions of his fatellites, this opinion might be either eftablished or rejected: but at all events fuch a series of obfervations could not fail to throw great light on the fubject.

The very great distance of the planet Saturn, and L the tenuity of its light, do not permit common obfervers to distinguish those varieties which it is probable are on its furface. Herschel's telescopes fhew belts on its furface. Thefe are generally pa rallel to the ring. Saturn is found to revolve on an axis perpendicular to the plane of the ring, and its diameter is fhorter than that which is measured in the plane of the ring in the proportion of 10 to II. The ring itself is inclined to the ecliptic; in confequence of which, its apparent

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parent figure is continually varying. When the line of its nodes points directly towards the Earth, the ring, presenting its edge to the obferver, becomes invifible to common telescopes: if the fame line points directly towards the Sun, the ring becomes invifible for want of illumination: and laftly, if the plane of the ring paffes between the Sun and the Earth, the ring cannot then be feen, because its dark fide is towards us. At all other, times its figure is that of an oval, which is broader or narrower accordingly as the line of the nodes is farther from or nearer to the above positions.

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CHA P. XIII.

OF THE LENGTH OF DAYS AND NIGHTS, AND OF

THE SEASONS.

WE have seen that every planet which is accesfible to obfervation has a revolution on its axis ; the intention of which is, undoubtedly, to give alternate night and day to every part of their furfaces. An inclination of the axis of any planet to the axis of its orbit, by caufing the length of days and the intensity of heat to vary, will occafion a viciffitude of feafons. On this account Jupiter, whose axis is nearly perpendicular to the plane of its orbit, has equal days and nights on every part of its furface at the fame time, the days being four hours and twenty-eight.minutes, and the nights of the fame length. But the planets Mars and Venus, whofe axes are

inclined

inclined to the planes of their respective orbits, have each an annual change of feafons and length of days. The Earth, for the fame reafon, has a fimilar viciffitude, the explanation of which will render it unneceffary to enlarge on the circumstances of the other planets,

For this purpose it will be useful to define thofe N imaginary circles, which aftronomers and geographers have invented for the purposes of methodizing and facilitating the communication of science.

On the Earth a great circle, fupposed to be o drawn at an equal distance from each pole, is termed the Equator: lefs circles drawn parallel to the equator are called Parallels of Latitude; and great circles interfecting the equator at right angles, and confequently paffing through the poles, are called Meridians. But when the meridian of a place is fpoken of, it is ufually understood to be a femicircle paffing through the given place, and terminating at the poles. The other half which completes that whole circle, is then called the oppofite meridian.

In the heavens, a great circle, parallel to the P equator, is termed the Celeftial Equator; but the lefs circles parallel to it are called Parallels of Declination; and the great circles interfecting it at right angles, and paffing through the celeftial poles of the Earth, are called Hour Circles, or circles of right afcenfion.

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The ecliptic is that great circle in the heavens, in which the Sun describes its apparent annual