method, the tables of the Sun's mean motion are con-, ftructed. Though the planets above described perform their periods round the Sun, or rather round the centre of gravity, yet many of the planets seen from the Earth will appear to move in a contrary motion to the order of the figns; particularly the inferior planets; and fometimes they may appear stationary, or not to move at all, for feveral nights together. But these appearances are nothing but optical deceptions, arifing partly from the motions of the planets, and partly from the motion of the Earth on which we are placed; for we always judge a planet to be in that part of the ecliptic which is on the oppofite fide of the planet to us; this is called its Geocentric Longitude: but the part of the ecliptic in which the planet is feen by an obferver, supposed to be placed in the Sun, is called the Heliocentric Longitude. And the longitude of any planet or ftar is an arch of the ecliptic, counted from the beginning of Aries to the place where the ecliptic is cut by a circle perpendicular to the ecliptic, and paffing through the ftar or planet. SECT. II. OF THE SECONDARY PLANETS. THE fecondary planets, or fatellites, are certain planets which perform a revolution round any other planet, as the Moon does round the Earth. They are called fatellites, because because they are always found attending their primary pla nets, and making the tour about the Sun together with them. There are but four primary planets that are certainly known to have fatellites: viz. the Earth, Jupiter, Saturn, and the Georgium Sidus; though fome have imagined they have difcovered fatellites attending fome of the other planets, as hath been hinted in the last section; but these observations have not been fufficiently confirmed. The Earth is attended by one fatellite, called the Moon, and marked. She performs her revolution round the Earth in an elliptic orbit, the mean eccentricity of which is one eighteenth part nearly of her mean distance from the Earth, or about 13,000 miles; her mean diftance from the Earth being 60 femidiameters of the Earth; or about 240,000 miles. The mean time of one revolution of the Moon about the Earth, or from one New Moon to another, when the overtakes the Sun again, is 29 days 12 hours 44 minutes 3 feconds 11 thirds. But the mean time in which he moves once round her whole orbit is 27 days 7 hours 43 minutes 8 feconds, which is at the rate of about 2290 miles in an hour. For the Moon has completed one revolution about the Earth before he comes again in conjunction with the Sun; because, while the Moon is performing her revolution, the Earth has advanced about a 13th part of the ecliptic forward. The Moon turns once round her own axis exactly in the time that the goes round the Earth. This is the reafon the fame fide of the Moon is always turned towards the Earth; and day and night in the Moon, taken together, are just as long as a lunar month. The diameter of the Moon is to that of the Earth, as 20 to 73; therefore it is equal to 2180 miles. The farface of the Moon is to that of the Earth as 3 is to 40' 1 to 13 nearly; therefore, the Earth reflects 13 times or as as as much light upon the Moon, as fhe does upon the Earth, when he is at her full. The folid content of the Moon is to that of the Earth as 3 is to 146: the denfity of the Moon's body is to that of the Earth as 5 is to 4; therefore, her quantity of matter is to that of the Earth as 1 is to 39 nearly. The force of gravity on her furface is to that on the Earth as 100 is to 293. The axis of the Moon is almost perpendicular to the plane of the ecliptic; there fore, fhe has little or no difference of seasons. The mean apparent diameter of the Moon is 31 minutes 16 feconds. The various phases and appearances of the Moon have puzzled all the aftronomers of antiquity. Her wanings and increafings, her various pofitions with regard to the Earth, and her frequent eclipfes, were matters of constant admiration. The Moon being a dark spherical body, and fhining only with the borrowed light of the Sun, can only have one half of her body illuminated at the fame time, the oppofite half remaining in its native darkness; therefore, as the Moon performs a revolution round the Earth, fhe will fometimes turn the whole of her illuminated face towards the Earth; at which time the appears perfectly round, and is a full moon: at other times only a certain portion of her illuminated face will be turned towards the Earth; she will then appear either horned, half round, or gibbous, according to the quantity of her illuminated part which is feen by us. To illuftrate this, let ABCDEFGH reprefent the orbit of the Moon, (fig. 9, plate 17.) Now, when the Moon is at A, in conjunction with the Sun, her dark fide will be turned towards the Earth, and therefore the will be invifible, as at a, which is then called the New Moon. When she arrives at B, or has run through one eighth part of her orbit, one quarter of her illuminated face will be turned towards the Earth; fhe will then appear horned, as at b. When the arrives at C, one half of her illuminated face face is turned towards the Earth, as at c, when the is said to be in her quadrature. When the arrives at D, which is called her fecond octant, three parts of her illuminated face will be turned towards the Earth, and the will appear gibbous, as at d. When fhe arrives at E, the whole of her illuminated face is turned towards the Earth, and the appears quite round, as at e, when he is faid to be a full Moon. As the proceeds through the other half of her orbit, she decreases again from e to a, and nearly in the fame ratio, as the increased, in the former half of her orbit. And the Earth has all the fame appearances to an observer in the Moon, as the Moon has to us, but in a contrary order: viz. the Earth being at the full to them, when the Moon changes to us, and vice versa; as is evident from a view of the figure. The motions of the Moon are all very irregular; the only equable motion she has, is the rotation on her own axis in the space of a month, being the time in which the moves round the Earth; which is the reason that he always expofes the fame face towards the Earth. The orbit of the Moon is very changeable, and does not long preferve the fame figure; for though the orbit of the Moon be an ellipfe, having the Earth in one of her foci thereof; yet the eccentricity is fometimes greater than at other times. The plane of the Moon's orbit is inclined to that of the ecliptic, in an angle of five degrees. The face of the Moon has the appearance, when viewed through a telescope, of being diverfified with hills and val lies; this is alio proved to be the cafe, from the edge or border of the Moon appearing jagged, efpecially about the line which feparates the illuminated part of the Moon from the dark fide thereof. The fpots alto of the Moon, which are taken for mountains, are found to caft a triangular fhadow in the direction oppofite to the Sun; and thofe parts parts which are taken for vallies or cavities are always dark on that fide next the Sun, and illuminated on the oppofite fide, which is agreeable to experience. Sometimes the tops of the mountains are feen illuminated by the Sun, white their bafes are in the dark fide of the Moon; and by these means we have a good method of taking the height of the lunar mountains. Thus, let ED (fig. 14) be the Moon's diameter, ECD the line dividing the dark from the illuminated part of the Moon; and A the top of a hill in the dark part, just beginning to be illuminated: with a telescope, take the proportion of AE to the diameter ED, then there are given the two fides AE, EC, of the right-angled triangle AEC; the fquares of the two fides of the right-angled triangle being added together give the fquare of the hypothenuse AC, from the fquare root of which, fubtracting BC, the radius, there remains AB, the height of the mountain. From late obfervations, Dr. Herschel has difcovered that very few of the lunar mountains exceed half a mile in perpendicular height. The fame gentleman has alfo obferved three volcanoes in the Moon, which he thus defcribes: "I perceived (April 19th, 10 hours 36 minutes, fidereal tine) three volcanoes in different parts of the dark part of the New Moon: two of them are either already nearly extinct, or otherwise in a flate of going to break out; which, perhaps, may be decided next lunation: the third shews an actual eruption of fire, or luminous matter; its light is much brighter than the nucleus of the comet which Mr. Mechain difcovered at Paris the 10th of this month." The following night he discovered it burn more violently; and by meafaring, he found the fhining or burning matter to be more than three miles in diameter. The actual fire or eruption of a volcano exactly refembled a fmall piece of burning charcoal, when it is covered by a very thin coat of white afhes; and it had a degree of brightness, VOL. II. G g about |