round the earth, besides attending the earth in its annual journey round the sun. The surface of the moon is greatly diversified with inequalities, which through a telescope have the appearance of hills and valleys. Astronomers have drawn the face of the moon as viewed through a telescope, distinguishing the dark and shining parts by their proper shades and figures. Each of the spots on the moon has been marked by a numerical figure, serving as a reference to the proper name of the particular spot which it represents ;* as Herschel's volcano; 1, Grimaldi; 2, Galileo, &c.; so that the several spots are named from the most noted astronomers, philosophers, and mathematicians. The best and most complete picture of the moon is that drawn on Mr. Russel's lunar globe. Dr. Herschel informs us that, on the 19th of April, 1787, he discovered three volcanoes in the dark part of the moon; two of them appeared nearly extinct, the third exhibited an actual eruption of fire, or luminous matter. On the subsequent night it appeared to burn with greater violence, and might be computed to be a bout three miles in diameter. The eruption resembled a piece of burning charcoal, covered by a thin coat of white ashes: all the adjacent parts of the volcanic mountain were faintly illuminated by the eruption, and were gradually more obscure at a greater distance from the crater. That the surface of the moon is indented with mountains and caverns, is evident from the irregularity of that part of her surface which is turned from the sun; for, if there were no parts of the moon higher than the rest, the light and dark parts of her disc, at the time of the quadratures would be terminated by a perfectly straight line; and at all other times the termination would be an elliptical line, convex towards the enlightened part of the moon in the first and fourth quarters, and concave in the second and third; but, instead of these lines being regular and well defined when the moon is viewed through a telescope, they appear notched and broken in innumerable places. It is rather singular that the edge of the moon, which is always turned * Vince's Astronomy. towards the sun, is regular and well defined, and at the time of full moon no notches or indented parts are`seen on her surface. In all situations of the moon, the elevated parts are constantly found to cast a triangular shadow with its vertex turned from the sun; and, on the contrary, the cavities are always dark on the side next the sun, and illuminated on the opposite side: these appearances are exactly conformable to what we observe of hills and valleys on the earth; and even in the dark part of the moon's disc, near the borders of the lucid surface, some minute specks have been seen, apparently enlightened by the sun's rays; these shining spots are supposed to be the summits of high mountains, which are illuminated by the sun, while the adjacent valleys nearer the enlightened part of the moon are entirely dark. *Supposing this to be the fact, astronomers have determined the height of some of the lunar mountains. The method made use of by Riccioli (though it gives the true result only at the time of the quadratures) is here explained, because it is much more simple than the general method given by Dr. Herschel in the Philosophical Transactions for 1780. Let ADB (Plate IV. Fig. 7.) be the disc or face of the moon at the time of the quadratures, ACB the boundary of light and darkness; MO a mountain in the dark part, the summit M of which is just beginning to be enlightened, by a ray of light SAM from the sun. Now, by means of a micrometer, the ratio of MA to AB may be determined; and as AC is the half of AB, and MAC a right-angled triangle, by Euelid 1 and 47th/AC+AM2=CM from which take CO=AC, and the remainder MO, is the height of the mountain. Riccioli observed the illuminated part of the mountain St. Catharine, on the fourth day after the new moon, to be distant from the illuminated part of the moon about one-sixteenth part of the moon's diameter, viz. MA one-sixteenth of AB, or one-eighth of AC; now, if we take the moon's diameter 2144 miles, as we have before determined, the height of this mountain will be miles! Galileo makes MA =1-20th of AB; and Hevelius makes MA=1-26th of AB; the former of these will give the height of the mountain 5 miles, and the latter 3 miles. Dr. Herschel thinks, "That the heights of the lunar mountains are in general greatly overrated, and that the generality of them do not exceed half a mile in their perpendicular elevation." On the contrary, M. Schroeter, a learned astronomer of Lilienthal, in the duchy of Bremen, says, that there are mountains in the moon much higher than any on the earth, and mentions one above a thousand toises higher than Chimboraco in South America. The same author has likewise lately published a new work on the heights of the mountains of Venus, some of which he makes upwards of twenty-three thousand toises in height, which is above seven times the height of Chimboraco! 8 Whether the moon has an atmosphere or not, is a question that has long been controverted by various astronomers; some endeavour to prove that the moon has neither an atmosphere, seas, nor lakes; while others contend that she has all these in common with our earth, though her atmosphere is not so dense as ours. It cannot be expected in an introductory treatise, where general received truths only ought to be admitted, that we should enter into the discussion of a controverted question; however, it may be proper to inform the student, that the advocates for an atmosphere, if we may be allowed to reason from analogy, have the advantage over those who contend that there is none. It is admitted on all hands, that the moon has mountains and valleys, like the earth, and appears nearly the same with respect to shape and the nature of her motions; may we not then fairly infer that she is similar to the earth in other respects. V. OF MARS . Mars appears of a dusky red colour, and though he is sometimes apparently as large as Venus, he never shines with so brilliant a light. From the dulness and ruddy appearance of this planet, it is conjectured that he is encompassed with a thick cloudy atmosphere, through which the red rays of his light penetrate more easily than the other rays. This being the first planet without the orbit of the earth, he exhibits to the spectator different appearances to Mercury and Venus. He is sometimes in conjunction with the sun, like Mercury and Venus, but was never known to transit the sun's disc. Sometimes he is directly opposite to the sun, that is, he comes to the meridian at midnight, or rises when the sun sets, and sets when the sun rises; at this time he shines with the greatest lustre, being nearest to the earth. Mars, when viewed through a telescope, appears sometimes full and round, at others, gibbous, but never horned. The foregoing appearances clearly shew, that Mars moves in an orbit more distant from the sun than that of the earth. The apparent motion of this planet, like that of Mercury and Venus, is sometimes direct, or from east to west; at others retrograde, or from west to east; and sometimes he appears stationary. Sometimes he rises before the sun, and is seen in the morning; at others he sets after the sun, and of course is seen in the evening. Mars revolves on his axis in 24 hours 39 minutes 22 seconds; and his polar diameter is to his equatorial diameter as 15 to 16, according to Dr. Herschel; but Dr. Maskelyne, who carefully observed this planet at the time of opposition, could perceive no difference between his axes. The inclination of the orbit of Mars to the plane of the ecliptic is 1° 51'; the place of his ascending node about 180 in Taurus;* his horizontal parallax is said to be 23' 6 he performs his revolution round the sun in 686 days 23 hours 15 minutes 44 seconds; and his apparent semi-diameter, at his nearest distance from the earth, is 25''; consequently his mean distance from the sun is 144907630+ miles; his diameter 4318 miles; and his magnitude a little more than 14th of that of the earth.‡ This planet travels round the sun at the rate of 55223 miles per hour; and the parallax of the earth's annual * The longitude of the ascending node of Mars for the beginning of the year 1750 was 17° 38′ 58′′ in Taurus, and its variation in 100 years is 46′ 40′′. Vince's Astronomy. + For, 686 days 23 hours 15 min. 44 sec.=59354144 seconds, the square of which is 3522914409972736, this divided by 995839704797184 the seconds in a year (see the note page 127) gives 3.537632, the cube root of which is 1.523716, the relative distance of Mars from the sun. Hence, 1.523716 × 23882.84-36390.6654 distance of Mars from the sun in semi-diam. of the earth, and 36390.6654 × 3982=144907629.6 miles the mean distance of Mars from the sun. Now, if the horizontal parallax of Mars at the time of opposition be 23" .6 as stated by M. de la Caille, we have (see Plate IV. Fig. 6) Sine PSO-sine 28" .6 Is to PO one semi-diameter As radius sine of 90° Is to SO=8741.93 semi-diam. 6.0583927 0 0000000 10.0000000 3.9416073 Hence. the distance of Mars from the earth at the time of opposition is 8741.93 of the earth's semi-diameters; 8741.98: 25′′:: 23882.84: 9" .15 the apparent diameter of Mars if seen from the earth at a distance equal that of the sun; then 32′ 2′′: 886149:9′′ .15: 4218 miles the diame ter of Mars. The cube of 7964, the diameter of the earth, is 505119057344 : and the cube of 4218, the diameter of Mars is 75044648232; the quotient produced by dividing the former by the latter, is 6.73. viz. the magnitude of the earth is nearly seven times that of Mars. For, 113: 355:: 144907630 X 2: 910481569 miles the circumference of the orbit of Mars, and 686 days 23 h. 15 m. 44 sec.; 910481569 m.:: 1 h.: 55223 miles. orbit, as seen from Mars, is about 41 degrees. As the distances of the interior planets from the sun are found by their elongations, so the distances of the exterior planets may be found by the parallax of the earth's annual orbit.* VI. OF THE NEW PLANETS, CERES, PALLAS, JUNO, AND VESTA. 1. On the first of January 1801, M. Piazzi, Astronomer Royal at Palermo, in the island of Sicily, discovered a new planet between the orbits of Mars and Jupiter (generally called Ceres Ferdinandia, from the island in which it was discovered, and Ferdinand IV. King of the Two Sicilies.) The elements of the theory of this planet are at present very imperfectly known: it appears like a star of the eighth magnitude (consequently it is invisible to the naked eye,) its distance from the sun is said to be about 24 times that of the earth, and its periodical revolutions nearly four years and eight months. This planet, called by some astronomers an asteroid, is not confined within the ancient limits of the zodiac. Its diameter, according to Dr. Herschel, is about 162 miles. 2. On the 28th of March 1802, Dr. Olbers of Bremen, while examining some of the stars near the new discovered planet, Ceres Ferdinandia, perceived a star of the seventh magnitude, situated near the northern wing of the constellation Virgo, which had the appearance of a planet. By continuing his observations, he * In Plate IV. Fig. 8. let S represent the sun, E the earth, and M Mars; now, as the earth moves quicker in its orbit than Mars. the planet Mars will appear to go backward when the earth passes it. Thus, when the earth is at E, Mars will appear among the fixed stars at m; but as the earth passes from E to e, Mars will appear to go from m to n, though he is in reality travelling the same way as the earth from M to 0 The place m where Mars is seen from the earth among the fixed stars, is called his Geocentric place, but the place P, where be would be seen from the sun, is called his Heliocentric place, and the arc mP, which is the difference between his apparent and true place, is called the Parallax of the earth's annual orbit Now as this angle may be determined from observation, and is known to be about 41°; in the right angled triangle SEM. we have given SE—23882.84 semi-diameters, the distance of the earth from the sun, the angle SME measured by the arc m P=41°, to find SM-36403.49 semi-diameters of the earth, the distance of Mars from the sun. |