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VENUS.

VENUS is the next planet in order, which revolves round the sun from west to cast, moving at the rate of eighty thousand miles per hour, and performing her periodical revolution in 224, 16, 49, at the distance of 68,891,486 miles from the sun. She has all the phases and peculiarities of motion which we have observed in Mercury; from which we conclude, in the same manner, that she is a solid globe, shining only with borrowed light from the sun; moving in an orbit included by the orbit of the earth; having an inferior and superior conjunction with the sun, but no opposition; putting on all the phases of the moon; sometimes appearing on the east side of the sun, when she becomes our evening star, and again, on the west side of him, when she is our morning star; being each for 290 days together; appearing stationary between her conjunctions, at her eastern and western elongations, when she is distant from the sun about 48°; and passing over the face of the sun, when her inferior conjunction happens about the time when she is in her nodes, that is about 14° of Gemini and Aquarius.* Her transits do not happen so frequently as those of Mercury. They happen in the months of May and November, when the earth is in her nodes, and generally in eight years after one transit, wanting two days, there will be another, and then not again for near a century: for eight years contain 2922 days; which are

* N.B. The place of the descending node of Venus in 1786, August 25, was 8, 14o, 44', 38", by Mr. Bugge, and the annual motion of the nodes is 30" 37. The inclination of her orbit S 23' 38" 6.

nearly equal to thirteen revolutions of Venus, or 2920 days. Three only have yet been observed, viz. in 1639, 1761, 1769, and the next will be seen in Nov. 1874; after which no other will happen till May 1996, and in eight years more there will be another in May 2004.

That her orbit includes the orbit of Mercury is evident from this, that her greatest elongation is about 48°, whereas that of Mercury is but about 23°.

From the spots observed on her surface, astronomers have determined her rotation round her axis to be performed in 24 days and 8 hours; so that she has but 9 days in her year; for dividing 224d 16h, by 24d 8h, the quotient is 9 nearly. This quarter of a day will make every fourth year a leap year in Venus, as it is with us, which will be more fully explained hereafter.

By the same spots, we have found that her axis, on which her diurnal rotation is performed, is inclined to the axis of her annual orbit, in an angle of 75 degrees, her north pole inclining to the 20th degree of Aquarius, whereas that of our earth inclines to the first degree of Cancer. This will make her summers to be at the same time with our winters, and vice versa. As her axis lies almost in the plane of her orbit, differing from it only 15 degrees, her north pole going foremost in her annual course, her motion resembles that of a ball shot from a rifle, which has a rotatory motion round that axis, which lies in the direction of its motion. Hence those stars, which to us seem to have the slowest motion, appear to her inhabitants to move with the greatest velocity, and vice versa, her north pole being but a few degrees out of the ecliptic.

As the sun alters his declination 75 degrees in Venus north and south of her equator, her tropics must

be within 15 degrees of her poles, and her polar circles at the same distance from her equator. This will occasion a remarkable peculiarity in her seasons. At her equator, and polar circles, there will be two springs, two summers, two autumns, and two winters; because, when the sun is at her tropics, his rays will be more oblique at her equator, than they are with us in the middle of winter, which will occasion a winter there when with us it would be the middle of summer. And the case is nearly the same at her polar circles. At her poles, there is one spring and autumn, a summer as long as them both, and a winter as long as the other three seasons. The difference of heat and cold will be much greater in Venus at her poles, than with us, as the sun comes within 15 degrees of the zenith and is again 75 degrees below the horizon. As there are but 9 days in her year, in which time the sun changes his declination 150 degrees, if he pass vertically over any place on any day, he will pass many degrees from it on the next, which will give the heated regions time to cool. From this great change of declination in the course of a single day, the longitude may be as easily found in Venus, as the latitude is with us.

That Venus is surrounded with an atmosphere of considerable density, sufficient to refract the sun's rays, was very evident from her appearance, at her internal contact with the limb of the sun, at her last transit; for when she was fully advanced upon the sun's disk, still his limb did not appear quite to surround her, for a few seconds, until her atmosphere also advanced upon him. She appeared to adhere to his limb by a black and dark ligament, which after a few seconds suddenly broke, and left the sun's limb perfectly bright, when

that part of her atmosphere passed his limb, which was dense enough to refract his light.

As she is furnished with the regular returns of day and night, with all the variety of seasons, and an atmos phere for the support of aniinal life, she is probably designed to be the residence of living creatures, whose constitutions are adapted to the temperature of their habitation.

As the orbit of Venus is inclined to the orbit of the earth in an angle of 3°, 23′ 38′′, she would appear to a spectator at the center of the sun, to be in the ecliptic, when she was at her node, and to be continually inereasing her distance from the ecliptic, until she arrived at her limit, or 90 degrees from her node, where it would amount to 3°, 23', 5", which is then greatest; from thence it would decrease again till it vanished at her node. The like appearances would take place in the other half of her orbit. These variable distances from the ecliptic, being measured on a secondary of the ecliptic are called her heliocentric latitudes, when supposed to be viewed from the center of the sun; but the same arches of the secondary, when viewed from the center of the earth, are called her geocentric latitudes, and must appear of different lengths, according to the earth's different distances from Venus. The same difference takes place between the heliocentric and geocentric latitudes of any other planet.

The geocentric latitude of a planet therefore will be continually variable on two accounts, viz. first, it will increase or decrease directly, with the heliocentric latitude; and secondly, it will appear greater as the earth's distance from the planet grows less: so that it will be directly as the heliocentric latitude, and inversely as the earth's distance from the planet. When

Venus is in her inferior conjunction she is nearer to the earth than she is to the sun, and therefore her geocentric latitude will then be greater than her heliocentric latitude. The same will be the case with all the superior planets at their oppositions. But when Mercury is in his inferior conjunction, he is still nearer to the sun than he is to the earth, and therefore his geocentric latitude at that time will be less than his heliocentric latitude; and the same will be the case with all the superior planets at their conjunctions with the sun.

Venus does not appear brightest, when most of her enlightened disk is turned towards us. Her brightness depending on the quantity of light reflected to the eye, and this quantity being directly as the enlightened part of her disk, and inversely as the square of her distance from the earth, she will appear brightest when this quantity becomes a maximum.

THE EARTH.

THE earth on which we live is the next planet which revolves round the sun as the center of her motion. She revolves from west to east in 365d 6 9 from any star to the same again, at the distance of 95,173,117 miles from the sun, moving at the rate of 68,217 miles every hour in her orbit, which is nearly 150 times as fast as a cannon ball. The plane of its orbit, called the ecliptic, we make the standard with which the orbits of the other planets are compared. Besides her annual motion in her orbit, she turns round on her axis once in every 23h 56' 4', which produces an apparent motion of all the celestial bodies from east to west in the same time. Her axis is inclined to the axis of her orbit in an angle of 23° 28', and points to a star, nearly, which

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