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436
ASTRONOMY.
on them at firſt; between which power and force there is ſo exact an adjuſtment, that they continue in the ſame tracks without any ſolid orbits to confine them.
Mercury, the neareſt planet to the ſun, goes round him (as in a circle marked Plate XXXIX. fig. i.) in 87 days 23 hours of our lime nearly; which is the length of his year. But, being ſeldom ſeen, and no ſpots appearing on his ſurface or diſk, the time of his rotation on his axis, or the length of his days and nights, is as yet unknown. His diſtance from the ſun is computed to be 32 millions of miles, and his diameter 2600. In his courſe round the ſun, he moves at the rate of 95 thouſand miles every hour. His light and heat from the ſun are almoſt ſeven times as great as ours; and the ſun appears to him almoſt ſeven times as large as to us. The great heat on this planet is no argument againſt its being inhabited; ſince the Almighty could as eaſily ſuit the bodies and conſtitutions of its inhabitants to the heat of their dwelling, as he has done ours to the temperature, of our earth. And it is very probable that the people there have ſuch an opinion of us, as we have of the inhabitants of Jupiter and Saturn; namely, that we muſt be intolerably cold, and have very little light at ſo great a diſtance from the ſun.
This planet appears to us with all the various phaſes of the moon, when viewed at different times by a good teleſcope; excepting only that he never appears quite full, becauſe his enlightened ſide is never turned directly towards us but when he is ſo near the ſun as to be loſt to our ſight in its beams. And, as his enlightened ſide is always toward the ſun, it is plain that he ſhines not by any light of his own; for if he did, he would conſtantly appear round. That he moves about the ſun in an orbit within the earth’s orbit is alſo plain, (as will be (ſhewn afterwards), becauſe he is never ſeen oppoſite to the ſun, nor above 56 times the ſun’s breadth from his centre.
His orbit is inclined ſeven degrees to the ecliptic; and that node from which he aſcends northward above the ecliptic is in the 14th degree of Taurus; the oppoſite, in the 14th degree of Scorpio. The earth is in theſe points on the 6th of November and 4th of May, new ſtyle; and when Mercury comes to either of his nodes at his inferior conjunction about theſe times, he will appear to paſs over the diſk or face of the ſun, like a dark round ſpot; but in all other parts of his orbit his conjunctions are inviſible, becauſe he either goes above or below the ſun.
Mr Whiſton has given us an account of ſeveral periods at which Mercury may be ſeen on the ſun’s diſk, viz. in the year 1783, Nov. 12th, at 3 h. 44 m. in the afternoon ; 1786, May 4th, at 6 h. 57 m. in the forenoon; 1789, Dec. 6th, at 3 h. 55 min. in the afternoon; and 1799, May 7th, at 2 h. 34 in. in the afternoon. There will be ſeveral intermediate tranſits, but none of them viſible to us.
Venus, the next planet in order, is computed to be 59 millions of miles from the ſun ; and by moving at the rate of 69 thouſand miles every hour in her orbit, (as in the circle marked ♀), ſhe goes round the fun in 224 days 17 hours of our time nearly. But though this be the full length of her year, as ſhe performs only 91⁄4 revolutions on her own axis in that time, her year conſiſts only of 91⁄4 days; ſo that in her, every day and night together is as long as 241⁄3 days and nights with us. This odd quarter of a day in every year makes every fourth year a leap-year to Venus; as the like does to our earth. Her diameter is 7906 miles; and by her diurnal motion the inhabitants about her equator are carried 43 miles every hour, beſides the 69,000 above mentioned.
Her orbit includes that of Mercury within it; for at her greateſt elongation, or apparent diſtance from the ſun, ſhe is 96 times his breadth from his centre; which is almoſt double of Mercury’s. Her orbit is included by the earth’s; for if it were not, ſhe might be ſeen as often in oppoſition to the ſun, as ſhe is in conjunction with him; but ſhe was never ſeen 90 degrees, or a fourth part of a circle, from the ſun.
When Venus appears weſt of the fun, ſhe riſes before him in the morning, and is called the morning-ſtar; when ſhe appears eaſt of the ſun, ſhe ſhines in the evening after he ſets, and is then called the evening-ſtar; being each in its turn for 290 days. It may perhaps be ſurpriſing at firſt, that Venus ſhould keep longer on the eaſt or weſt of the ſun, than the whole time of her period round him. But the difficulty vaniſhes when we conſider, that the earth is all the while going round the ſun the ſame way, though not ſo quick as Venus; and therefore her relative motion to the earth muſt in every period be as much ſlower than her abſolute motion in her orbit, as the earth during that time advances forward in the ecliptic, which is 220 degrees. To us ſhe appears, through a teleſcope, in all the various ſhapes of the moon.
The axis of Venus is inclined 75 degrees to the axis of her orbit; which is 511⁄2 degrees more than our earth’s axis is inclined to the axis of the ecliptic; and therefore her ſeaſons vary much more than ours do. The north pole of her axis inclines toward the 20th degree of a quarius, our earth’s to the beginning of Cancer; conſequently the northern parts of Venus have ſummer in the ſigns where thoſe of our earth have winter, and vice verſa.
The artificial day at each pole of Venus is as long as 112-y natural days on our earth.
The fun’s greateſt declination on each ſide of her equator amounts to 75 degrees; therefore her tropics are only 15 degrees from her poles, and her polar circles as far from her equator. Confequently, the tropics of Venus are between her polar circles and her poles; contrary to what thoſe of our earth are.
As her annual revolution contains only 91⁄4 of her days, the fun will always appear to go through a whole ſign, or twelfth part of her orbit, in little more than three quarters of her natural day, or nearly in 183⁄4 of our days and nights.
Becauſe her day is ſo great a part of her year, the ſun changes his declination in one day ſo much, that if he paſſes vertically, or directly over head of any given place on the tropic, the next day he will be 26 degrees from
