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437
ASTRONOMY
from it; and whatever place he paſſes vertically over when in the equator, one day’s revolution will remove him 361⁄4 degrees from it. So that the ſun changes his declination every day in Venus about 14 degrees more at a mean rate, than he does in a quarter of a year on our earth. This appears to be providentially ordered, for preventing the too great effects of the ſun’s heat, (which is twice as great on Venus as on the earth), ſo that he cannot ſhine perpendicularly on the ſame places for two days together; and by that means the heated places have time to cool.
If the inhabitants about the north pole of Venus fix their fouth or meridian line through that part of the heavens where the ſun comes to his greateſt height, or north declination, and call thoſe the eaſt and weſt points of their horizon, which are 90 degrees on each fide from that point where the horizon is cut by the meridian line, thefe inhabitants will have the following remarkable things.
The ſun will riſe 221⁄2 degrees north of the eaſt; and going on 1121⁄2 degrees, as meaſured on the plane of the horizon, he will croſs the meridian at an altitude of 121⁄2 degrees; then making an entire revolution without ſetting, he will croſs it again at an altitude of 481⁄2 degrees; at the next revolution he will croſs the meridian as he comes to his greateſt height and declination, at the altitude of 75 degrees; being then only 15 degrees from the zenith, or that point of the heavens which is directly over head; and thence he will defeend in the like ſpiral manner, croſſing the meridian firſt at the altitude of 481⁄2 degrees; next at the altitude of 121⁄2 degrees; and going on thence 1121⁄2 degrees, he will let 221⁄2 degrees north of the weft; ſo that, after having been 45⁄8 revolutions above the horizon, he deſcends below it to exhibit the like appearances at the ſouth pole.
At each pole, the fun continues half a year without ſetting in ſummer, and as long without riſing in winter; conſequently the polar inhabitants of Venus have only one day and one night in the year, as it is at the poles of our earth. But the difference between the heat of ſummer and cold of winter, or of mid-day and mid-night, on Venus, is much greater than on the earth; becauſe in Venus, as the ſun is for half a year together above the horizon of each pole in its turn, ſo he is for a conſiderable part of that time near the zenith; and during the other half of the year always below the horizon, and for a great part of that time at leaſt 70 degrees from it. Whereas, at the poles of our earth, although the ſun is for half a year together above the horizon, yet he never aſcends above, nor deſcends below it, more than 231⁄2 degrees. When the fun is in the equinoctial, or in that circle which divides the northern half of the heavens from the ſouthern, he is ſeen with one half of his diſk above the horizon of the north pole, and the other half above the horizon of the ſouth pole; ſo that his centre is in the horizon of both poles: and then deſcending below the horizon of one, he aſcends gradually above that of the other. Hence, in a year, each pole has one ſpring, one harveſt, a ſummer as long as them both, and a winter equal in length to the other three ſeaſons.
At the polar circles of Venus, the ſeaſons are much the ſame as at the equator, becauſe there are only 15 degrees betwixt them; only the winters are not quite ſo long, nor the ſummers ſo ſhort; but the four ſeaſons come twice round every year.
At Venus’s tropics, the ſun continues for about fifteen of our weeks together without ſetting in ſummer, and as long without riſing in winter. Whilſt he is more than 15 degrees from the equator, he neither riſes to the inhabitants of the one tropic, nor ſets to thoſe of the other; whereas, at our terreſtrial tropics, he riſes and ſets every day of the year.
At Venus’s tropics, the ſeaſons are much the fame, as at her poles; only the ſummers are a little longer, and the winters a little ſhorter.
At her equator, the days and nights are always of the ſame length, and yet the diurnal and nocturnal arches are very different, eſpecially when the ſun’s declination is about the greateſt; for then his meridian altitude may ſometimes be twice as great as his midnight depreſſion, and at other times the reverſe. When the fun is at his greateſt declination, either north or ſouth, his rays are as oblique at Venus’s equator, as they are at London on the ſhorteſt day of winter. Therefore, at her equator there are two winters, two ſummers, two ſprings, and two autumns every year. But becauſe the ſun ſtays for ſome time near the tropics, and paſſes ſo quickly over the equator, every winter there will be almoſt twice as long as ſummer; the four ſeaſons returning twice in that time, which conſiſts only of 91⁄4 days.
Thoſe parts of Venus which lie between the poles and tropics, and between the tropics and polar circles, and alſo between the polar circles and equator, partake more or leſs of the phenomena of theſe circles as they are more or leſs diſtant from them.
From the quick change of the ſun’s declination it happens, that if he riſes due eaſt on any day, he will not ſet due weſt on that day, as with us; for if the place where he riſes due eaſt be on the equator, he will ſet on that day almoſt weſt-north-weſt, or about 181⁄2-degrees, north of the weſt. But if the place be in 45 degrees north latitude, then on the day that the fun rifes due eaſt he will ſet north-weſt by weſt, or 33 degrees north of the weſt, and in 62 degrees north latitude. When he riſes in the eaſt, he ſets not in that revolution, but juſt touches the horizon 10 degrees to the weſt of the north point, and aſcends again, continuing for 31⁄4 revolutions above the horizon without ſetting. Therefore, no place has the forenoon and afternoon of the ſame day equally long, unleſs it be in the equator, or at the poles.
The fun’s altitude at noon, or at any other time of the day, and his amplitude at rifing and letting, being very different at places on the ſame parallel of latitude, according to the different longitudes of thoſe places, the longitude will be almoſt as eaſily found on Venus as the latitude is found on the earth; which is an advantage we can never enjoy, becauſe the daily change of the ſun’s declination is by much too ſmall for that important purpoſe.
On this planet, where the ſun croſſes the equator in any year, he will have 9 degrees of declination from that place
Vol. I. No. 19. 3 5S
