SECTION IX. 243 north and south poles, they would have only one day of six months long, and one night of six months, in a whole year, ac- cording as the sun is on this or the other side of the equator; for the sun moving slowly in the ecliptic on the north side of the equator half a year, would be all that time above the horizon to the inhabitants at the north pole, though it went round them daily : and the sun moving in the ecliptic on the south side of the equator half a year, would be below their horizon all that time. The same might be said concerning the inhabitants of the south pole. The two equinoctial days, or when the sun is in the points aries, or libra, the day and night are equal all the world over ; and this is true in a sense to those who live under thepoles ; for thecentre of the sun is in their horizon. Thus half the sun is above their horizon, and half below it for 24 hours together. Thus, though the polar inhabitants begin to lose the sun at the autumnal equinox, theyare not in utter darkness all the time of the sun's absence : For the twilight lasts till the sun is 18 degrees below their horizon, and that is till he has 18 degrees of declination. The inhabitants of the north pole are therefore without the twilight only from the 2nd of November till the 18th of January. Let it be noted also that the refraction of the rays through the thick air or atmosphere makes the sun appear above their horizon several days sooner, and disappear several days later, than otherwise it would do. It may be added in favour of their habitations too, that the moon whentyshe is brightest, (viz.) from the first quarter to the last, does not set during their middle of winter: For in that part of her month she is most opposite to the sun, and is therefore in that part of the heavens whichis most distant from the sun while he never rises. The parallels of the sun's declination in this position of the sphere are all parallel to the horizon ; and are the same with the parallelsof his altitude, and therefore his highest altitude with them can never exceed 231 degrees. The stars that they could see would be always the same, making perpetual revolutions round them, and never set nor rise, nor be higher or lower. And the planets during half their periods will be above their horizon, as Saturn 15 years, Jupiter 6, Mars, 1, 85.c. III. An oblique sphere, figure x. is where the latitude or elevation of the pole is at any number of degrees less than 90. Therefore all the inhabitants of the earth (except under the equator and the poles) have an oblique sphere. Here the equator and all the parallels of declination cut the horizon obliquely, therefore the sun and stars always rise and set at oblique angles with the horizon. Asone pole of the world is always in their view, and the @ 2
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