SECTION XXI. 317 It is true, that in a considerable length of time these tables will want further correction, because of those 44 minutes which are really wanting to make up the super-added day in the leap- year. But these tables will serve sufficiently-for any common operations for forty or fifty years to come, provided you always consult that table which is applicable to the current year, whe- ther it be a leap -year, or the first, the second, or the third year after it. IV. Observe also these tables of the sun's declination are sometimes reduced (as it were) to one single scale. And for this purpose men generally choose the table of declination for the second aftef leap-year, and this is called the mean declination, that is, the middle between the two leap - years. This is that ac- count of the sun's place and declination, &c. which is made to be represented on all mathematical instruments, viz. globes, quadrants, projection of the sphere, and graduated scales, &c. and this serves for such common geometrical practices in astronomy without any very remarkable error. Concerning the table of the fixed stars, let it be remember- ed that they move slowly round the round the globe eastward in circles parallel to the ecliptic, and therefore they increase their longitude 50 seconds of a minute every year, that is, one degree in seventy-two years. But their latitudenever alters, because they always keep at the same distance from the ecliptic. Let it be noted also, that this slow motion of the fixed stars causes their declination and their right ascension to vary (though very little) everyyear. Their right ascension necessarily changes because their longitude changes, though not exactly in the same quantity. And though their latitude never alters, because lati- tude is the distance from the ecliptic, yet their declination must alter a little, because it is their distance from the equator. But the tables of their right ascension, which I have here exhibited, will serve for any common practices for at least twenty years to come, and their declination for near fifty years, without any sen- sible error in such astronomical essays as these. It may be proper here to give notice to learners, that the same stars may have north latitude and south declination; such are all those that lie between the equator and the southern half of the ecliptic ; but all those stars which lie between the equator and the northern half of the ecliptic, have south latitude and north declination.
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