EQ 



EQU 



always in the same ratio to each other, as 

 the simple quantities before multiplica- 

 tion : thus, if 6 and 8 are multiplied by 4, 

 the equimultiples 24 and 32 will be to 

 each other as 6 to 8. 



EQUINOCTIAL, in astronomy, a great 

 circle of the celestial globe, whose poles 

 are the poles of the world. It is so call- 

 ed, because, whenever the sun comes to 

 this circle, the days and nights are equal 

 all over the globe ; being the same with 

 that which the sun seems to describe at 

 the time of the two equinoxes of spring 

 and autumn. All stars directly under this 

 circle have no declination, and always 

 rise due east, and set full west. The 

 hour circles are drawn at right angles to 

 it, passing through every fifteenth de- 

 gree ; and the parallels to it are called 

 parallels of declination. 



EQUINOX, the time when the sun en- 

 ters either of the equinoctial points, 

 where the ecliptic intersects the equi- 

 noctial. It was evidently an important 

 problem in practical astronomy, to de- 

 termine the exact moment of the sun's 

 occupying these stations ; for it was na- 

 tural to compute the course of the year 

 from that moment. Accordingly, this 

 has been the leading problem in the as- 

 tronomy of all nations. It it susceptible 

 of considerable precision, without any 

 apparatus of instruments. It is only ne- 

 cessary to observe the sun's declination 

 on the noon of two or three days before 

 and after the equinoctial day. On two 

 consecutive days of this number, his de- 

 clination must have changed from north 

 to south, or from south to north. If his 

 declination on one day was observed to 

 be 21' north, and on the next 5' south, it 

 follows that his declination was nothing, 

 or that he was in the equinoctial point 

 about 23 minutes after 7 in the morning 

 of the second day. Knowing the pre- 

 cise moments, and knowing the rate of 

 the sun's motion in the ecliptic, it is easy 

 to ascertain the precise point of the 

 ecliptic in which the equator intersected 

 it. By a series of such observations 

 made at Alexandria, between the years 

 161 and 127 before Christ, Hipparchus, 

 the father of our astronomy, found that 

 the point of the autumnal equinox was 

 about six degrees to the eastward of the 

 star called spica virginis. Eager to de- 

 termine every thing by multiplied obser- 

 vations, he ransacked all the Chaldean, 

 Egyptian, and other records to which 

 his travels could procure him access, for 

 observations of the same kind ; but he 

 does not mention his having found any. 



VOL. V. 



He found, however, some observations of 

 Aristillus and Timochares, made about 

 150 years before. From these it appeared 

 evident that the point of the autumnal 

 equinox was then about eight degrees 

 east of the same star. He discusses 

 these observations with great sagacity 

 and rigour ; and, on their authority, he 

 asserts that the equinoctial points are not 

 fixed in the heavens, but move to the 

 westward about a degree in 75 years, or 

 somewhat less. 



This motion is called the procession of 

 the equinoxes, because by it the time and 

 place of the sun's equinoctial station pre- 

 cedes the usual calculation : it is fully 

 confirmed by all subsequent observations. 

 In 1750, the autumnal equindx was ob- 

 served to be 20 21' westward of spica 

 virginis. Supposing the motion to have 

 been uniform during this period of ages, 

 it follows that the annual precession is 

 about 50" and one-third ; that is, if the 

 celestial equator cuts the ecliptic in a 

 particular point on any day of this j ear, 

 it will on the same day of the following 

 year cut it in a point 50" and one-third to 

 the west of it, and the sun will come to 

 the equinox 20' 23" before he has com- 

 pleted his round of the heavens. Thus 

 the equinoctial, or tropical year, or true 

 year of seasons, is so much shorter than 

 the revolution of the sun, or the sidereal 

 year. It is this discovery that has chiefly 

 immortalized the name of Hipparchus, 

 though it must be acknowledged that all 

 his astronomical researches have been 

 conducted with the same sagacity and in- 

 telligence. It was natural, therefore, 

 for him to value himself highly for the 

 discovery. It must be acknowledged to 

 be one of the most singular thai has been 

 made, that the revolution of the whole 

 heavens should not be stable, but its axis 

 continually changing. For it must be 

 observed, that since the equator changes 

 its position, and the equator is only an 

 imaginary circle, equidistant from the 

 two poles, or extremities of the axis, 

 these poles, and this axis, must equally 

 change their positions. The equinoctial 

 points make a complete revolution in 

 about 25,745 years, the equator being all 

 the while inclined to the ecliptic in near- 

 ly the same angle. Therefore the poles 

 of this diurnal revolution must describe 

 a circle round the poles of the ecliptic, 

 at the distance of about 23 degrees in 

 25,745 years ; and in the time of Timo- 

 chares, the north pole of the heavens 

 must have been 30 degrees eastward of 

 where it now is. 



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