CELESTIAL LATITUDE AND LONGITUDE. 99 



Stars is called the celestial equator. Now the celestial equa- 

 tor does not coincide with the ecliptic, but makes an angle 

 with it of twenty-three degrees and twenty-eight minutes, 

 that is, the axis of the earth is not perpendicular to the plane 

 of the ecliptic, but is inclined twf^nty-three degrees and 

 twenty-eight minutes. Thus we have two great circles, the 

 ecliptic and equator, passing through the heavens eastwardly 

 and westwardly, from either of which the latitude of the hea- 

 venly bodies might be estimated. But astronomers have se- 

 lected the ecliptic for this purpose, and have supposed lines 

 or circles to cross it at right angles, as the meridians do the 

 equator ; which lines or circles are called secondaries to the 

 ecliptic. The points where all the secondaries meet, are 

 called i\\Q poles of the ecliptic ; which points are twenty-three 

 degrees twenty-eight minutes from the celestial poles. Hence 

 the latitude of a heavenly body is its distance from the 

 ecliptic, measured on a secondary to the ecliptic ; and like 

 latitude on the earth, it can never exceed ninety degrees. 

 The longitude of a heavenly body is the distance of a se- 

 condary to the ecliptic, reckoned from some given uniform 

 secondary, called the prime secondary. But the longitude 

 of heavenly bodies, unlike longitude on the earth, is reckon- 

 ed only easticard; it may extend, therefore, to three hun- 

 dred ajid sixty degrees. It is usually stated in signs, degrees, 

 minutes, and so forth ; and the prime secondary, from which 

 it is reckoned, cuts the ecliptic in the beginning of the sign 

 Aries, a point where the celestial equator crosses the ecliptic. 

 If a secondary, for instance, passing through a heavenly 

 body, cuts the ecliptic eighteen degrees in the sign Capri- 

 corn, then, since the first point of Capricorn is nine signs 

 eastward from the first point of Aries, the longitude of that 

 body is nine signs, eighteen degrees. But it is often impor- 

 tant to know the distance of a heavenly body from the celes- 

 tial equator, as well as from the ecliptic. This distance is 

 its declination, and is reckoned on a meridian, as latitude is 

 on the earth. Its distance from the beginning of Aries, 

 reckoned on the equator, is its right ascension ; which, like 

 celestial longitude, is reckoned through the whole circle, or 

 three hundred and sixty degrees. Two planets are said to 

 be in conjunction with each other, when they have the same 

 longitude, or are in the same degree of the ecliptic on the 

 same side of the heavens, though their latitude be different. 



