268 Astronomy. [Lecture 17 



cording as the body is on the north or south side of the 

 ecliptic. 



The longitude of a heavenly body is an arch of the 

 ecliptic intercepted between the first of aries and the point 

 of it, which is cut by a secondary to the ecliptic passing 

 through the heavenly body. 



The armillary sphere is an instrument composed of the 

 principal circles which are usually drawn upon an artificial 

 globe. 



The colures are two secondaries to the equinoctial ; the 

 one passing through the equinoctial points, and called the 

 equinoctial colure, the other passing through the solstitial 

 points, and called the solstitial colure. 



The ecliptic is a great circle of the sphere, in which the 

 sun always appears to move, so called because eclipses ge- 

 nerally happen when the moon is in or near this circle. 

 The obliquity of the ecliptic is the angle it makes with 

 the equator, which is now about twenty-three degrees 

 twenty-eight minutes. This angle varies within very 

 narrow limits. 



The equinoxes are the two points where the ecliptic 

 cuts the equator, so called because when the sun is in 

 either of these situations the days and nights are equal to 

 each other all over the globe. 



The geocentric place of a planet is that position which 

 it has when seen from the earth, or, strictly from the 

 earth's centre. 



The terminator is that great circle which divides the 

 enlightened hemisphere from the dark hemisphere of any 

 planet. 



The heliocentric place of a planet is that in which it 

 would appear to a spectator placed in the sun's centre. 



The sextile is an aspect of two heavenly bodies when 

 they are sixty degrees distant from each other, and is de- 

 noted in an ephemeris by #. 



