ASTRONOMY. 41 



two points of it that are intercepted by the ecliptic, 

 called the equinoctial points. The solstitial colure 

 is the other great circle at right angles to the 

 equator, cutting it in the solstitial points. It 

 passes through the poles of the ecliptic. 



If smaller circles of the sphere be described 

 touching the solstitial points, and at right angles 

 to the axis, as M C, B D, they are tropics, of which 

 that on the south side of the equator is called the 

 tropic of Capricorn, and that on the north side of 

 the equator the tropic of Cancer. The two polar 

 circles F G, I K, are at the same distance from the 

 two as the tropics are from the equator, that is, 23-§-°. 



It is necessary here to mention the difference 

 between what is called the sensible and rational 

 horizon. If you suppose that part of the surface 

 of the earth on which you stand to be a plane, and 

 to be extended every way till it reach the heavens, 

 this plane forms the sensible horizon. The rational 

 horizon is a circle whose plane is parallel to the 

 former, but passing through the centre of the 

 earth. Though the globe of the earth appears so 

 large to those who inhabit it, yet it is so small 

 when compared with the immense sphere of the 

 heavens, that the distance between the sensible 

 and rational horizons is nothing in comparison 

 with it. 



The zodiac is a broad portion of the heavens, 

 which stretches about 8° on each side of the 

 ecliptic ; it is divided into twelve parts, called 

 signs, and each sign into 30°. If you imagine a 

 number of great circles of the sphere standing at 

 right angles to the plane of the ecliptic, and, con- 

 sequently, intersecting each other in its poles, 

 these are called circles of celestial longitude, and 



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