. Astronomy for high schools and colleges . as the angle subtended by an object is small, wemay regard it as varying directly as the linear magnitudeof the body, and inversely as its distance from the ob-server. A line seen perpendicularly subtends an angleof 1° when it is a little less than 60 times its length dis-tant from the observer (more exactly when it is 5Y-3lengths distant) ; an angle of V when it is 3438 lengthsdistant, and of V when it is 206265 lengths numbers are obtained by dividing the number ofdegrees, minutes, and seconds, respectively, in the cir-cumference, by
. Astronomy for high schools and colleges . as the angle subtended by an object is small, wemay regard it as varying directly as the linear magnitudeof the body, and inversely as its distance from the ob-server. A line seen perpendicularly subtends an angleof 1° when it is a little less than 60 times its length dis-tant from the observer (more exactly when it is 5Y-3lengths distant) ; an angle of V when it is 3438 lengthsdistant, and of V when it is 206265 lengths numbers are obtained by dividing the number ofdegrees, minutes, and seconds, respectively, in the cir-cumference, by 2 X 3-14:159265, the ratio of the circum-ference of a circle to the radius. PLANES AND CIRCLES OF A SPHERE. o Planes and Circles of a Sphere.—Let Fig. 1 representthe outline of a sphere, of which 0 is the centre. Imaginea plane ^ ^ to pass through the centre O and cut thesphere. This plane will divide the sphere into two equalparts called liemispheres. It will intersect the sphere ina circle A E B F^ called a great circle of the Fig. 1.—sections op a sphere by planes. Through 0 let a straight line P 0 P he passed per-pendicular to the plane. The points P and P\ in whichit intersects the surface of the sphere, are everywhere 90°from the circle A E B F, They are called poles of thatcircle. Imagine another plane C E D F^ to cut the sphere in agreat circle. Its poles will be Q and Q. The following relations between the angles made by thefigures will then hold ; I. The angle P Q hetween the poles will he equal to theinclination of the planes to each other, II. The arc B D^ which measures the greatest distancehetween the two great circles^ will he equal to this sameinclination, III. The points Eand F^ in which the two great circlesintersect each other^ are the poles of the great circle P Q AC P QB D, which pass through the poles of the first circle. © m 2© or s SYMBOLS AND ABBREVIATIONS. SIGNS OF THE PLANETS, ETC. The Earth. Mars. Jupit
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