Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: of all parallels still nearer to that pole. Also the parallel HL, whose distance from the invisible pole is equal to the latitude, is entirely below the horizon; and the same is true of all parallels still nearer to that pole. Hence, in the United States, stars within a certain distance of the north pole never set, and stars at an equal distance from the south pole never rise. The circle BO is called the circle of perpetual apparition, because the stars which are included within it never set. The radius of this circle is equal to the latitude of the place. The circle HL is called the circle of perpetual occultation, because the stars which are included within it never rise. The radius of this circle is also equal to the latitude of the place. The celestial sphere here described is called an oblique sphere, the diurnal motion being oblique to the horizon. Whether the sphere be right or oblique, one half of the celestial equator will be below the horizon, and the other half above it . Every object on the equator will therefore be above the horizon during as long a time as it is below, and will rise and set at the east and west points. 34. Effects of centrifugal force.—We have discovered that the earth has a globular figure, and that it rotates upon its axis once in 24 sidereal hours. But, since the earth rotates upon an axis, its form can not be tliat of a perfect sphere; for every body revolving in a circle acquires a centrifugal force which tends to make it recede from the centre of the circle. Every particle, P, upon the earth's surface acquires, therefore, a force which acts in a direc- ng.7. N tion, EP, perpendicular to the axis of rotation. This centrifugal force, a which we will represent by PA, may be resolved into two other forces PB and PD, one acting in ...