. Astronomy for high schools and colleges . lieavenly bodies are continually changing, and no onealtitude or azimuth is true except for a particular momentand for a particular place. Other circles of referencemust, therefore, be sought when we desire to make use ofco-ordinates which shall be permanent. § 4. THE DIURNAL MOTION. The diurnal motion is that apparent motion of the sun,moon, and stars from east to west, in consequence ofwhich they rise and set. The term diurnal is applied because the motion repeatsitself day after day. The diurnal motion is caused by a daily revolution ofthe earth o
. Astronomy for high schools and colleges . lieavenly bodies are continually changing, and no onealtitude or azimuth is true except for a particular momentand for a particular place. Other circles of referencemust, therefore, be sought when we desire to make use ofco-ordinates which shall be permanent. § 4. THE DIURNAL MOTION. The diurnal motion is that apparent motion of the sun,moon, and stars from east to west, in consequence ofwhich they rise and set. The term diurnal is applied because the motion repeatsitself day after day. The diurnal motion is caused by a daily revolution ofthe earth on an axis passing through its centre, called theaxis of the earth. This axis intersects the earth at twoopposite poles called the north and south poles of theearth. If the earths axis be continued indefinitely in both direc- 16 ASTBOmMT. tions, it intersects the celestial sphere in two oppositepoints, called celestialjpoles. The north celestial pole corresponds to the north end ofthe earths axis; the south celestial pole to the south Fig. 5. The Earths Axis and the Plane of the Equator.—iVP, the North Pole; SP,the South Pole; E Q, plane of the Equator ; e g, the terrestrial Equator. The plane J^ Q, passing through the centre of the earthat right angles to the axis, is called the plane of theequator. The plane of the equator intersects the surface of theearth in a circle e q, called the terrestrial or geographicaleqicator, passing through certain countries and oceans, astaught in geography. When the plane of the equator is continued out in-definitely so as to reach the celestial sphere it meets thelatter in a great circle, called the celestial equator. The celestial equator is everywhere half way betweenthe two celestial poles and 90° from each. The celestialpoles are, therefore, the poles of the celestial equator. THE DIURNAL MOTION. ^1 Apparent diurnal motion of tJie celestial sphere.—The observer on tlie earth being nnconscious of its revo-lution, the celestial sp
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