OBSERVER'S RATIONAL HORIZON
Is a grate circle on the celestial sphere every point on which is 90 degrees away from his Zenith.The plane of the observer's rational horizon,therefor passes through the center of the earth.
VISIBLE HORIZON
Is a small circle on the earth surface bounding the observer's field of vision at sea.
The radius of the visible horizon increase as the observer's height of eye increase.
Is a small circle on the celestial sphere,the plane of which passes through the observer's rational horizon.
