An equatorial coordinate system (1) which has the plane of the equator and the plane of the local meridian as its geometric elements. A point in both planes is specified as the center to which directions are referred. The coordinates of a celestial body are the angle (called the hour angle) from the plane of the local meridian to a plane perpendicular to the equatorial plane and passing through the celestial body and the center, and the angle from the equatorial plane to the line from the center to the body. The hour angle is accounted positive when measured in a clockwise direction.