In celestial navigation, a mathematical expression describing a direction with relationship to the center of the galaxy. A heading is composed of two numbers measuring an azimuth value and an elevation value in degrees. A heading of 000, mark 0 describes a direction toward the geometric center of the galaxy. In terms of navigation on a planet's surface, this is analogous to describing a direction in degrees from north, in which case a course of 5 degrees would be slightly to the right of a direction directly toward the planet's north pole. A heading differs from a bearing in that it has no relationship to the current attitude or orientation of the spacecraft.