Final answer:
A chart position enclosed by a semi-circle represents a running fix, which is used in navigation to determine the position of a moving ship by advancing lines of position to a common time.
Step-by-step explanation:
A chart position enclosed by a semi-circle is a running fix. This is a method used in navigation to establish the position of a moving vessel when it has been impossible to obtain a fix. By marking a series of bearings or lines of position at different times, and then advancing these lines along the vessel's known course and speed to a common time, the intersection of these lines suggests the current location on a chart. This technique is especially useful when the navigator has not been able to obtain multiple bearings at the same time.