单项选择题
The angle at the pole measured through 180°from the prime meridian to the meridian of a point is known as（）.
A.the departure B.the polar arc C.longitude D.Greenwich hour angle
单项选择题 The altitude at LAN may be observed by starting several minutes in advance and continuing until a maximum altitude occurs. This procedure should not be used（）.
单项选择题 Superior conjunction occurs when（）.
单项选择题 Sidereal time is used by navigators when（）.
