单项选择题
An amplitude of the Sun in high latitudes（）.
A.is most accurate before sunrise B.is most accurate after sunset C.should only be observed when the Sun's lower limb is above the horizon D.is most accurate when the Sun's center is observed on the visible horizon
单项选择题 Altair is found in what constellation?（）
单项选择题 After Venus passes the point of greatest elongation east in its orbit，the first position in which the elongation will be zero is（）.
单项选择题 A variable star is one that（）.
