单项选择题
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
A.Hercules B.Cygnus C.Aquila D.Capricorn
A.superior conjunction B.inferior conjunction C.opposition D.None of the above; the elongation will never be zero