单项选择题

In general，the most effective period for observing stars and planets occurs during the darker limit of（）.

A.sunset
B.civil twilight
C.nautical twilight
D.astronomical twilight

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单项选择题

Elongation becomes zero at（）.

A.opposition
B.west quadrature
C.apogee
D.inferior conjunction

单项选择题

During one synodic rotation，a body makes one complete turn relative to the（）.

A.Earth
B.Sun
C.stars
D.vernal equinox

单项选择题

Diurnal aberration is due to（）.

A.motion of the Earth in its orbit
B.rotation of the Earth on its axis
C.the body's orbital motion during the time required for its light to reach the Earth
D.a false horizon