單項選擇題A star that suddenly becomes several magnitudes brighter and then gradually fades is a（）.

A.double star
B.variable star
C.nova
D.nebula


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1.單項選擇題A star is observed at lower transit. The line of position derived from this sight is（）.

A.on the prime vertical
B.a latitude line
C.a longitude line
D.of no special significance

2.單項選擇題A sidereal day is shorter than a solar day. This difference is due to（）.

A.irregularities in the daily rotational rate of the Sun
B.the space motion of the solar system
C.the precession of the equinoxes
D.the use of different reference points

3.單項選擇題A sidereal day is approximately how much shorter than a solar day?（）

A.4 minutes
B.8 minutes
C.12 minutes
D.16 minutes

4.單項選擇題A sextant having an index error that is off the arc has a（）.

A.positive correction
B.dip error
C.negative correction
D.semidiameter error

5.單項選擇題A semidiameter correction is applied to observations of（）.

A.Mars
B.the Moon
C.Jupiter
D.All of the above