单项选择题
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
单项选择题 A sidereal day is approximately how much shorter than a solar day?（）
单项选择题 A sextant having an index error that is off the arc has a（）.
单项选择题 A semidiameter correction is applied to observations of（）.
