单项选择题

In the horizon system of coordinates what is equivalent to the equator on the Earth?（）.

A.Prime vertical circle
B.Principal vertical circle
C.Parallels of altitude
D.Horizon

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单项选择题

A.be circumpolar
B.have a declination equal to or greater than your latitude
C.have a GHA of 180°
D.have the SHA equal to or less than the LHA

单项选择题

A.sunrise
B.1200 LMT
C.sunset
D.2400 LMT

单项选择题

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