On the celestial sphere, what are the circles that pass through Zenith from the celestial sphere called?

Latitude

Horizon

Vertical circles

Longitude

Correct Answer : Vertical circles

Explanation : Vertical circles are the circles on the celestial sphere that passes through Zenith or Nadir from the celestial horizon. They are half circles made on the celestial sphere. Latitudes & Longitudes are the features pertaining to the planet Earth. Horizon is the great circle halfway of zenith & nadir.

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How many rotations are required to transform the ECI coordinate system to the perifocal system?

Correct Answer : Option (D) : 3

Explanation : The coordinate transformation from XYZ frame to X’Y’Z’ frame is carried out by a single rotation about one axis. The other two axes remain in the same plane. But to transform from the ECI coordinate system to the perifocal system, there have to be successive 3 rotations. The first rotation is about Z axis, second is through X’ axis and the final rotation is bout Z’’ axis.