How is the Z axis determined?

In the direction of the Winter Solstice

In direction of the Celestial South Pole

In direction of the position of the moon

In direction of the Celestial North Pole

Correct Answer : In direction of the Celestial North Pole

Explanation : The Ecliptic Coordinate system defines its Z axis in the direction of the Celestial North Pole. It is chosen as the reference because it is constant in the Celestial sphere.

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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.