why does circle of ilumination do not coincide with the axis of the earth ??????
Figure 3.1 of the chapter, Motions of the Earth, illustrates that the earth is inclined at an angle of 23.5 degrees from the vertical line. If the earth was not inclined at an angle of 23.5 degrees from the vertical then the Circle of illumination and and the axis of earth would have coincided. Thus the Inclination of the earth's axis with respect to the vertical line is the reason for circle of illumination not coinciding with the axis of earth.