पञ्चसिद्धान्तिका (आचार्य वराहमिहिर - सूर्य, रोमक, पौलिश, वासिष्ठ एवं पितामह सिद्धान्त)

IV. 21 IV. THREE PROBLEMS 93 Fig. IV. 6 NESWZ is the sky-sphere. It is the sky as seen by an observer on the earth who fancies himself stationary, though taking part in the rotation of the earth, and thinks that the stellar sphere is rotating on the axis joining the celestial poles, NP, SP. NESW is the horizon marked by the cardinal points, North, East, South, West. (Note that the east and west points are interchanged so as to appear as we see them when looking up at the sky.) Z is the zenith, corresponding in the sky to the observer's position on the earth, and is the point where the line from the centre of the earth, through the observer, joins the sky-sphere. NNPZOS is the meridian. EZW is called the prime vertical. ENP is part of what is called unmaṇḍalam. ZSH is part of the vertical circle from the zenith to the horizon passing through the sun, moon, etc. (Note that the prime vertical is the vertical circle passing through the east and west points, and that the meridian itself can be considered as a vertical circle passing through the North and South points.) ZS is the zenith distance of S, and HS is the altitude. S₁SS₂ is the diurnal circle, the apparent path of S daily, due to earth's rotation. The stellar sphere (Fig.4) can be recognised here by the celestial equator ErROW, by the ecliptic EcrSC, by the north celestial pole NP, by the position of the Sun S, etc. and by the declination circle. NPSR, of which SR is the arc of declination. Because of the position of the observer on the earth with reference to the terrestrial North pole, the celestial North pole (NP) seems lifted up along the meridian from the north-point (N) so that its altitude is equal to the latitude of the place, and by this the celestial equator is depressed southward by the same amount from the prime vertical.