सिद्धान्तशिरोमणि: गणिताध्याय (भास्कराचार्य - ग्रहगणित, मध्यमाधिकार व स्पष्टाधिकार सटीक)

86
south pole respectively. These two points evidently do
not move, though the earth is herself rotating in the
clockwise direction. If O be the centre of the earth, OP₁
produced meets the skies at a point called the north celes-
tial pole and OP₂ produced meets the skies in the point
called the south pole. It so happens that the north celes-
tial pole is very near a star which is called the pole star.
This star is known as the Dhruva-Tāra in as much as it does
not appear to move at all while all the heavens (i.e. all the
stars of the sky) appear to be revolving round the earth
rising and setting as seen at any place. (The word Dhruva
means fixed). Aryabhaṭāchārya mentioned in so many
words that it is the earth that really rotates and so the
stars which are themselves fixed appear to be going round
the earth in circles parallel to the circle ABC. अनुलोमगतिः
नौस्थः पश्यत्यचलं विलोमगं यद्वत्। अचलानि भानि तद्वत्समपश्चिमगानि
लङ्कायाम्॥ (Explained under verse 7 Bhagaṇādhyāya). The
celestial Equator is the great circle which is the circle
of intersection of a plane perpendicular to the polar axis
and passing through the earth's centre with the celestial
sphere (celestial sphere is the sphere-like surface which shape
the Sky takes and on which the stars and the planets appear
to be studded.) Similarly the terrestrial equator ABC is
the circle of intersection of the earth's globe with the same
plane. Thus the terrestrial and the celestial equators are
concentric coplanar circles with the earth's centre as the
common centre. A great circle of a sphere is a circle whose
plane passes through its centre. Thus ABC is a great circle
on the earth's surface, because its plane passes through the
earth's centre. Similarly the celestial equator is a great
circle of the skies. The circle QST is called a small circle,
just as the diurnal circle traced by any star in its diurnal
rotation is also a small circle parallel to the celestial equa-
tor. Thus small circles, an infinity of them can be drawn
parallel to the terrestrial equator ABC and they will be in
decreasing dimension as we proceed towards the pole.
Hence the Sphutaparidhi or the rectified circumference