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

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In the beginning of the Triprasnādhyāya, Bhāskara says
“साक्षे देशे खगोलवलयानां, तिरश्चीनभगोलवलयानां च संपातात्त्र्यस्राणि
क्षेत्राण्युत्पद्यन्ते, तान्यक्षक्षेत्रसंज्ञानि” ie. In a place having a
latitude the diurnal paths of stars and planets will be
inclined to the fundamental circles of the celestial sphere
namely horizon, meridian and prime vertical and so their
intersection gives rise to what are called latitudinal tri-
angles, in which the angles would be ϕ, 90-ϕ and 90
where ϕ is the latitude. Thus in fig. 21 the projections
of the triangles ESB, EDB, DSB, EDF, EBF, ESF and
EQG on the meridian plane will be all triangles in which
the angles will be ϕ, 90-ϕ and 90° so that they are all
latitudinal triangles. These are all similar to the funda-
mental gnomonic triangle gmn of fig. 18 wherein also the
angles are ϕ, 90-ϕ and 90°. Here, there is one important
point to be observed. We have said “their projections
are all similar”. In fact the corresponding spherical
triangles enumerated above are all apparently similar,
though they are not viewed in modern astronomy as
regular spherical triangles, because all the three sides of
the triangles are not arcs of great circles. It is not possi-
ble to apply Napier’s rules to these triangles for the reason
mentioned above. None the less, the property of simil-
arity of the projected right angled triangles is made use
of in Hindu Astronomy to obtain the magnitudes of the
sides of the projected triangles.
EW is called the prāk-pratīchī sūtra, SS′ the Uda-
yāsta sūtra; similarly if lines through F, B, D, A parallel
to EW be drawn, these sūtras will intersect the meridian
planes in points which constitute the projected triangles
mentioned above. Let us study these triangles which we
connote by the same letters with lowered indices. Thus
for example E₁ S₁ B₁ is the projected triangle of ESB
where of course E₁ will be the centre of the celestial
sphere. The lines drawn parallel to SS′ or EW through
B, D etc. will be denoted as BB′, DD′, FF′ etc.
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