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

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∴ Kalā = (H cos h × H cos δ) / R II
∴ Iṣṭa-yaṣṭi = [(H cos h × H cos δ) / R] × [(H cos φ) / R] III
Now Unmaṇḍala-Śaṅku is derivable from the sixth
latitudinal triangle in which Krāntijyā is the Karṇa and
Unmaṇḍala-Śaṅku is the Bhuja. Comparing it with the
second latitudinal triangle Krāntijyā / R = U. S. / (H sin φ) where
U. S. is Unmaṇḍala-Śaṅku.
∴ U. S. = (Krāntijyā × H sin φ) / R = (H sin φ H sin δ) / R IV
∴ Iṣṭa-Śaṅku as per the above formulation is given by
Iṣṭa-Śaṅku (I. S.) =
(H sin δ H sin δ) / R + (H cos φ H cos δ H cos h) / R² = H cos z V
[चित्र: गोलीय त्रिभुज PZS (Spherical triangle PZS) — शीर्ष: Z (zenith), P (celestial pole), S (celestial body / Sun); भुजाएँ: ZP = 90 - φ, ZS = z, PS = 90 - δ; कोण: ∠PZS = 90 - A, ∠ZPS = h, ∠PSZ = η]
(Ref. fig. 55) Fig. 55 Formulae for △PZS
Z = zenith ; P = celestial pole. S = celestial body, say,
the Sun.
z = zenith-distance of the celestial body S.