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

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the rising hour angle and ℎ = L P̂ Q and Chara by the arc
EM and Sūtra = H sin (MN + EM) = H sin (unnata +
chara) = H sin EN = H sin (90 - ℎ) = H cos ℎ.
Verse 55. Sūtra multiplied by Kujyā and divided by
Charajyā will be also Kalā and Kalā multiplied by the Koti
and divided by the Karṇa of any latitudinal triangle will
be Iṣṭa yaṣṭi.
Comm. In as much as Kalā is the corresponding line
in the diurnal circle to the Sūtra in the plane of the
celestial equator
Sūtra / Kalā = R / (H cos δ) = Charajyā / Kujyā ∴ (Sūtra × Kujyā) / Charajyā = Kalā
In verse 33, we saw that
Dinārdha Sanku — Unmandala Sanku = Yaṣṭi.
This Iṣṭa-yaṣṭi will be therefore the perpendicular dropped
from 𝑛, the Sun's position in the diurnal circle on the
plane parallel to the horizon and passing through the
head of the Unmandala Sanku ie. passing through B and
parallel to the horizon in fig. 21. Since the angle between
the diurnal plane and the vertical plane of the yaṣṭi is
equal to φ the latitude, the Iṣṭayaṣṭi forms a latitudinal
triangle with the Kalā, it being the Koti or side opposite
to the angle 90 - φ and the Kalā being the hypotenuse,
∴ Yaṣṭi / Kalā = cos φ = Koti of a latitudinal triangle / Karṇa of a latitudinal triangle
∴ Iṣṭayaṣṭi = Kalā × (Koti of a latitudinal triangle) / Karṇa
When the Sun is on the meridian, Iṣṭayaṣṭi becomes yaṣṭi
of verse 33.
The formula for Iṣṭayaṣṭi is therefore
(Kalā × H cos φ) / R = (H cos δ · H cos ℎ) / R × (H cos φ) / R
from formula (27)
= (H cos φ · H cos δ · H cos ℎ) / R² 27'