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

284
PZ = colatitude ; PS = north-polar-distance or co-decli-
nation.
η is called the parallactic angle ; h = hour-angle of S.
(90 — a) = The complement of the Hindu
azimuth a being measured from the East point.
cos (90 — δ) = cos (90 — φ) cos z + sin (90 — φ) sin z cos (90 — a)
sin δ = sin φ + cos z + cos φ sin z sin a (1)
cos z = cos (90 — φ) cos (90 — δ) + sin (90 — φ) ×
sin (90 — δ) cos h
= sin φ sin δ + cos φ cos δ cos h (2)
cos (90 — φ) cos (90 — a) = sin (90 — φ) cot z — sin (90 — a) cot h
ie. sin φ sin a = cos φ cot z — cos a cot h (3)
cos (90 — φ) cos h = sin (90 — φ) cot (90 — δ) — sin h ×
cot (90 — a)
ie. sin φ cos h = cos φ tan δ — sin h tan a (4)
sin z sin (90 — δ)
─────── = ───────────── ie. sin z cos a = sin h cos δ (5)
sin h sin (90 — a)
This means in modern terms
cos z = sin φ sin δ + cos φ cos δ cos h V which we
derive from the triangle PZS.
Verse 58. To get H cos z from h the hour-angle or
nata Kāla.
The H. vers (Nata) is called Sara (CQ of fig. 52) (29)
Antyā — Śara = Iṣṭāntyā ie. FQ — CQ = FC = AN
(fig. 51)
H cos δ Śara × Kujyā
Śara × ───────── = ───────────── = phala (CQ) (fig. 53)
R Charajyā
(30)

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Hṛti — phala = Iṣṭa-Hṛti ie. fq — cq = fc = an
(fig. 53)
Comm. (Ref. fig. 52). Nata = arc QN
∴ H. vers (Nata) = QC. (Called Śara) = H. vers (h)
Antyā — Śara = FQ — CQ = FC = AN = Iṣṭāntya II
Śara × (H cos δ / R) = phala = cq (fig. 53) = (Hvers h × H cos δ) / R
Hṛti — phala = fq — cq = fc = an = Iṣṭa-hṛti IV
Verse 59. (Phala × Koti of a latitudinal triangle) / Karṇa
= Ūrdhwa V
= βr (of fig. 54)
Comm. (Ref. fig. 54) βr is the corresponding line in
the plane of the meridian corresponding to phala in the
plane of the diurnal circle. As the angle between these
two planes is the latitude itself, by the principle of ortho-
gonal projection namely.
Magnitude of a projected segment = cosine of the
dihedral angle × the magnitude of the segment projected,
since Ūrdhwa is the orthogonally projected segment of
phala, so, Ūrdhwa = phala × cos φ
= (phala × H cos φ) / R VI (31)
= (phala × Koti of a latitudinal triangle) / (Karna of the latitude triangle) as stated
Thus Ūrdhwa = βr (of fig. 54).
Verse 60. Ūrdhwa is also given by
Ūrdhwa = [U.S. (Unmandala Śanku) × Śara] / Charajyā
Dinārdha-Śanku (D.S.)—Ūrdhwa = Iṣṭa-Śanku (I.S.) =
H cos z.

286
Comm. Since U.S. is the projected segment of
Charajyā on a vertical plane and since the diahedral angle
between the planes is φ the latitude
U.S. / Charajyā = Cos φ and so Śara × cos φ = (Śara × H cos φ) / R
= Ūrdhwa.
From fig. 54, Dinārdha-Śanku — Ūrdhwa = o'q — βr
(fig. 54) = o'β' = yX = Iṣṭa-Śanku.
In modern times, this means,
Śara = H. vers (h) = (R — cos h) = 1 — cos h in modern terms
phala = (Śara × H cos δ) / R = (1 — cos h) × cos δ ,, ,,
phala × (H cos φ) / R = Ūrdhwa = (1 — cos h) cos δ × cos φ ,, ,,
Dinārdha Śanku — Ūrdhwa = Iṣṭa Śanku H cos z
= cos z (in modern terms)
= H cos (φ — δ) (taking northern declination and following
Hindu convention with respect to signs)
= Dinārdha·Śanku
= cos (φ — δ) in modern terms.
∴ cos (φ — δ) — (1 — cos h) cos φ cos δ = cos z
ie. cos φ cos δ + sin φ sin δ — cos φ cos δ + cos φ
cos δ cos h = cos z
ie. cos z = sin φ sin δ + cos φ cos δ cos h as before.
Verse 61. Computation of H cos z (Mahā Śanku)
through Antyā and Hṛti.
Let the Dinārdha Śanku (D.S.) be computed through
Iṣṭāntyā and Iṣṭa Hṛti and therefrom Iṣṭa Śanku. From
the Śanku, Drik·jyā ie. H sin z and the shadow (KH sin z) / R
could be computed —H sin z should not be computed from
Hṛti.

287
Comm. We computed Dinārdha Śaṅku by the formula.
D.S. = (Antyā × U.S.) / Charajyā = (Hṛti × Koṭi of a L.T.) / (Karṇa of L.T.)
under verse 36 ; similarly
Iṣṭa Śaṅku (I.S.) will be given by
Iṣṭa Śaṅku = (Iṣṭāntya × U.S.) / Charajyā = (Iṣṭa Hṛti × Koṭi of a L.T.) / (Karṇa of the L.T.)
But Iṣṭāntyā = [R (sin φ sin δ + cos φ cos δ cos h)] / [cos φ cos δ]
(as under verse 56)
∴ Iṣṭa Śaṅku = [R (sin φ sin δ + cos φ cos δ cos h)] / [cos φ cos δ]
× (R sin δ sin φ) / (R tan φ tan δ) from formulae (13) and (19)
= R (sin φ sin δ + cos φ cos δ cos h) = R cos z = H cos z
Or again Iṣṭa Hṛti = (R cos z) / (cos φ) from formula (11)
∴ Iṣṭa Śaṅku = (R cos z / cos φ) × (H cos φ / R) = R cos z = H cos z
Having got H cos z, using the formula H sin² z = R² —
H cos² z, H sin z ie. Dṛk-jyā can be computed. Also K =
12R / (H cos z) and S = (KH sin z) / R give the Chāyā Karṇa and Chāyā.
Bhāskara cautions us that H sin z cannot be computed
from Hṛti as mentioned in verse 37. because there in that
verse, the H sin z computed is that at noon alone.
Verse 62. Alternate method of obtaining K.
The Chāyākarṇa when the Sun is on the unmaṇḍala
multiplied by Kujyā or that when the Sun on the prime-
vertical multiplied by Taddhṛti, or again that when the
Sun is on the meridian multiplied by Hṛti, divided by
Iṣṭa Hṛti, will be equal to the Iṣṭa-Karṇa K.

288
Comm. Equation (23) under verse 41 is
(Iṣṭa Śaṅku / Iṣṭa Hṛti) = (D.S. / Hṛti) = (S.S. / Taddhṛti) = (U.S. / Kujyā) = cos φ I
and 12 / K = (H cos z) / R (under verse 40)
ie. K = 12R / Iṣṭa Śaṅku which means
Dinārdha Karṇa = 12 R / D.S. ; Sama Karṇa = 12R / S.S.
and unmaṇḍala Karṇa = 12 R / U.S.
Substituting for the numerators in I
12R / (K × Iṣṭa Hṛti) = 12R / (DK × Hṛti) = 12R / (S.K × Taddhṛti)
= 12R / (U.K. × Kujyā) II
∴ Iṣṭa Karṇa × Iṣṭa Hṛti = S.K. × Taddhṛti
= U.K. Kujyā
∴ Iṣṭa Karṇa = (Uumaṇḍala Karṇa × Kujyā) / Iṣṭa Hṛti
= (Sama Karṇa × Taddhṛti) / Iṣṭa Hṛti = (Dinārdha Karṇa × Hṛti) / Iṣṭa Hṛti
Verse 63. Just a caution.
If in any context where the word ūna-yuta has been
used, the quantity to be subtracted exceeds the quantity
from which it is to be subtracted, it goes without saying
that subtraction should be reversely effected and in the
place of addition subsequently prescribed subtraction should
be done and Vice-versa.
Comm. Bhāskara gives three examples to illustrate
his point. In verse 54, while defining Sūtra (H sin 90 – h)
we are asked to subtract chara from unnata when δ > 0

289
Fig. 56
and add Chara to Unnata when δ < 0 and take the H sine
of the result. Let us first consider the case when δ > 0.
(Refer fig. 56) when the Sun is at ☉, Iṣṭa Śaṅku is H sin ☉L;
and Unmandala Śaṅku is H sin BM. When the
Sun is at ☉₁, Iṣṭa Śaṅku = H sin ☉₁N. In the first case
Iṣṭayaṣṭi = (H sin ☉L — H sin BM) which will be the
orthogonal projection of ☉B on the meridian plane.
Unmandala Śaṅku and the Iṣṭa Śaṅku in the position ☉₁
are similarly the orthogonal projections of BM and ☉₁N
on the same plane. In the position ☉ Iṣṭa Śaṅku =
Unmandala Śaṅku + Iṣṭayaṣṭi, whereas in the position
☉₁, Iṣṭa Śaṅku = Unmandala Śaṅku — Iṣṭayaṣṭi which is
now downwards. Thus in the place of addition we have
subtraction of Iṣṭayaṣṭi. This reversion has arisen out of
37

290
the fact that in the position ☉, Sūtra is the H sine of
(KA — KE) whereas in the position ☉₁ Sūtra is the H sine
of EC ie. H sine of (KE — KC) ie. in the former position
Sūtra = H sine (Unnata-Chara) and in the lattter Sūtra =
H sin (Chara-Unnata). Thus a reversion in subtraction
here, effects a reversion of addition of the Iṣṭayaṣṭi.
Similar is the case in the other cases cited by Bhāskara.
Analytically this happens so because cos h, when h > 90,
becomes negative and adding cos h tantamounts to
subtracting sin θ where h = 90 + θ.
Verse 64. Another point to be observed.
Hvers (90 + θ) = R — H cos 90 + θ̅ = R + H sin θ.
The Unmandala Sanku is not observable when δ is
south in as much as it is below the horizon ; none the less
it may be computed for the purposes of effecting proportion.
Fig. 57
Hvers (CG) = Hvers (CÔG) = GH
Hvers (EG) = Hvers (EÔG) = Hvers (90 + θ)
= R + OH' = R + EL = R + H sin ☉
as defined by Bhāskara,

291
Comm. Under verse 58, we had to form Hvers h; a
doubt might arise as to what this Hvers h would be if
h > 90. Hence Bhāskara defines it and the definition is
clear from fig. 57. The Unmandala S'anku has the
formula R sin δ sin φ so that either when δ is negative or
when φ is negative, it will be negative which means it will
be in the opposite direction ie. vertically downwards.
Since negative latitudes are not considered by the Hindu
astronomers, the other case alone is considered. Even
from a figure it is evident that when the Sun is in the
south of the Equator, the Unmandala S'anku is vertically
downwards. In proportions like I given under verse 62,
we can use the magnitude of this Unmandala S'anku also
and it does not vitiate the results, when we take its
numerical value.
Verse 65. The Sun crosses the prime-vertical when
his northern declination falls short of the latitude. Then
alone there is sense in giving the magnitude of his shadow
at that moment. When the Sun does not cross the prime-
vertical at all, the Sama S'anku which could be computed
out of its formulation, though it does not exist, under the
Sun, yet, it could be used in proportions (like I under
verse 62) and no blunder is committed.
Comm. This is a beautiful example cited by Bhāskara
where he intuitively uses the so-called principle of
geometrical continuity. We have formulated Sama
S'anku as (R sin δ) / (sin φ) ie. H cos z = (R sin δ) / (sin φ) . We have a real
value of z when δ > φ, for, then only H cos z < R. When
φ > δ, then H cos z > R which is impossible, for, no
Hindu sine or Hindu cosine could be greater than R just
as no modern sine or cosine could be greater than unity.

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TERMINOLOGY
N.B.—In as much this Tripraśnādhyāya has a good number of technical terms whose understanding is
necessary to understand the Hindu methods of solving diurnal problems, we shall collect here all
such technical terms under this heading for guidance.

293

294

295

297

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| Cara Cāpa | Arc of the celestial Equator bet. the East point and foot of the declination circle | | | |
| Carajyā | H sine of the above or the corresponding line of Kujya in the plane of the celestial Equator | 13 | 13-17 | (R · H sin φ · H sin δ) / (H cos φ · H cos δ) | R tan φ tan δ |
| Natakāla | Hour angle h | | | | |
| Unnata | Time elapsed after rise | | | | |
| Śara | Corresponding line of phala in the Equatorial plane | 29 | 58 | H vers (h) | R (1 — cos h) |
| Phala | ⊥ᵃʳ from the culminating point on a line through the celestial body parallel to Udayāstastūtra | 30 | | (H vers h · H cos δ) / R | R cos δ (1 — cos h) |
| Ūrdhwa | Orthogonal projection of phala on the plane of the meridian | 31 | 59 | (H vers h · H cos φ · H cos δ) / R² | R cos φ cos δ (1-cosh) |

299
Yet (R sin δ) / (sin φ) will have a value greater than R ie.
even though the Sama-S'anku is never born so to say,
it has a magnitude. ‘तत्कथमिदं वन्ध्यासुतवत्’ Bhāskara
exclaims with respect to the magnitudes of Sama-S'anku
and Taddhṛti as well, both of which are not there, yet,
both of which have magnitudes greater than R. So
he says “those magnitudes of the Sama-S'anku and
Taddhṛti are like the sons of a barren lady”. Then
he says ‘तदपि प्रदर्श्यते’ ie. ‘We shall show how they arise.’
Here he uses his intuition of the principle of geo-
metrical continuity. Even when the diurnal circle does
not cut the prime-vertical, their planes intersect, out-
side the sphere and the perpendicular dropped from the
point of intersection on the plane of the horizon is the
Sama S'anku which has a magnitude greater than R.
Similarly the Taddhṛti could be seen what it is now.
These magnitudes can enter into a proportion like the I
in verse 22, and do help us to get the other real magnitudes
like the Unmandala S'anku etc.
Verses 66, 67 and first half of 68. To obtain the
time from the shadow.
Iṣṭāntyakā = (U.K. × Carajyā) / I.K. = (D.K. × Antyā) / I.K.
= (k × R²) / (R cos δ × I.K.) ; Rvers⁻¹ (Antyā − I. A.) = h
Dinārdha − h = Unnatakāla
where K = Karṇa, k = Vishuvat Karṇa, I.A. = Iṣṭāntya.
Comm. We had under verse 62.
Iṣṭa-Karṇa × Iṣṭa-Hṛti = D. K. × Hṛti = S. K. ×
Taddhṛti = U. K. × Kujyā multiplying throughout by
R / (R cos δ)

300
Iṣṭa-Karṇa × I. A. = D. K. × Antyā = U. K. × Carajyā
so that I.A. = (D.K. × Antyā) / I.K. – (U.K. × Carajyā) / I.K. I
But U.K. = 12 R / U.S. (verse 40)
∴ U.K. × Carajyā = (12R × Carajyā) / U.S.
= (12R × Kujyā × R) / (U.S. × H cos δ) since Carajyā = (Kujyā × R) / (H cos δ)
But Kujyā and U.S. are the Karṇa and Koṭi of the
seventh latitudinal triangle so that Kujyā / U.S. = k / 12
Comparing with the first fundamental latitudinal triangle.
∴ U.K. × Carajyā = (12R² × k) / (H cos δ 12) = kR² / (H cos δ)
Hence substituting in I
I.A. = (U.K. × Carajyā) / I.K. = kR² / (H cos δ × I.K.)
Thus we have proved the first part of the statement.
Having obtained I.A., from fig. 52 we have Antyā – I.A.
=(FQ–AN)=CQ. The Utkrama Cāpa of CQ = NQ = h
and Dinārdha – h = Unnatakāla. Let us see what this
procedure means in practice. Since on any day at any
place, φ and the declination of the Sun are known we can
compute all the magnitudes given in the verse or more
easily H cos δ so that from the formula Iṣṭāntyā =
12R² / (H cos δ × I.K.) where K = √(S² + 12), the shadow being
observed Iṣṭāntya could be computed in no time. Also
the Antyā of the day R+(H tan φ tan δ) can be computed
so that the segment CQ can be got. The inverse Hversine
of this is h. The arc CQ above was symbolized as Sara.

301
Bhāskara’s proof of I.A. = kR² / (H cos δ × I.K.) proceeds
from first principles as follows :—(i) If by k we have 12 as
the Koṭi what have we for R? The result is H cos z,
Mahāśaṅku. ∴ Mahā Śaṅku = 12 R / I.K. From Mahā Śaṅku
we pass on to Iṣṭa Hṛti with which it forms a latitudinal
triangle. If by the gnomon of 12 units we have k the
Viṣuvat Karṇa, what have we by Mahā Śaṅku? The
result is (12 R / I.K.) × (k / 12) = kR / I.K. Again from the Iṣṭa-Hṛti we
pass on to I.A. by multiplying by R / (H cos δ) so that I.A. =
kR² / (H cos δ × I.K.) as given.
Second half of verse 68. The inverse Hversine if a
quantity x greater than R, is 5400 + H sin ⁻¹ (θ) when
x - R = θ.
Comm. Since Hvers (90 + θ) = R + H sin θ = x (say)
90 + θ = Hvers ⁻¹ (R + H sin θ) =
But 90° are equal to 5400 asus and θ = H sin ⁻¹ (H sin θ)
= H sin ⁻¹ (x - R) = त्रिज्याधिकभागस्यक्रमचापम्
∴ 5400 + त्रिज्याधिकभागक्रमचापम् = Utkrama Cāpa of a
त्रिज्यादिक quantity.
Verse 69. Alternate method of obtaining the time
that has elapsed after Sunrise noting the shadow S.
Subtract Charajyā from or add it to Iṣṭāntyā according
as δ is north or south. Obtain inverse H sine of the
remainder and add the Caracāpa to this inverse H sine.
Then we have the unnatakāla by converting the result into
time.

302
Comm. Ref. fig. 52. I.A. = AN. I.A. - Carajyā =
B.N. Hvers⁻¹ (BN) = arc EN. Arc EN + Caracāpa =
EN + EM = MN. This converted into time is evidently
the Unnatakāla because the arc MN of the equator is the
arc intercepted between the feet of the declination
circles at rising and at the time concerned M being the
foot of the rising declination circle and N the foot of that
at the time in question. The convention of signs is
clear.
Verse 70 and first half of 71. To obtain the Sun's
longitude from the shadow S.
The gnomonic shadow at noon, being multiplied by R
and divided by K, the inverse H sine of the result gives
the meridian zenith-distance. This being decreased or
increased by the latitude gives the Sun's declination
according as the extremity of the shadow is north or
south. From the declination, we have the Sun's longitude
by the formula H sin δ = (H sin λ H sin ω) / R .
Comm. We have from the triangle formed by the
gnomon and the shadow S, S / K = sin z or SR / K = H sin z
∴ H sin⁻¹ (SR / K) = z. Since we are directed to take
the mid-day shadow, we have the meridian zenith-distance
and from the formula z + δ = φ we have δ. If the
extremity of the shadow be north, the Sun is south of
the zenith, and then φ ~ z = δ. The word वियुक्ताः is
used to signify difference which is positive. If z > φ
then the declination is south and vice-versa. If the
extremity of the shadow is on the south, the Sun is on
the north of the zenith. in which case φ + z = δ.
Second half of verse 71. To obtain φ from δ.

308
If the zenith-distance and the declination are of the
same direction, their difference, otherwise their sum will
be the latitude.
[Fig. 58] Comm. Suppose the zenith-
distance is north and decli-
nation also north, then
clearly from fig. 58, in this
position S₁ of the Sun QS₁ —
ZS₁ = ϕ = δ — Z (1) Again
in the position S₃ of the Sun,
zenith-distance is south and
declination is also south ; so,
here also difference gives ϕ
ie. ZS₃ — QS₃ = Z — δ = ϕ.
(3) In the position S₂, how-
ever, Z is south and δ is north, so that their sum is
equal to ϕ. It will be noted that the Hindu convention
of signs does not contemplate negative declination and
also it uses the word 'difference' to signify the positive
difference, as for example, in the first two cases δ ~ Z is
taken as ϕ. The modern formula Z + δ = ϕ applies
universally with the convention that δ is +ve if north,
ϕ is +ve if south.
Verses 72, 73. To obtain the Bhuja from the shadow.
Karṇa Vrittāgrā = (A × K) / R where A = Agrā and K
the Chāyākarṇa. This Karnāgrā is to be taken as
belonging to the opposite hemisphere to the Sun. Calling
this Karnāgra as a and the equinoctial shadow as s, a ⨦ s
according as δ is south or north gives the Chāyābhuja b.
Thus a ⨦ s = b. If the extremity of the shadow be on the
north and δ be north, then b + a = s ; if δ be south
b ~ a = s. If b be north, b ~ s = A, otherwise ie. if
south b + s = a. (R × a) / K = Agrā and
(Agrā × Koṭi of latitudinal triangle) / (Karṇa of the latitudinal triangle) = H sin δ.