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

GRAHŌDAYĀSTĀDHIKĀRA
Verse 1 and first half of verse 2.
The Udayalagna of a planet is termed Prāk-Dṛk-graha
and the Astalagna is termed the Paschima-Dṛk-graha.
If the Prāk-Dṛk-graha happens to be less than the current
lagna the planet had already risen. If it be greater, the
planet is still to rise. Similarly if the Paschima-Dṛk-graha
is less than the current lagna, the planet had already set ;
otherwise is yet to set.
Comm. The words Prāk-Dṛk-graha and Paschima-
Dṛk-graha are coined to indicate the points of intersection
of the ecliptic with the eastern and western horizon
respectively when the planet is rising or setting, when the
planet has latitude. When the planet has no latitude
Prāk-Dṛk-graha coincides with the rising planet and the
Paschima-Dṛk-graha with the setting planet. When the
planet has a latitude, the foot of the latitude, which
signifies the planet’s position on the ecliptic differs from
the two Dṛk-grahas. In this case ie. when the planet has
a latitude the Prāk-Dṛk-graha’s longitude will be less than
that of the planet’s longitude whereas the Paschima-Dṛk-
graha’s longitude will be greater than that of the planet.
If x and y be the differences of the longitudes of the planet
and those of the Dṛk-grahas respectively, it is clear from a
figure that tan x = (tan β) / (tan θ) and tan y = (tan β) / (tan ϕ) where β is the
latitude of the planet and θ and ϕ the angles which the
ecliptic makes with the horizon at the planet’s rising and
setting respectively.
Since θ and ϕ are then respectively the zenith-
distances of the pole of the ecliptic in each case, and since