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