Step 1: Concept Clarification
The objective is to identify the ancient Indian scholar who employed the concept of "Lunar mansions" (Nakshatras) for the specific purpose of predicting seasonal rainfall.
Step 2: In-depth Analysis
Although various ancient texts discuss agriculture and weather, the most comprehensive and systematic treatise on astrology, astronomy, and meteorology is the Brihat Samhita, authored by Varāhmihira (6th century CE).
Varāhmihira allocated several chapters to rainfall prediction (Garbhalakshana, Pravarshana). His methodologies involved extensive observations, encompassing the positions of the Sun and Moon in various Nakshatras (lunar mansions), cloud formations, wind direction, and biological indicators.
While Parāśara's Krishi Parashara also addresses rainfall, Varāhmihira's work is recognized as the most authoritative and detailed regarding the use of astronomical phenomena like lunar mansions for this specific application.
Step 3: Conclusive Determination
Varāhmihira stands as the foremost ancient authority who extensively utilized lunar mansions for the prediction of rainfall.