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वह दोहरा निर्देशांक जाल जो ज्योतिष गणनाओं को संभव बनाता है
In Module 1.1, we learned that the ecliptic is a great circle — the Sun's apparent annual path through the sky. Now we need to measure positions along this circle. The ancient world settled on dividing the circle into 360 degrees (अंश, Amsha).
Why 360? Because it's extraordinarily divisible. 360 has 24 divisors: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120, 180, 360. This makes it easy to divide the sky into equal parts. The Babylonians may have originated this choice (their year was ~360 days), but Indian astronomers adopted and perfected it.
Each degree is further divided into 60 arcminutes (called Kala, कला in Sanskrit) and each arcminute into 60 arcseconds (Vikala, विकला). This sexagesimal (base-60) subdivision is shared between Indian and Mesopotamian astronomy.
The Surya Siddhanta (Ch.1) defines the Bhachakra (भचक्र) — the "star circle" or zodiac — as a 360° circle centered on the ecliptic. The term Amsha (अंश) means "part" or "degree" — literally a 1/360th part of the circle. Parashara (BPHS Ch.1) opens with: "The zodiac comprises 360 degrees, 12 signs of 30° each, and 27 nakshatras..." — establishing both coordinate grids in the very first chapter.
Here is the key insight that distinguishes Indian astronomy: the same 360° ecliptic circle is measured by two overlapping grids simultaneously:
360° ÷ 12 = 30° per sign
Based on the Sun's monthly movement (~1° per day × ~30 days = ~30°). The Sun spends approximately one month in each sign. This is the solar measurement.
360° ÷ 27 = 13°20' per nakshatra
Based on the Moon's daily movement (~13° per day). The Moon spends approximately one day in each nakshatra. This is the lunar measurement.
This is NOT arbitrary. The Sun moves ~1°/day, so it crosses 30° in ~30 days (one month). The Moon moves ~13°/day, so it crosses 13°20' in ~1 day. The solar grid (Rashis) divides the year into 12 months. The lunar grid (Nakshatras) divides the month into 27 days. Together, they form the Panchanga — the complete measurement of celestial time.