What is a Zodiac in Vedic Astrology ?

This video explains why the zodiac is about 18 degrees wide i.e. 9 degrees above and 9 degrees below the ecliptic. It also explains how zodiac signs or Rashis and Nakshatras starts from the same point and relationship between Rashi and Nakshatra. It also explains how Rashi covers 9 padas spread across 3 Nakshatras.