Rasi is a grouping system to segment the Nakshaktras (distant stars) in space. The space is viewed as a circle of 360 deg and each rasi would have 30 deg each, that makes 12 rasis. In each rasi a group of 2.5 nakshaktras are placed. To be precise 13d 20m for each nakshaktra.
According to Jyotisa sutras, a rasi shows a persons innate nature while the nakshaktras their qualities.
Om Tat Sat