Nakshatra is 27th part of the sidereal Zodiac in the Hindu or Vedic Astrology. The Moon complete one round in approximate 27 days. So 1 day's average motion is 13degree 20 minute. Which is span of 1 Nakshatra. The name of each nakshatra is based on the prominent star in that area.
Starting point of Nakshatra is same as of Signs. So 0degree is starting point of Aries sign as well as of Ashwini nakshatra.
Nakshatras are ruled by a planet, as Ashwini ruled by Ketu, Bharni by Venus, Krittika by the Sun....
1st 9 nakshatra complete 1st cycle of 9 planets rularship. As well as 1st 9 nakshatra cover 120 degrees (1/3 of circle) of Zodaic, Aries to Cancer. From Leo to Scorpio 2nd cycle complete and from Sagittarius to Pisces last 3rd cycle complete.
So every planet is rular of 3 Nakshatra, after each 120 degrees.
As Ketu is lord or rule Ashwini, Magha & Mula.
Vimshottari Mahadasha or another dasha system is calculate on degrees of the Moon Nakshatra at the birth time/any specific time.