A nakshatra (Sanskrit nakṣatra) is a lunar mansion: one of the 27 segments into which Indian astronomy divides the band of sky through which the Moon travels. The Moon takes about 27.3 days to complete one sidereal circuit of the zodiac, so it spends roughly one day in each nakshatra. Long before the twelve signs (rashis) entered Indian astronomy, the Vedic texts tracked time, rituals and seasons by the Moon's position among these star groups, and they remain the foundation of the panchanga, of muhurta (electional timing) and of horoscopy today.
Size and structure: 13°20′ each, four padas of 3°20′
Dividing the 360° circle by 27 gives each nakshatra an equal span of 13°20′. The series begins at 0° sidereal Aries with Ashwini and ends at 360° with Revati. Each nakshatra is split into four quarters called padas of 3°20′ each, giving 108 padas in all. Because 108 is also the number of navamsas (ninths of a sign), every pada corresponds to one navamsa sign, which is why the navamsa chart (D9) can be read directly from nakshatra padas. Nine padas make one rashi of 30°, so the boundaries of the signs and nakshatras coincide every 2¼ nakshatras: Aries holds Ashwini, Bharani and the first pada of Krittika, Taurus holds the rest of Krittika, Rohini and half of Mrigashira, and so on.
Each pada also has a traditional sound or syllable, and the Moon's pada at birth is widely used to choose the first letter of a child's name. In older Vedic lists, which begin with Krittika (the Pleiades), each nakshatra is identified with a junction star (yogatara); the modern sidereal zodiac is anchored to one of them, Spica (Chitra), placed at 180° in the Lahiri ayanamsa used by most Indian panchangas.
Ruling planets and the Vimshottari dasha
Every nakshatra has a planetary lord. The nine lords repeat in a fixed order three times around the zodiac: Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn and Mercury. This sequence drives the Vimshottari dasha, the 120-year system of planetary periods described in Brihat Parashara Hora Shastra. The nakshatra holding the Moon at birth fixes which planet's period (mahadasha) a life begins with, and how much of it remains, in proportion to the distance the Moon still has to travel in that nakshatra.