An imaginary sphere covering the sky with the Earth at its centre is known as the celestial sphere.
The ecliptic of the celestial sphere is divided into 27 equal parts known as nakshatras. Hence, their angular region is 360÷27=13o20′.
The moon remains in one nakshatra for 1 day as it undergoes an angular displacement of 13o20′ in one day. The Sun remains in one nakshatra for 365÷27=13.5 days.
Nakshatras are given names on the basis of imaginary figures formed by joining stars or by luminous stars belonging to the nakshatra. Pushya, Swati and Ardra are some well-known nakshatras.