What are Nakshatras?

The Nakshtras, also called lunar mansions or constellations. In Jyotish or Indian astrology there are 27-nakshatras prescribed by Parasara (ancient sage who founded Vedic astrology).

The first nakshatra, Ashwini, starts at Aries 0’00’’ degrees and ends at Aries 13’30’’.
The next nakshatra, Bharani, starts at Aries 13’20’’ and ends at Aries 26’40’’.
Then Krittika starts at Aries 26’40’’ and ends at Taurus 10’00’’.
Then Rohini from Taurus 10’00’’ to 23’20’’ and so on, until Revati, which extends from Pisces 16’40’’ to Aries 0’00’’

Two nakshatras take up 26’40’’, which is 3’20’’ short of one full sign ( a full sign is 30’00’). Therefore, 3’20’’ is 1/4 of a nakshatra, and is called a nakshatra-pada, or simply a pada(means step). As such, Aries contains Ashwini, Bharani, and the 1st pada of Bharani. Taurus contains padas 2- 4 of Krittika, Rohini, and the first 2 padas of Mrgasira. Each nakshatra therefore contains 9 padas (note that 3’20’’ x 9 = 30 degrees, or one full sign).
The notation Ashwini-1 means the 1st pada of Ashwini, Ashwini-2 the 2nd pada, and so on. Aries then contains Ashwini, Bharani, and Krittika-1, and Taurus contains Krittika-2,3,4, Rohini, and Mgrasira-1,2.

Four signs contain 9 x 4 = 36 padas, or 9 full nakshatras. This happens 3 times, as indicated by the breaks in the nakshatra table below. Aries - Cancer contain Ashwini - Ashlesha, Leo - Scorpio contain Magha - Jyeshta, and Sagittarius - Pisces contain Mula - Revati.
Just like the signs, each nakshatra has a ruler or owner. Unlike the signs, Rahu and Ketu are assigned rulerships. Ashwini is ruled by Ketu, Bharani by Venus, Krittika by Sun, then Mars, Rahu, Jupiter, Saturn, and Mercury. The cycle then repeats in the same order two more times. Note the order of nakshatra rulership is the same as the mahadasa order in the vimshottari dasha scheme, as it must be, since this is how the vimshottari dasha order is defined.
A table summarizing the nakshatras is given below.

The nakshatras act as a filter through which the grahas act. Take Sun at Aries 10-00; this corresponds to Ashwini, which is ruled by Ketu. Sun therefore has an influence from Ketu, and we need to look at the disposition of Ketu to fully understand the influence of Sun.
We continually emphasize the word graha here and in other articles in this web site. It is explained there that the word graha means to grasp, or lay hold of. The entity that the grahas are grasping onto are the nakshatras.

When one graha is in a nakshatra ruled by another graha, it acts to integrate the influences of the two grahas. This integration is often important for providing balance. For example, Jupiter is an expansive graha, and this is taken to be "good". On the other hand, too much expansion can result in obesity, or excessive accumulation. Saturn, on the other hand, indicates constriction, and is often considered "bad". When Jupiter and Saturn balance each other it can provide for a very strong foundation which can result in "healthy growth", or growth (expansion) with balance.

Following table will show certain characteristics of 27 Naksharta
in Vedic Astrology

Note: Those in italic are derived from Brihat Samhita through a study of transit effects, the other indications are directly stated from other works.