The 27 nakshatras or lunar mansions are a very important part of Vedic or Hindu astrology,
perhaps more so than the solar zodiac signs themselves, as it is the Moon and not the Sun that takes precedence in Indian
astrology. Whereas here, in the West, we are known by our Sun sign, in India you are known by your Moon sign. This emphasis
on the Moon can be seen in their system of of 27 nakshatras or mansions which are based on the 27.3 day period that it takes
the Moon to traverse all twelve signs of the zodiac. It's as if the Moon has its own zodiac and why not? It is as prominent
in the night time sky as the Sun is in the day.
Each nakshatra is 13°20' wide and begins with 0 Aries and runs counterclockwise as can be seen in the above diagram. Each nakshatra is further divided into 4 parts of 3°20' each out of a total of 108. These 27 divisions and their 108 subdivisions form a complex system of divinement in Indian astrology. The mansion containing yor Moon and its Lord planet is most important. For example, if your Moon is at 6° Cancer then your lunar Lord is Saturn. Each mansion also brings in the influences of the stars for that particular area of space. Some of these stars are linked or associated with the lord(s) of that mansion. The lunar zodiac of 27 mansions or signs works independently of the solar zodiac. Is based on the stars or constellations with the solar zodiac as a reference or background.
Vedic astrology uses three basic charts: the lunar chart with your Moon sign as house 1, the solar chart with your Sun sign as house 1, and the normal or most common Lagna chart with your Ascendant sign as your first house. All three are valid and yield different information about yourself.
The biggest difference in Vedic astrology is that their zodiac signs are 24 degrees behind our zodiac signs. In other words, their 0 degree Aries is our 24 degrees Aries. This is because they base their zodiac on the stars or constellations rather than the equinoxes of Western astrology. Both systems were roughly in alignment over 2000 years ago but because of Precession(the apparent slow backward movent of the stars), the two slowly drifted out of alignment. The Vedics use the the opposite point of the star Spica(now at 24 Libra) to start the zero Aries point of the zodiac and add successive thirty degree segments from there, whereas Westerners always start the zodiac at the vernal equinox where the Earth's equatorial plane cuts the Sun's plane or ecliptic. This has been a great source of friction and controversy between East and West and there appears to be little give and take on either side.
Personally, I rely on the Western equinox or season-based astrology as it makes more sense to me, and there is evidence to support that originally the Vedic scripts recommended this as well. The current Lahiri or sidereal Indian astrology is not as ancient as most believe. In fact it can be traced back to the 1950's When Lahiri standardized it along with the 27 nakshatras. The twelve sign zodiac is an ancient Greek import(around 100 BC). It was in the 16th century that the astronomer Ganesa Daivajna made the constellations the signs and the opposite point of Spica the starting point -a mistake which has dogged Hindu astrology ever since. Not only were the constellations uneven, but the starting point was arbitrary. Lahiri partially corrected this through his sidereal division(12 equal spaces of 30 degrees each) but when he tried to go further to fix the starting point(by reintroducing the more ancient tropical zodiac based on Dikshita's book: History of Indian Astronomy Part II), he was met by stiff opposition from the Vedic astrologers. And so the Hindu zodiac remains off by 24 degrees.
The Surya Siddharta, a fundamental Indian text on astronomical astrology states that the signs are tropical and mathematical divisions rather than constellations. In the 14th chapter it unequivocally states that the signs begin or end from "equinox to equinox and from solstice to solstice". Also, in the 21st chapter of the Srimad Bhagavatam, another ancient text, it makes it clear that Aries and Libra begin at the equinoxes when daylight and darkness are equal in length. For more on this you can visit Vic DiCara's excellent site and more specifically http://www.vicdicara.com/12-signs-of-the-zodiac-1
As to the idea that there are thirteen zodiac signs rather than twelve -this is absurd, based once again on the faulty notion that the stars or constellations(of which there are thirteen) equate with the signs. There is NO CONNECTION between the two -except symbolic. The invisible signs and the visible constellations were indeed synchronized some two thousand years ago but have slowly drifted apart. By 200 AD the difference became obvious and a split occurred between those who kept the constellations as the signs and those who understood Precession and an equinox and solstice based system. Besides, astrology can only work if it is an EVEN-NUMBERED system, making possible the aspects and elements.
There are other components of Vedic astrology, however, that I prefer over Western astrology such as the whole sign approach where houses and aspects are concerned, their inclusion of the stars, and their belief in karma and reincarnation. I wish, however, they would be more open to the reality of the outer planets beyond Saturn which they do not accept on account of their invisibility and distance and because it would disrupt the traditional planetary scheme, all of which to me are weak and faulty reasons. Instead, they emphasize Rahu and Ketu or the north and south lunar nodes which are practically given planetary status even though they are not. It is true that Western astrology confuses itself with scores of planetary bodies and house systems but it is equally true that Vedic astrologers in general refuse to expand from their limited seven-planet system. They are stuck in the past and refuse to acknowledge the growth or unfoldment of the world which is reflected in the discovery of new planets.
Twelve, not seven, is the universal or complete number, and the process of seven to twelve planets(one for every sign) is also the process of incomplete humanity to complete humanity. In fact, there was a time, long forgotten in the ancient past, when astrology consisted of 12 planets and twelve signs, and the Sun's orbit was exactly 360 days and Moon's orbit was exactly 30 days, but catastrophic cosmic events occurred which shattered this perfect system, and it is only now in the last few hundred years that it is slowly being rebuilt.
Eventually, East and West, male and female, inner and outer will merge and complete and so will our astrologys, and only then will it be as it should.