NASA:

The constellations are different sizes and shapes, so the Sun spends different lengths of time lined up with each one. The line from Earth through the Sun points to Virgo for 45 days, but it points to Scorpius for only 7 days. To make a tidy match with their 12-month calendar, the Babylonians ignored the fact that the Sun actually moves through 13 constellations, not 12. Then they assigned each of those 12 constellations equal amounts of time. Besides the 12 familiar constellations of the zodiac, the Sun is also aligned with Ophiuchus for about 18 days each year.

For the whole article, see:
http://spaceplace.nasa.gov/starfinder2/en/

Astro Dienst - astro.com

Western astrology mostly uses the tropical zodiac, in which 0° Aries is fixed at the vernal point. The vernal point is the point where the Sun is located at the spring equinox.

By contrast, sidereal astrology uses a sidereal zodiac whose initial point is defined relative to the fixed stars. Sidereal astrology has a western as well as an eastern tradition. The former claims to go back to the Babylonian and Hellenistic traditions, whereas the latter originates from the Indian tradition, which has become known as "Vedic" astrology in recent years. (Since Vedic spirituality does not have anything to do with astrology, this is actually a misnomer, and I shall henceforward call it "Indian astrology" or "sidereal astrology".)

For the whole article, see:
http://www.astro.com/astrology/in_ayanamsha_e.htm