I couldn't believe it at first, but NASA has some webpages in its site dedicated to astrology (OK, it's of the skeptics type). That seems a good place to begin to study astronomy correlated to astrology.
http://spaceplace.nasa.gov/starfinder2/en/A criticism which springs up soon is the way the Chaldeans astronomers chose to split the zodiacal belt into 12 equal parts, instead than 13 unequal parts. Also, another criticism is that because of the precession of the equinoxes the limits between the zodiacal signs have shifted.
When the Babylonians first invented the 12 signs of zodiac, a birthday between about July 23 and August 22 meant being born under the constellation Leo. Now, 3,000 years later, the sky has shifted because Earth's axis (North Pole) doesn't point in quite the same direction.
Now Mimi's August 4 birthday would mean she was born "under the sign" of Cancer (one constellation "earlier"), not Leo.
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.
