A quick google brings up this:
So, once again, it takes no account for precession, or the fact that Uranus, Neptune and Pluto have been discovered since the charts were originally conceived of.
Thanks vbloke. Interesting discussion. For the first time, I had to use google for this thread.
For starters, here is a comparison of Vedic and Western astrology by a professor. He says both the systems are very different.
http://sify.com/lifestyle/astrology/fullstory.php?id=13308506&t=Astro%20Facts
I really donot know the concept of precession. I did a google on "precession astrology" and the first website seem to mention the same question that you had posted. Reading first line on the following link
"
Astrology claims that someone born on 30th March has the sun at 10° Aries, whereas in reality on 30th March the sun is clearly in the fixed star constellation of Pisces."
tells me that Vedic Astrology is not being discussed here. There is no relation of Sign (which in based on Sun in Western and on Moon in Vedic) and the fixed set of dates in Vedic Astrology.
http://66.102.9.104/search?q=cache:kHgRYcBcgKEJ:www.astro.com/astrology/in_praezession_e.htm+precession+astrology&hl=en
I am sure you can understand this link better than me because this seem to apply to Western Astrology only.
If Vedic astrology takes positions of planets into account, how come Mercury has an effect, but the much more massive Uranus and Neptune don't?
I donot know the answer to this. But in every Vedic horoscope, there are two more psuedo planets or nodes, called Rahu and Ketu (UMBRA and PENUMBRA) that are also considered in addition to the planets that you have mentioned. They are treated almost like planets.
http://sify.com/lifestyle/astrology/fullstory.php?id=13308506&t=Astro%20Facts
The above link talks about it as well.
"Vedic system uses mainly the two luminaries and five other planets but include two “CHAYA” shadow planets viz Rahu and Ketu. Their importance can be judged from Physics definition of UMBRA and PENUMBRA points when two luminaries interact. The importance of these becomes evident by the fact that an eclipse occurs whenever luminaries and these fall in a straight line."
Which shows that thousands of years ago they were aware of Solar and lunar eclipses.
In addition, this a mythological representation of eclipses which was written thousands of years ago. Rahu and Ketu (Umbra and penumbra, modern language) are in a same line when we have eclipses.
In Vedic Astrology, Rahu and Ketu occupy totally opposite houses to each other. e.g. if Rahu in in 5th house, then Ketu is in 11 th house and so on.
http://www.sanskrit.org/Astronomy/Rahu.html
Also, I am not very sure but maybe because Neptune, Pluto and Uranus are the outer most and the most far away planets, they may have lesser effects. (Just guessing!)
If this is true, then Vedic astrology is twice as inaccurate as western astrology, as western astrology is based on charts that are 2600 years old.
That may not be true to draw this conclusion. Thousands of years ago, they seem to know about the planets, stars, positions, movements, eclipses which we have discovered only later on. I am really amazed.