Here are a few good questions (with my replies) about the relationship of the sidereal nakṣatra (Vedic fixed stars) to the tropical rāśī (zodiac signs).
Question: You present the idea that nakshatras and rasis are not intrinsically connected, but I have read that Chandra [the Moon] is exalted in Rohini [Aldebaron]. So does the point of exaltation move with the nakshatra or stay in Taurus? In other words, are exaltation and debilitation related to signs or nakshatras?
My reply: Indian tradition has tried to correlate planetary exaltation degrees to the nakṣatra, because, in the last dozen or so centuries, they operated under the assumption that the stars (nakṣatra) are permanently fixed to the signs (rāśī). To make this correlation work, they had to depart from the globally accepted definition of exaltation points for three planets.
They had to change the Sun’s exaltation point from 19º to 10º so that it fell in Aśvinī nakṣatra (home of his sons, the Asvin) rather than Bharaṇī (the home of Yama, of the sunless netherworld). They had to change Saturn’s exaltation from 21º to 20º so that it fell in Svātī nakṣatra (home of Vayu, the god of wind, compatible with Saturn’s Vata/wind medical nature) instead of Viśākhā. They had to change Jupiter’s exaltation degree from 15º to 5º so that its debilitation would not be in Śravaṇa (home of Viṣṇu, a God worshipped by Jupiter) or in the very auspicious Abhijit (home of Brahmā, god of sacred texts and knowledge).
If instead we note that the most ancient, Ṛg Vedic Indians did comprehend the precession of equinoxes causing the drift between stars and signs – at least up until the onset of the Dark Ages roughly 1,500 years ago – then we might also be inspired to do the same, and would then not feel the need to impose the principles of signs (like exaltation and debilitation) onto the stars.
QUESTION: Are exaltation and debilitation determined by sign, not nakṣatra?
Exaltation and debilitation are the highest and lowest states of dignity (“dignity” means how well we behave, how much we control our meaner tendencies and exhibit our gracious side.) Dignity is a result of how a planet feels in the environment created by a sign. Look carefully at the rules of evaluating dignity and you will see that this is an indisputable truth. For example, planets are very dignified in their own signs, and are rather undignified in signs that belong to their enemies, etc.
Therefore exaltation and debilitation are determined by sign, not by Nakṣatra.
Question: As far as I understand, the nature of each nakshatra reflects the nature of sign (or vice versa).
This is another relic of the Dark Ages concept that the signs and stars are the same, or permanently related. I have written a book, 27 Stars, 27 Gods (available through my site) which demonstrates how all the real symbolic and interpretive traits of a nakṣatra are a direct result of the gods (devatas) who dwell there.
Question: I thought this explained why the nature of a nakṣatra differs in the parts that occupy different neighboring signs.
This is yet another relic of the Dark Ages way of trying to merge two things that do not merge.
Question: Does it mean that nakshatra can change some of its characterization due to new position in Tropical rasi?
No, the two systems, sidereal nakṣatra and tropical rāśī are two different systems. The use of the two can and probably should blend, but the definitions of the key principles of the two are not meant to blend.