I recently skimmed Cosmos and Psyche, and read the first hundred pages as well as the last few chapters and one in the middle on Moby Dick. Very interesting book, and does achieve its purpose of convincing the reader to take another look at astrology. That is not to say, we should believe in astrology, but we should study our environs for patterns. This has been done since the beginning of man and looking to the skies for patterns and correlations with earthly and human phenomena is only one type of data retrieval for the discernment of patterns.
Tarnas's primary point is that synchronicity exists. Jung's original coining of the term was result of merely slowing down long enough and examining every thought, situation and observable phenomena to see a pattern. I predict that one day synchronicity, synonymous with the Chineses understanding of the Tao, Greek conception of cosmic sympathy, Hermetic doctrine of macro/microcosm, midieval concept of unus mundus, will be as commonly accepted and discussed as gravity in the collective culture. Many choose to go fast and ignore pattern, merely dismissing all unexplainable pattern as coincidence. Whereas coincidence is merely what is left over after a bad theory. As such all "coincidence" should be studied and folded into one's worldview.
Synchronicity requires data, and given that all the data mankind has acculumated was observed and derived from earth, representing less than 1/1000th of the cosmos, ought to ignite the curiosity of any sentient being. More data can yeild new patterns and new conclusions. I'm not necessarily saying that the traditional field of astrology ought to be accepted wholly, just that it ought to be practiced in its most literal sense (study the stars) and remain open to the possibility of cosmic synchronicity.
Tarnas begins by noting that astrology is like any other field or theory in that in goes in and out of favor over time. We're currently in a very objectivist, science-driven, phase of humanity and it is only reasonable that it has fallen out of favor. But like Aristarchus' idea of a sun-centered cosmos, it went out of favor, and experienced a massive resurgence after Copernicus was able to utilize the modern observation techniques and geometry to prove that the earth was not the center of the universe. Even after proving it, it would be hundreds of year before this idea was accepted.
Also, important to note is that after Copernicus, a worldview that does not consider the cosmos must be totally errant. ne must resolve the issue that we are apparently the only life form amidst a very cold (literally and perhaps figuratively), violent and centerless universe.
I think Tarnas' primary contribution is in exposing the fallacy of the prevailing objective worldview, as objects absolutely cannot be percieved without the subjective framework of the observer. That is, we have a massive disconnect occurring between our objective understanding of physics and our subjective understanding of psychology. The result of which is that we project patterns and do not earnestly observe them. Failure to understand this removes the mystery from the cosmos, as each scientific advance only furthers the utter lack of meaning in the vast void of space. Keeping in mind the subjectivity of observation allows us to ask questions regarding why we see a particular pattern. Failure to do so yields a robotic and incomplete understanding of reality.
This "objective" worldview that seek to de-anthropomorphize the cosmos is itself of anthropomorphism (conforming space to just another human idea of objectivism). It is dangerous to employ massive amounts of data, that we now have thanks to the Hubble and others, utilizing a limited, self-isolating set of assumptions (i.e. that the objective physics/subjective psychology doesn't need to be resolved).
In this regard, Tarnas agrees with Jung in stating that depth psychology is the direct successor to philosophy (and science, I believe), in that it seeks to extend the rational mastery of the cosmos to the inner world of psychology. It extends the range of inquiry dramatically, if not infinitely.
Lastly, Tarnas did a great job of saying that astrology is not prescriptive, but descriptive. There are patterns in the sky that correlate with personal and cultural events. Many attempt to use these patterns to predict, but ultimately astrology fails at this in a specific sense, but succeeds in a more general achetypal sense.