Hi Prudence,
Sidereal means a few different things to astrologers, depending on their school, but they all have in common that the chart has taken into account
precession, which refers to a wobble in the Earth's rotation that makes the Vernal Equinox point (and the whole ecliptic and starry sky with it) go "backwards" over a period of about 26,000 years.
The essential effect of taking a sidereal approach is, that "what you see is what you get" - if you went out and looked at the sky, the planet really is "in" the Zodiacal constellation corresponding to the Zodiac sign it is named for.
My own "system" (hardly more than a framework at this point) is only mine - I haven't studied any schools of sidereal astrology. I see astrology as "real" - real physical forces, some very strong (like the Sun and Moon) but most very subtle but persistent. All of these forces act to create a complex web of energy that affects everything in the solar system, including Earth and everything on it, to a greater or lesser degree. When we are born (which I define astrologically for a person as the first unaided breath, or for ideas and organizations the moment of the creation of that idea in the minds of the people making it) the character of that moment is "imprinted" in us. The quality of that energy should be modifed by various other more immediate forces of the same character, such as strong electricity (at least, that's what I would expect - I would love to study people born close to electrical generators or during thunderstorms, and perhaps those born close to the Earth's magnetic poles).
For me, the Zodiac constellations, for which the signs were named, are convenient ancient fictions that cannot possibly exert any force. The signs are essentially descriptions of Earth energy at the time the Zodiac was fixed - around 2000 years ago. This is the reason that the Tropical zodiac Sun-signs still seem so appropriate, at least in the northern hemisphere, even though the constellations they referred to all that time ago have precessed almost a complete sign. I.e. Leos still exhibit classical Leo traits, even though most of them are born with the Sun in sidereal Cancer. The answer is that the Earth season is still the same, and imprints its qualities the same way no matter where the Vernal Equinox point is.
This only works for the Sun sign of course, since it is the Sun which determines the seasons. The other planets, for me, are independent of the signs or constellations, and only their aspects should be considered.
For things like the galactic center, I can't see how one can make a tropical use of it, without believing that the Zodiac constellation is actually one of the real actors in the chart - which I don't think tropicalists believe.
That is, the galactic center is a real energy source in a real constellation, not in the fictional tropical zodiac - in my humble opinion. Sagittarius the constellation (sidereal) cannot give this reality to Sagittarius the sign (tropical) as it precesses. There is not a "virtual" galactic center that makes the rounds once every 26,000 years. That is, in my opinion, because the signs refer to earthly seasons, and can be held not to precess, but the galactic center, since it exerts a real force, cannot. The galactic center really precesses with the constellation Sagittarius, and exerts its force from there.
Thus while Pluto is in the tropical Sagittarius, it is not in the real Sagittarius, and is not therefore transiting the GC. Whatever force Pluto might be exerting, and whatever force the GC is exerting, are not in conjunction at this moment.
See - it is much easier to be a siderealist!
