PATRICK in Easton, Pennsylvania, wants to know "the whole thing about how Pluto rules Scorpio, like now that it's been demoted. And who rules what?"

Honestly, Patrick? I don't care about the demotion. Pluto is too small, oblique, and insignificant to make much of a ruler ...like Bush for instance! Jokes aside, Pluto wasn't used millennia ago when natal astrology was perfected, and I rarely use it in my natal readings. Here's the original Western model for calculating that planetary rulership stuff anyhow, I feel the information should be allowed to speak for itself:

Western Cosmology has a standard Eurasian double yin-yang polarity pattern in both elemental etiology and planetary rulership. We start at the Cancer-Leo border, assigning the Sun and the Moon the first polarity, as single-sign rulers of Leo and Cancer respectively, since they are exceptionally potent luminaries.

We continue from proximal to distal from this zodiac point, and in the solar system thru the five visible planets, assigning a yin and yang sign to each.

In the zodiac, this proceeds bilaterally toward the polar opposite at Capricorn-Aquarius, respectively yin and yang Saturn domains. Thus, Mercury fits Gemini as its yang aspect and Virgo as the yin. Venus receives yang Libra and yin Taurus. Mars takes super-yang Aries and smoulderingly-yin Scorpio. Sagittarius and Pisces for lovely old Jove, and sad old crusty Kronos takes equally crusty Capricorn and quirky Aquarius. Got that, Patrick?

These seven luminaries are the only ones visible with the naked eye, and so we recognize this system of allocation as a TOTALLY culturally arbitrary construct, of course, a given. The West ignores Uranus and Neptune pretty much right up until Kepler, not really noticed. "Perception is projection," Laura Cerwinske used to say. Uranus and Neptune, were invisible to the naked eye, imperceptible with the five senses, and consequently irrelevant! No soup for you!

[There is, however, archaeological evidence emerging that ignorance of Uranus and Neptune was a largely European flaw. Mayan and Vedic astronomers may have been aware of these two large and incognegro gas giants, possibly identified in India as  the original Ketu and Rahu. Both civilizations had calculus long ago, and both incorporated two extra, invisible planets in their astronomical models. This hypothesis obviously cries out for for further research, but I digress.]

When we discovered Uranus, Neptune and Pluto in the West, we were hard pressed to find a dominion for them, and so we disrupted a perfectly good symmetry established by some very devout rabbis some 4000 years ago. Conversely today, we are now finding hundreds of Pluto-sized planetoids at the edge of our solar system. And so, Patrick from Easton, I say for Pluto the jig is up!
In transits and progressions, yes, Pluto can be deadly. But in birth charts? All marbles rule, or none rule! Modern astrologers should therefore sh*t or get of the pot on this whole rulership issue.

Oh yes, this particular method of arranging pairs among 2+5 luminaries and 12 archetypes only works in a base-12 cultural zodiac. We see this duodecimal bias not just in Greek pentaphobia/philia, but pretty much everywhere between Iceland and Japan. We'll look into Mesoamerica next year, as they use a base-20 system for their zodiac. (Read: Platonic Solids)

This is about as dry and technical as I care to get at this hour, Patrick from Easton. Pluto never ruled Scorpio in the first place. Beyond that, you are now on your own.