American astrologer Noel Tyl has re-stated Lilly's definition of peregrination. He says that a planet is peregrine when it is not ruling or dignified in the sign it inhabits and when it makes no Ptolemaic aspect to any other planet. That means no conjunction, sextile, square, trine or opposition.
Aspects modify the way a planet expresses its archetype. In most charts, aspects form a web of reactions. Contact with one planet will set off a chain of contacts that may move all over the chart. But when a planet makes no major aspects it becomes a singleton and functions on its own. Noel Tyl observes that peregrine planets can run away with a chart because they're not in relationship with anything else that's going on.
Tyl also describes planets which are in aspect to each other but not to any other planets as "peregrine islands." I have a friend with one of these; I've attached his chart to this post. His Sun, at 0° Libra is tightly conjunct Mercury, at 0° Libra in the 7th house. There are no other planets in his horoscope that have any contact with this island and so, while there's a lot going on in the chart, it doesn't actually "reach" his Sun in meaningful ways. It's very difficult integrate this unbounded ego with the rest of the chart.
My friend is a good guy, but one soon realizes that everything he talks about refers to him in some way. He's the star of every story, usually the hero of the events, but sometimes he depicts himself as the one most victimized by heretofore unexpected, unrecognized forces whose actions could not have been anticipated ahead of time. While he is kind, generous and pleasant to be around, extending himself to others is something he does because he benefits in some way: he receives praise, attention, admiration and so on--things that confirm and enhance his ego in fairly simple, direct ways.
Many fine astrologers reject the notion of peregrination because their experience is that there's always a way to include an apparently isolated planet by some minor aspect. That makes a lot of sense to me, but if it's necessary to use very minor, less significant aspects to integrate a planet like the Sun or Moon, it's not clear to me that the planet will always be truly woven into the chart as a whole. Sometimes, however, this may be possible if the singleton is in close aspect to the midpoint between other planets.
In my own case, my Moon in Capricorn is peregrine. Astro.com refuses to show this peregrination even when the chart orbs are reduced to 80%. I will note that the only way to include my Moon is to use a 10° orb and reach to the outer limit to create a trine between the Moon and Sun. For myself, I don't think this makes much difference in the chart. The Moon is in detriment in the first house, near the cusp of the second. Sagittarius rises with Saturn conjunct the Ascendant. Pluto and Sun in Virgo and Mercury in Leo are in close conjunction in the ninth house. Neptune in Scorpio is the only planet in a water sign, Jupiter is the only planet in air. Everything else is evenly divided between earth and fire. My Sun, Moon, Mars and Pluto are all in earth signs and Mars is inhibited significantly in Taurus. The chart describes me as intellectual and pragmatic in general, not very emotional and slow to anger.
You bet! I was just wondering. It certainly makes sense to include aspects. I'm down with that. My chart would rock if that were the case. My Mercury would still be hating it, but whatevuh 
