Synastry is quite simply chart comparison. The synastry charts that computers generate twine together two charts in one space, one laid inside the circle of the other. In this way we can see how the placements of one chart aspect the placements in the other. This seems simple as it is one chart, however when we analyze the synastry chart we are really looking at two, each individual chart with the placements of the other superimposed.
Elsa said the best thing I’ve ever heard about synastry: that another person’s natal placements are “like permanent transits to your chart.” This is why we’re looking at how the other person’s planets aspect those of an individual. You’ve heard Pluto transiting your moon is tough? Well if this person’s Pluto conjuncts your Moon, interacting with them will feel like a permanent Pluto-Moon transit. Some people have charts where they already experience a similar energy and they can handle it. For others it’s just too much.
Of course everyone is looking for the “good” aspects, perhaps a Venus-Moon contact where the esthetics of the other feel good. They like the same things you need. Would it surprise you to hear that most astrologers I’ve read cite Saturn aspects as the most indicative of a long-lasting relationship? A lifelong Saturn transit doesn’t sound so dreamy but it can be the glue that holds things together. All work and no play makes Jack a dull boy. But without any stressful aspects Jack’s not likely to get off his duff and get to the finish line either.
Synastry can be used with the charts of partners, parents and children, heck if you know your cat’s birth info I don’t see why you couldn’t do that. I’ve done the synastry between my daughters trying to figure out why two such lovely young ladies would interact like rabid wolves. You can also do a sort of synastry between your chart and an event chart, like a wedding chart. It’s not exactly the same but it’s the same principle. It’s just like assessing transits, comparing them to your own chart. Because that’s what synastry is, in nuts and bolts, assessing the transit-like energy you encounter when you are in any sort of a relationship with another.
Have you ever compared your chart to someone else’s?