In the vast world of astrology, we hold many, many theories about romantic compatibility to be loosely, kinda-sorta true. There's no rule that applies perfectly to everyone's love lives, but there are some that hold more water than others. Those not-quite-golden compatibility theories usually have something to do with our planetary placements and those of our partner.
As you may already know, your sun sign (the sign that most of us read our horoscope for), can only reveal so much about you. And when we're talking about matters of the heart, it's important to look at your moon sign, Venus sign, and, especially for our purposes here, your Mars sign. This sign determines your temper, actions, and sex drive — and you can find it if you pull up your birth chart.
In her writing about compatibility astrology (aka synastry), astrologer Annie Heese highlighted that finding your Mars sign is conjunct your partner's sun sign as particularly exciting. ("Conjunct" simply means that two celestial bodies are in the same sign.) This aspect, Heese wrote, suggests "a real physical connection" between the two of you, to the point that you recognize them as just your "type." Similarly, astrologer Larry Schwimmer noted in an article for HuffPost that having the same Mars sign as your partner's sun sign is a recipe of sexual compatibility. So, if you're super attracted someone whose sun sign is Sagittarius, there's a chance that your Mars sign is Sag, too.
Or, to speak from personal experience, before I was even aware of Heese and Schwimmer's thoughts on the matter, I was chatting with another astrologer about my birth chart. He noticed that my Mars was in Cancer, but didn't make much of it and pretty quickly moved on to focus on all the Capricorn activity I've got going on. It was only when I mentioned that my partner's sun sign is Cancer that he made a big deal out of my Mars placement. "Oh, that makes perfect sense, then," he told me. "That's what drew you to him in the first place."
To be clear, he didn't say that I'm with my partner solely because his sun and my Mars are both in Cancer. And he certainly didn't suggest that I'm attracted to every single Cancer who crosses my path. But he hinted that this coincidence could have played a role in the initial spark between me and my partner.
If you're looking for further "proof" that this theory works IRL, keep in mind that we're dealing with a single idea within an imperfect science — a lot of anecdotal evidence is about as much as you'll find. So, don't stress if you don't have an instant connection with someone whose sun sign matches your Mars sign — or, for that matter, if you're happily dating or sleeping with someone who turns out to have a completely different sun sign from your Mars sign. This is just one factor in the massive equation that is sexual chemistry. Plus, the heavens don't always work instantly, so give things time.