As an astrologer, I love to consider a person’s natal moon phase—aka the phase that the moon was in when they were born—while reading their birth chart because it says so much about their innermost desires and life trajectory. (To look up your own moon phase, plug your birthday into a moon-phase calculator.) As TikTok would have it, this moon phase can also signal your compatibility with a potential or current partner….but, cosmically, there’s more to that story.

For the uninitiated, the TikTok trend involves using a video editor to superimpose photos of two people’s moon phases in order to see if they fit together like puzzle pieces, forming a complete full moon—not unlike those cute best friend heart necklaces of the early 2000’s. This is all visual poetry, even more spellbinding when set to the sound of “Every Summertime” by NIKI, which accompanies the trend.

While this easy “test” certainly oversimplifies astrological compatibility (more on that below), there is plenty you can learn about yourself and a partner based on your moon phases. And some of this intel could prove useful when considering your compatibility.

How moon phases can affect compatibility

Moon-phase compatibility may be a new trend, but it has its roots in ancient astrological practices. To explore its potential accuracy, it’s helpful to consider the different elements at play.

At a broad level, the moon in astrology represents the inner life, emotions, feelings, and the heart. It’s how we find happiness and security. Therefore, the moon absolutely impacts romantic compatibility.

In fact, in a compatibility reading, an astrologer will typically consider which of the 12 zodiac signs a person’s moon falls in to determine their deepest motivations, and which of the 12 astrological houses, to explore focus areas. And then they’ll compare this intel with the same information from the person’s partner.

Each moon phase reflects a particular emotional theme or undercurrent for people who are born under it.

From there, an astrologer might also consider both people’s natal moon phases, which is where the TikTok moon-phase compatibility trend comes into play. There are eight moon phases in total: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent. And each of these reflects a particular emotional theme or undercurrent for people who are born under it.

For example, someone born under a full moon might be emotionally expressive, while someone born under a waning crescent is more likely to embrace closure. First quarter and third quarter moons handle conflict best, but third quarter moons are more likely to adapt with internal shifts versus external actions.

By exploring moon phases, we can certainly gain rich vocabulary to describe what we like or don’t like about the way a partner processes emotions. But the TikTok trend goes a step further, promising that certain moon-phase combinations—the ones that puzzle together neatly to form a full moon—are bound to reflect soulmates and that the rest reflect couples who aren’t meant to be together. In practice, though, things aren’t so clear-cut.

Why the TikTok moon-phase compatibility test falls short

While there’s obvious appeal to such a quick “yes” or “no” answer on the question of compatibility, I find that assessing compatibility comes with a lot more gray area. Certain birth-chart combinations do seem to portend lots of trouble, whereas others hold a ton of starstruck potential, but most actually fall somewhere in the middle. And the people involved might wind up compatible or… not so much based on which beneficial aspects in their charts prove most appealing and which challenging themes turn out to be most problematic.

That’s all to say, even the four moon-phase pairings destined to be perfect fits in the TikTok compatibility trend aren’t without their drawbacks. Below, I’ve broken down the general upsides and downsides to each:

• New moon and full moon: The dreamer (new moon) and the doer (full moon) often balance each other out—but the full moon’s high-energy vibe could also overpower the new moon’s calmer, meditative nature.
• Waxing crescent and waning gibbous: The rising star (waxing crescent) and the mentor (waning gibbous) share a preference for growth and ease. But in some cases, the thoughtful waning gibbous could grow frustrated with the waxing crescent’s carefree curiosity and spontaneity.
• Waning crescent and waxing gibbous: The thinker (waning crescent) and the coach (waxing gibbous) share an emphasis on transformation, but tensions could arise from the waning crescent’s need for internal processing.
• First quarter and third quarter: The activist (first quarter) and the philosopher (third quarter) both have a high tolerance for tension and may lean into conflict. But while the first quarter seeks to produce external change, the third is primarily working through an internal change of heart.

The potential points of compatibility (and incompatibility) in these and other moon-phase pairs also depend on other elements of both people’s birth charts. Each of us contains multitudes, and our moon phases reflect just one way—among countless ways—to determine compatibility.

Other major relationship markers on the birth chart include the element (fire, earth, air, or water) of each of your personal planets, and especially Venus, the planet of love. For instance, if you have Venus in Aries and your partner has Venus in Leo, that means you both have Venus in the fire element—so, the different ways you express love are likely to work together. Another potential sign of fated chemistry? Any planet (but especially the sun or moon) located in the same sign as a partner’s north node (a point in the sky associated with destiny).

The added allure of moon-phase compatibility over the above is that it’s simple to grasp, regardless of astrology knowledge; after all, moon phases are easily identifiable in the night sky. And given the astrological relevance of a person’s moon phase for their emotional vibe, there’s certainly no harm in trying out the TikTok trend with a current or prospective partner, just to see if it might hold true in your life. If so, keep it close to your heart, and bring it up the next time cupid strikes. It’s thought that the moon exists to illuminate life’s great mysteries.