What the Vertex is
The Vertex is a calculated point where the ecliptic crosses the prime vertical in the west. The prime vertical is a great circle in the sky that runs from due east, straight overhead, to due west.
Put simply, the Ascendant marks where the zodiac meets your eastern horizon, and the Vertex marks where it meets this east-west circle on the western side. Its opposite point is called the Anti-Vertex.
Why it usually sits in houses 5 to 8
Because the Vertex is defined on the western side of the sky, it nearly always lands in the western half of the chart wheel, which in most charts means somewhere from the fifth house to the eighth.
That placement is part of why astrologers link it with other people. The western half of the chart, around the Descendant, is traditionally associated with partners, meetings and relationships rather than with the self.
Sensitive to both time and place
The Vertex depends on your exact latitude as well as on local sidereal time, so it shifts quickly as Earth turns. A small change in birth time can move it noticeably, and the same clock time in two different cities gives two different Vertex degrees.
Without a birth time it cannot be calculated at all. If your time is rounded, treat the degree as approximate and pay more attention to the house than to the exact minute of arc.
How astrologers interpret it
The Vertex is often described as a point of encounters that feel fated or unplanned, people and situations that seem to arrive from outside. Its sign is read as the flavor of those meetings, and its house as the area of life where they are said to cluster.
Relationship astrologers pay attention when one person's planets fall on another person's Vertex. That is a reading technique, not a promise about any relationship.
Behind the calculation
We convert your birth time to UTC with the historical time zone of your birthplace, then use Astronomy Engine to find local sidereal time and the tilt of Earth's axis. From those, plus your latitude, we work out where the ecliptic crosses the prime vertical in the west.
For context, compare the Vertex with your Ascendant on the rising sign calculator. To test it against another person, a synastry chart lays both charts together, and your birth chart shows the full wheel.