← Reference · 01 of 07

How to read the wheel

A birth chart is a map of the sky from one place at one moment. Everything on it is a position, and every position is checkable.

The circle is the ecliptic — the plane of the Earth's orbit, which from here looks like the path the Sun traces through the year. It is divided into twelve 30° signs starting at the March equinox. That starting point is why this is called the tropical zodiac: it is anchored to the seasons rather than to the constellations, which have drifted about a sign away over two thousand years.

The bodies are placed by ecliptic longitude, measured in degrees from that same equinox point. When the chart says the Sun is at 23°30' Pisces, it means 353°30' along the circle. Nothing about the placement is interpretive; it is where the object was.

The wheel is rotated so the Ascendant — the degree rising over the eastern horizon — sits at the left. That rotation is what turns a sky map into a chart for a person: it depends on the exact minute and the exact place, and it is why a birth time is worth so much more than a birth date.

The lines across the middle are aspects: pairs of bodies separated by particular angles. A trine is 120°, a square 90°, an opposition 180°. They are drawn when the separation falls within an allowed margin, called the orb, and the tighter the orb the stronger the claim.