The Ecliptic Plane
The ecliptic plane is the geometric plane of Earth’s orbit around the Sun. From Earth, the Sun appears to trace a great circle along the sky called the ecliptic. The zodiac is measured along this path, which is why the ecliptic underpins chart calculation and sign longitudes.
Why the ecliptic defines the zodiac
- Solar path: Signs are measured by ecliptic longitude along the Sun’s apparent path.
- Planet clustering: Most planets travel near the ecliptic, so zodiac constellations frame visible motion.
- Coordinate conversion: Astronomy’s equatorial coordinates are converted to ecliptic coordinates for charts.
Equinoxes, solstices, and seasons
Earth’s axis is tilted ~23.4°. Where the Sun crosses the celestial equator we get the equinoxes (March and September). At maximum north/south declination we get the solstices (June and December). These four stations divide the solar year and anchor seasonal timing.