The Ecliptic
The ecliptic is the apparent path the Sun follows across the sky over the course of a year; in reality it is the projection onto the celestial sphere of the plane of the Earth's orbit around the Sun. It is the basic reference line for sky coordinates and for the zodiac belt.
| In brief | The ecliptic is the apparent path the Sun traces across the sky over a year, that is, the projection of the plane of the Earth's orbit onto the celestial sphere. |
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| Astronomical account | The ecliptic is the great circle on the celestial sphere followed by the Sun's apparent annual motion, and it represents the plane of the Earth's orbit. It makes an angle of about 23.4 degrees with the celestial equator; the points of intersection are the equinoxes. Because the orbital planes of the planets lie close to it, they all appear in the neighbourhood of the ecliptic. |
| Use in astrology | In astrology all planetary positions are expressed in ecliptic longitude; the 12 signs are 30-degree slices of this circle. A birth chart too is in essence a diagram of the plane of the ecliptic drawn for a particular moment. |
| Evidence status | The ecliptic is an astronomical reference that is measured precisely and forms the basis of celestial mechanics; its existence and its geometry are not in dispute. The layer of meaning astrology builds upon this circle, however, is not a system supported by observational evidence. |
| Common misconceptions |
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Apparent path, real plane
The Sun does not of course wander among the stars; we are the ones who wander. As the Earth moves along its orbit we see the Sun in front of different stars each month, and the line joining these positions forms the ecliptic. Its name comes from eclipses: solar and lunar eclipses happen only when the Moon comes close enough to this plane.
The geometry of the seasons
The Earth's axis of rotation is tilted about 23.4 degrees from the perpendicular (the normal) to the plane of its orbit; put another way, the angle between the plane of the equator and the plane of the ecliptic is about 23.4 degrees. The ecliptic therefore crosses the celestial equator at roughly the same angle. The points of intersection are the equinoxes, while the Sun's northernmost and southernmost positions on the ecliptic correspond to the solstices. The seasons are the result of this geometry; the common explanation in terms of distance from the Sun is not correct.
Why does it matter for astrology?
Astrology's sign slices are defined by longitudes measured along the ecliptic, and every planetary position in a chart is calculated with this line as the reference. The ecliptic is thus one of the rare grounds on which astronomy and astrology share the same measuring infrastructure; the parting of ways begins not in the measurement but in its interpretation. For the belt as a whole you can look at the Zodiac record.
Related terms: Zodiac, Constellation, Precession