← Encyclopedia of Astrology · lesson 2/235
Ecliptic
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What it is
Watch where the Sun stands against the background stars and, over a year, it seems to make one complete loop of the sky. That loop is the ecliptic. In reality it is the Earth that goes round the Sun, so the ecliptic is the plane of the Earth's orbit seen from the Earth — but for astrology, which describes the sky as it looks from here, it is the Sun's path.
The name comes from eclipse: it is only when the Moon crosses this line, at new or full Moon, that an eclipse can happen. The two crossing points of the Moon's orbit are the lunar nodes.
Why it matters
The ecliptic is the spine of the zodiac. The twelve signs are laid end to end along it, and a planet's position in a chart — "Mars at 8° Leo" — is its ecliptic longitude: how far along this line it stands, counted from 0° Aries.
The planets do not sit exactly on the line. Their orbits are tilted a little against the Earth's, so each one wanders a few degrees north or south of it; that distance is its ecliptic latitude. The Moon can reach just over 5°, Venus and Mars rather more near their closest approaches, and Pluto, with its steeply tilted orbit, about 17°. An ordinary chart wheel flattens all of this onto the circle and shows longitude only.
The tilt
The ecliptic is tilted against the celestial equator — the Earth's equator projected onto the sky — by about 23°26', an angle called the obliquity of the ecliptic. It slowly changes over many thousands of years and is decreasing at present.
That tilt is the reason for the seasons, and it fixes four of the most important points in astrology:
- where the ecliptic crosses the equator going north, the March equinox, 0° Aries in the tropical zodiac;
- where it crosses going south, the September equinox, 0° Libra;
- its most northerly point, the June solstice, 0° Cancer;
- its most southerly point, the December solstice, 0° Capricorn.
The same tilt gives each planet a declination as well as a longitude. A planet on the ecliptic can be at most about 23°26' from the equator; one that wanders beyond that is said to be out of bounds.
How it is used
Almost everything in a Western chart is measured along the ecliptic: signs, aspects, house cusps (which are points where various great circles cut it), the Ascendant (the degree of the ecliptic rising on the eastern horizon) and the Midheaven (the degree crossing the meridian). Declination, measured from the equator instead, is the second coordinate system astrologers use, and it is where parallels come from.
In the tradition
Ptolemy treated the ecliptic as the "oblique circle" through the middle of the zodiac, and the solstice and equinox points as its natural beginnings — which is why, for him, the signs start where the seasons do. deVore calls it the via solis, the way of the Sun.