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Celestial equator
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What it is
Imagine the Earth's equator pushed outward until it meets the dome of the sky. The great circle it draws there is the celestial equator, also called the equinoctial. It divides the sky into a northern and a southern half, just as the terrestrial equator divides the globe, and it sits directly overhead for anyone standing on the Earth's equator.
Because the whole sky appears to turn about the Earth's axis once a day, the celestial equator is the circle of that daily turning. Stars on it rise due east and set due west, and spend exactly half the day above the horizon and half below, wherever on Earth you are.
Two circles, one tilt
Astrology works with two great circles at once. The ecliptic is the Sun's yearly path, along which the zodiac is laid. The celestial equator is the Earth's daily circle. They are tilted against each other by about 23°26', and they cross at two points:
- the March equinox, where the Sun passes from south to north of the equator — 0° Aries in the tropical zodiac;
- the September equinox, where it passes from north to south — 0° Libra.
On those two days the Sun is on the equator, day and night are roughly equal everywhere — hence equinox, "equal night" — and the Sun's declination is zero. Halfway between, at the solstices, the Sun reaches its greatest distance from the equator, about 23°26' north or south.
What is measured from it
The celestial equator is the reference circle for a second set of coordinates, alongside zodiacal longitude:
- Declination — how far a body lies north or south of the equator, in degrees. This is what parallels and contraparallels compare, and what decides whether a planet is out of bounds.
- Right ascension — how far round the equator a body lies, counted eastward from the March equinox, often in hours and minutes rather than degrees.
Right ascension matters behind the scenes. The Midheaven is found from the right ascension of the meridian (sidereal time), and most quadrant house systems, Placidus and Regiomontanus among them, are built by dividing time or space along the equator before projecting the result back onto the ecliptic. Primary directions, the oldest Western timing method, are also measured along it.
How it is read
Few astrologers read the equator as a symbol in its own right; it is a frame of reference. But it brings three things into a reading:
- The equinox points. The degrees where the ecliptic meets the equator, 0° Aries and 0° Libra, together with the solstice points 0° Cancer and 0° Capricorn, are treated as sensitive degrees — the so-called world points — especially in mundane astrology, where the Sun's ingress into Aries is cast as a chart for the year.
- North and south. A planet's declination says how high or low it rides, and by extension how strongly some astrologers read its expression.
- A second kind of contact. Two planets at the same distance from the equator are linked by declination even when the wheel shows no aspect between them.
In the tradition
Ptolemy begins the signs from the equinoxes and solstices — the places where the ecliptic and the equator meet or stand farthest apart — and so the celestial equator is written into the very definition of the tropical zodiac. deVore notes the older name, the equinoctial circle.