Ecliptic and Equatorial Compared
The difference between the two frames runs further than the difference made by any of the other three settings. This page sets out what changes between them and what does not.
What changes
| Ecliptic | Equatorial | |
|---|---|---|
| Measured along | celestial longitude | right ascension |
| Measured away | celestial latitude | declination |
| Coordinates called | longitude, latitude | right ascension, declination |
| Zodiac signs | shown | withheld entirely |
| Position format | sign coordinates or cycle angles | cycle angles only, in degrees |
| Parallels matched in | celestial latitude | declination |
| Transit dates | the conventional ones | can differ — by years for slow bodies |
The last two rows are the ones most easily overlooked, and each has a page of its own earlier in this category.
What does not change
The body's actual position. Nothing physical moves when the plane changes. The two frames are two descriptions of one unchanged sky, and moving between them is a change of measuring apparatus, not of the thing measured.
The location, zodiac and origin settings. They do exactly what they did. The plane is orthogonal to all three.
Anything horizon-based. The Ascendant, the Midheaven and the house cusps are built from your horizon and meridian and are not affected by which of the two sky-frames a position is reported in.
What the equatorial frame expresses
Declination. Parallels and contraparallels in the traditional sense are declination relationships, and declination is this frame's away-coordinate. The ecliptic frame has no way to state them.
The daily motion. Rising, setting, culmination and how long a body stays above the horizon are governed by declination and the observer's latitude. The sky turns about the celestial poles, and this is the frame built on them.
Right ascension as the timing measure. Some forecasting methods time transits by right ascension rather than by celestial longitude, so the positions they work from are stated in this frame. The two measures are not interchangeable: a body often reaches a given right ascension on a different date from the one it reaches the corresponding longitude, and for a slow high-latitude body that gap runs to years.
The vocabulary of astronomical sources. Star catalogues, ephemerides and observational software all work in right ascension and declination — bearing in mind that such a source will probably quote right ascension in hours where astrological software quotes degrees.
What the ecliptic frame expresses
The astrological tradition, entire. The signs are ecliptic, published ephemerides are ecliptic, and an unqualified position means ecliptic everywhere in the literature and in every other program. Output in the other frame does not contradict any of that; it is not commensurate with it.
Sign positions. They exist only in this frame, and not by omission: a sign name on a right ascension would be a claim about a circle the sign does not lie on.
Longitude as the timing measure. An ingress, an aspect perfecting, a transit to a natal degree — each is an arc along the ecliptic, and a date derived from one is an ecliptic date. It is commensurable with another date only where that one was produced in the same frame.
Where the distinction does not arise
Some calculations are defined on the ecliptic and nowhere else, so there is nothing left for the setting to decide. They are the ones built on natal chart structure — signs, houses, ecliptic arcs, a body's ingress into a house — none of which has an equatorial counterpart. Where software fixes the plane at Ecliptic for these, it is not a limitation of the calculation; it is declining to offer a setting that would have no meaning.
What a date leaves out
The same ambiguity closed the previous two categories, and here its consequences are largest.
A zodiac ambiguity is about 25°. An origin ambiguity is about a degree, for one body. A plane ambiguity can be years — and unlike the other two it leaves no signature in the output. Nothing about a date says which circle it was measured against.
Saved work generally keeps its own setting. What carries no trace of it is a date that has left the program: copied into notes, or set beside a date from somewhere else.
The four settings, together
That completes the four settings, and they can now be stated as one sentence each:
| Setting | Decides |
|---|---|
| Location | where you are standing — which fixes the shape of your sky and what time it is |
| Zodiac | where the counting starts along the ecliptic |
| Coordinate Origin | which point the direction to a body is measured from |
| Reference Plane | which circle the position is measured along, and away from |
None of them changes the sky. Each of them changes what the sky is reported as, so a position is fully stated only when all four are known.
Where to go from here
If any of it went past too quickly, the four categories are built to be read in order, and the introduction lays out the sequence.
The same four settings appear, under one name or another, in whatever ephemeris you use, and each of them now has a meaning.