Choosing a Reference Plane

Two settings, and a short answer: leave it on Ecliptic unless you are asking a question about declination or about the sky's daily motion.

Here is everything that moves when you do change it.

What changes

	Ecliptic	Equatorial
Measured along	celestial longitude	right ascension
Measured away	celestial latitude	declination
Column headings	Longitude, Latitude	Right Asc, 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 that catch people, and they have a page each behind them.

What does not change

The body's actual position. Nothing physical moves. The two frames are two descriptions of one unchanged sky, and switching 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 you are reporting positions in.

When to select Equatorial

For declination work. Parallels and contraparallels in the traditional sense are declination relationships, and declination is this frame's away-coordinate. This is the commonest good reason.

For questions about the daily motion. Rising, setting, culmination and how long a body stays above the horizon are all governed by declination and your latitude. The equatorial frame is where those questions are naturally posed.

For comparison with astronomical sources. Star catalogues, ephemerides and observational software all work in right ascension and declination. If you are checking Aquila's output against an astronomical reference, this is the frame that matches — bearing in mind that the reference will probably quote right ascension in hours where the applications quote degrees.

When not to

For ordinary astrological work. The tradition is ecliptic, the signs are ecliptic, published ephemerides are ecliptic, and every other astrological program means ecliptic by an unqualified position. Switching planes for no particular reason produces output that disagrees with all of them.

When you want sign positions. They are not available here, by design.

When comparing against your own earlier results, unless those were run in the same frame. A transit date from an equatorial run and one from an ecliptic run are not the same measurement, and for a slow high-latitude body they can be years apart.

When the setting is not offered

Some studies fix the plane at Ecliptic and hide the dropdown, because their calculations are defined on the ecliptic and nowhere else. 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. The absence is not a limitation of the calculation; it is the software declining to offer a setting that would have no meaning.

Recording the choice

The same warning that closed the previous two categories, and here it has the sharpest teeth.

A zodiac error is about 25°. An origin error is about a degree, for one body. A plane error can be years, and unlike the other two it leaves no signature in the output — a date is a date, and nothing about it says which circle it was measured against.

The plane is stored with every study and every chart, so saved work keeps its own answer. The risk is in what leaves the application: a date copied into notes, or compared against a date from somewhere else. Label it.

The four settings, together

That completes the four dropdowns, 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
Coord. Origin	which point the direction to a body is measured from
Ref. Plane	which circle the position is measured along, and away from

None of them changes the sky. All of them change what the sky is reported as — and knowing which is which is the difference between reading a chart and guessing at one.

Where to go from here

Back to the beginning, if any of it went past: the four categories are built to be read in order, and the introduction lays out the sequence.

Otherwise, to the applications themselves — Aquila and Compass — where the four dropdowns are waiting, and now say something.