The Three Origins Compared

There are three vantage points, but the distances between them are not of one kind. Two of them describe the same sky from points four thousand miles apart, and the third describes a different sky altogether.

The three, summarised

	Geocentric	Topocentric	Heliocentric
Measured from	Earth's centre	your exact spot	the Sun's centre
Uses your height	no	yes	no
Sun available	yes	yes	no
Moon available	yes	yes	no
Earth available	no	no	yes
Retrograde motion	yes	yes	never
Differs from geocentric by	—	up to 1° for the Moon, else negligible	everything

What each one describes

Geocentric describes the sky from a point nobody occupies, which is what makes its positions common property: one number for the whole planet. It is the frame of the astrological tradition, of every published ephemeris, and of any position quoted without qualification. The correction it leaves out is invisible for every body but one.

Topocentric describes the sky from a particular place on the surface. It is the more truthful of the two Earth-based frames, because it is measured from where an observer actually stood, and the difference it makes reaches a degree for the Moon and almost nothing for anything else. Questions about what a specific place could see are posed here: eclipse circumstances, occultations, a body clearing a horizon.

Heliocentric describes the planets' orbital relationships with the Earth's own motion taken out. It is not a more accurate version of the same enquiry; it is a different enquiry, and its output answers different questions.

What differs between them

Between Geocentric and Topocentric: nothing structural changes. The bodies and the calculations are the same, and the only meaningful difference is that the Moon moves by up to a degree. The two are directly comparable.

To or from Heliocentric: a great deal changes. The body list is rebuilt, so a selection containing the Sun, the Moon or the nodes does not survive the move, and retrograde phenomena vanish. Positions change by amounts that have nothing to do with precision, because they are answers to a different question.

The third column is a separate mode rather than a third point on a scale.

Where only one origin exists

Some calculations do not admit the choice at all, and the reason is always the same: the answer would be well formed and meaningless.

Heliocentric is the one that drops out, and there are three kinds of calculation it drops out of:

• Those whose subject is a geocentric appearance. Retrograde motion does not occur in a heliocentric frame, so a search for stations and retrograde cycles would have nothing to find in one.
• Those about an Earth-observed event. A lunar phase is how the Moon is lit as seen from here; a heliacal rising is about when a body first becomes visible to somebody standing on the ground. Neither survives being asked from the Sun.
• Those built on the angles and houses. The Ascendant, the Midheaven and the cusps derived from them are made from an observer's horizon and meridian, and the Sun's centre has neither. Anything keyed to them — primary directions, directions to the angles, house-based timing — has no heliocentric form.

Prediction as such is not on that list. Heliocentric transits and heliocentric planetary cycles are established techniques with a literature of their own. What does not survive the move to the Sun is the Earth-based frame a natal chart is built in, not the act of projecting forward from a birth moment.

Where fewer than three origins are offered for one of these, that is the reason, rather than any limitation of the calculation.

What a position leaves out

A position carries the same ambiguity about its origin as it does about its zodiac, but with smaller consequences.

Geocentric and topocentric positions differ enough to matter for the Moon and not enough to notice elsewhere, so a position separated from its origin is usually harmless and occasionally a degree wrong. Heliocentric positions are easy to recognise, because the Sun is missing from them, so they are hard to confuse with anything else.

The origin belongs with the chart that produced it. As with the zodiac, what carries no trace of it is a position that has travelled on its own.

What comes next

That completes three of the four settings. The fourth is the reference plane, which offers two options and looks like the narrowest of them. It decides which circle a position is measured along, and that choice can move the date of a transit by years.

Next: What a Reference Plane Is.