Choosing an Origin

Three vantage points, and a short answer: leave it on Geocentric unless you have a reason.

The reasons are worth knowing, and so is what happens when you act on them.

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

Which to use

Geocentric for essentially all astrological work. It is the tradition, it is what every published ephemeris and every other program means by an unqualified position, and the correction it leaves out is invisible for every body but one.

Topocentric when the Moon's position matters to better than a degree, or when the question is explicitly about what a particular place could see — eclipse circumstances, occultations, a body clearing a horizon. It is the more truthful Earth-based setting; it is simply not the conventional one.

Heliocentric when you want the planets' real orbital relationships with the Earth's own motion taken out. It is a different kind of enquiry rather than a more accurate version of the same one.

What switching costs you

Between Geocentric and Topocentric: nothing structural. The same bodies, the same studies, the same everything, with the Moon moved by up to a degree and nothing else meaningfully touched. You can switch freely and compare.

To or from Heliocentric: a great deal. The body list is rebuilt, so a selection containing the Sun, the Moon or the nodes cannot survive the move, and the applications will rebuild it for you. Retrograde phenomena vanish. Positions change by amounts that have nothing to do with precision — they are answers to a different question.

Treat the third column as a separate mode rather than a third point on a scale.

When the dropdown will not offer all three

Some studies restrict the choice, and every restriction is the same idea: the software declining to compute something that would be well-formed and meaningless.

Heliocentric is the option that gets withheld, and there are three kinds of study that withhold it:

• Those whose subject is a geocentric appearance. Retrograde motion does not occur in a heliocentric frame, so a study of 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.
• The predictive techniques. These are built on a natal chart and its angles, which are Earth-based constructions with no heliocentric counterpart.

If the option you expected is absent, that is the reason. It is not a limitation of the calculation.

Recording the choice

The same warning as the zodiac, with a narrower blast radius.

Geocentric and topocentric positions differ enough to matter for the Moon and not enough to notice elsewhere, so a mislabelled position is usually harmless and occasionally a degree wrong. Heliocentric positions are unmistakable — the Sun is missing — so they are hard to confuse with anything.

The origin is stored with every study and every chart. As before, the risk is in what leaves the application.

What comes next

Three settings down. The last one looks like the narrowest of the four — two options, a technical-sounding name — and it reaches further than any of them.

It decides which circle a position is measured along, and it can move the date of a transit by years.

Next: What a Reference Plane Is.