Geocentric
Geocentric measures every direction from the centre of the Earth.
It is the conventional default, and the frame nearly every chart ever cast has been computed in.
Nobody stands there
The obvious objection comes first: no observer has ever been at the centre of the Earth, so geocentric positions describe a view nobody has had.
That is true, but it is not a defect. The centre of the Earth is chosen because nobody is there: it is the one point on the planet that does not belong to any particular observer. A geocentric position is a statement about where a body is, agreed by everyone, rather than a statement about how it looked from one back garden.
Compare the two Earth-based frames:
| Geocentric | Topocentric | |
|---|---|---|
| Measured from | the Earth's centre | your exact spot |
| Needs a location | no | yes |
| Same for everyone | yes | no |
The geocentric position of Mars at a given instant is a single number. The topocentric position is a different number for every point on the planet.
Where the location still matters
A geocentric origin does not make the Location setting irrelevant, although it is easy to assume that it does.
The origin decides where a body's direction is measured from. It has nothing to do with the local frame — your horizon, your zenith, your meridian — which is still built from your latitude and longitude as the Locations category described.
So a geocentric chart still needs a place, because:
- The Ascendant and Midheaven come from your horizon and meridian.
- Every house cusp is derived from those.
- Rising and setting times are local by definition.
What Geocentric drops is the parallax correction. Everything else about the location is used as it always was.
Why it became the default
There are three reasons, in descending order of importance.
The correction it omits is almost always negligible. As the previous page established, moving from the Earth's centre to its surface shifts a planet by a fraction of an arcminute. For every body except the Moon, geocentric and topocentric positions agree to well within the width of a chart's own lines.
It is the tradition. Every ephemeris, every table, every text and every astrological convention of the last two thousand years is geocentric. A position quoted without qualification is geocentric, and output that matches the tradition is output that can be compared against it.
It is stable. A geocentric position does not depend on a location being entered correctly. If the latitude has a sign error in it, the planets are still right; only the angles and houses are wrong. That is a smaller failure, and an easier one to spot.
What it cannot tell you
Geocentric has one limitation, and a later page in this category, Retrograde Motion Is a Geocentric Effect, is given over to it.
Geocentric positions describe the sky as it appears from here, which includes appearances that are artefacts of the vantage point rather than facts about the bodies. The most conspicuous of these is retrograde motion, and it is not a real reversal of anything.
Seeing that clearly requires measuring from somewhere else.
What comes next
The next page takes the first of the two alternatives, Topocentric, which moves the origin about four thousand miles, from the centre of the Earth to the ground under your feet.
Next: Topocentric.