Topocentric
Topocentric measures every direction from the exact spot on the Earth's surface where the observer is standing.
The name is from the Greek topos, a place. Where geocentric positions belong to the planet, topocentric positions belong to a person.
This is the setting that uses your height
Every field in the location record finally does something here.
Geocentric calculation needs your latitude and longitude to build the local frame, but it measures directions from the Earth's centre, so your height never enters. Topocentric calculation measures from where you actually are, which means it needs to know how far from the centre that is — and that is latitude, longitude and height together.
This is the answer to a question the Locations category left open. The Height field exists for this setting, and for the handful of specialised calculations that reach for it directly.
What it changes
Almost nothing, and then one thing enormously.
| Body | Difference from geocentric |
|---|---|
| The Moon | up to about 1° — two full Moon-widths |
| Venus and Mars, at closest | well under an arcminute |
| The outer planets | a couple of arcseconds |
| The stars | nothing measurable |
That table is the previous page's parallax table with a different heading, because topocentric is the parallax correction. Selecting it is exactly the act of saying "apply the shift from the Earth's centre to my actual position".
For every body except the Moon, a topocentric chart is a geocentric chart with the numbers unchanged to the precision anyone reads them at.
When it is worth selecting
When the Moon matters and precision matters. A degree is large. If a lunar position is being used for anything where a degree would change the answer — a close aspect, a sign boundary, a house cusp the Moon is sitting near — the topocentric position is the one that describes what was actually overhead.
When the calculation is about a specific place seeing something. Eclipse circumstances, occultations, the exact moment a body cleared a horizon. These are questions about an observer, not about the planet, and they deserve an observer-centred origin.
When you want the honest answer to "where was it, from here?" Topocentric is the more truthful of the two Earth-based settings. It is not the traditional one, which is why it is not the default, but it is the one that describes the sky a person actually stood under.
When it is not
For comparison against published tables or other software. Those are almost universally geocentric. A topocentric Moon will disagree with them by up to a degree, and the disagreement is real rather than an error.
For anything but the Moon. The correction is below the noise. It costs nothing to apply and buys nothing either.
A caution about the location
Topocentric results are only as good as the location. A geocentric chart with a sloppy latitude still has the planets right; a topocentric one propagates the error into every position.
This is not an argument against the setting. It is an argument for entering locations carefully when you use it — and a reminder that the longitude sign trap has one more way to bite here than it does elsewhere.
What comes next
The third origin is not a refinement of the other two. It moves the vantage point ninety-three million miles, and what it changes is not a fraction of a degree — it is the contents of the chart.
Next: Heliocentric.