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
Here every field in the location record is used.
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
For almost every body it changes very little, and for the Moon it changes a good deal.
| 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 Parallax page's table with a different heading, because topocentric is the parallax correction. The topocentric frame is that shift applied, from the Earth's centre to the place the observer is standing.
For every body except the Moon, a topocentric chart is a geocentric chart with the numbers unchanged to the precision anyone reads them at.
What it describes
The Moon, to better than a degree. A degree is large enough to close an aspect, cross a sign boundary, or move the Moon off a cusp it was sitting near. Where a lunar position is doing work at that precision, the topocentric one is what was actually overhead.
What a particular place could see. This covers eclipse circumstances, occultations, and the moment a body cleared a horizon. These are questions about an observer rather than about a planet, and an observer-centred origin is where they are posed.
The sky a person actually stood under. Topocentric is the more truthful of the two Earth-based frames. It is not the traditional one, which is why it is not the default, but it is the one that answers "where was it, from here?" literally.
Where the correction vanishes
For every body but the Moon it is below the noise: it costs nothing to apply and changes nothing that can be read at the precision anyone works to.
Why the Moon disagrees with published tables
Those tables, and other software, are almost universally geocentric. A topocentric Moon differs from them by up to a degree, and the difference is real rather than an error in either.
It inherits the location's errors
Topocentric results are only as good as the location they were built from. A geocentric chart with a sloppy latitude still has the planets right; a topocentric one carries the error into every position.
This is not an argument against the frame, but it does mean the longitude sign trap reaches further here than anywhere else: a wrong longitude moves the Moon as well as the angles and houses.
What comes next
The third origin, Heliocentric, is not a refinement of the other two. It moves the vantage point ninety-three million miles, to the centre of the Sun, and changes which bodies a chart contains as well as where they are.
Next: Heliocentric.