What an Origin Is
The point a direction is measured from, a question that sits underneath every position, and the one the coordinate origin setting answers.
Parallax
Why the same body points in different directions from different vantage points, and why the size of the shift depends on distance.
Geocentric
Measuring from the centre of the Earth — the default setting, and the one that does not need to know where the observer is standing.
Topocentric
Measuring from where the observer actually stands — the setting that uses your latitude, longitude and height, and makes a noticeable difference only for the Moon.
Heliocentric
Measuring from the centre of the Sun — where the Earth becomes a planet, the Sun and Moon leave the chart, and the bodies show their real orbital positions.
Retrograde Motion Is a Geocentric Effect
No planet ever reverses. Retrograde motion is what overtaking looks like from a moving platform, and it is absent when positions are measured from the Sun.
The Three Origins Compared
What each of the three vantage points describes, what differs between them, and why some calculations exist in only one of them.