What an Origin Is
Every position on this site so far has been a direction. The celestial sphere kept direction and discarded distance; latitude and longitude named a direction from the centre of the Earth; the zodiac measured a direction around a circle.
A direction has to be measured from somewhere. That somewhere is the origin, and the third setting lets you move it.
Three places to stand
The Coordinate Origin setting offers three:
| Setting | Measure from |
|---|---|
| Geocentric | the centre of the Earth |
| Topocentric | the exact spot on the surface where the observer stands |
| Heliocentric | the centre of the Sun |
The diagram shows one planet seen from three vantage points, with a different direction to it from each. None of the three is the true direction. Each is correct for whoever is standing at that point.
Why this is not obvious
The origin is the easiest of the four settings to overlook, because nothing in an ordinary position announces it. A longitude of 15° Taurus looks complete, but it is 15° Taurus as seen from somewhere, and moving that point moves the answer.
Compare it with the other settings. The zodiac changes where the counting starts. The reference plane changes which circle you count along. The origin changes something more fundamental — where the observer is — and therefore what there is to count in the first place.
What actually differs
Two things change when the origin moves, and they are worth separating.
The direction to a body changes. This is the effect the diagram shows, and it has a name, parallax, which is the subject of the next page. How much it matters depends on how far away the body is.
The set of bodies changes. This is less obvious, and the change is larger. Measured from the Sun, the Sun has no direction, because that is where you are standing. The Earth, which has no direction from itself, has one from the Sun. The list of things a chart can contain is not the same in all three settings, so moving the origin changes what there is to choose from.
Nearer bodies shift more
A rule of thumb to carry through this category: the origin matters in inverse proportion to distance.
Move your vantage point a few thousand miles and a nearby object visibly shifts against its background, while a distant one does not shift at all. The stars are so far away that moving right across the Earth's orbit, a distance of nearly two hundred million miles, shifts them by only a second or two of arc.
Across the kinds of body, that works out like this:
- For the stars, the origin is irrelevant.
- For the planets, moving between the two Earth-based origins changes very little; moving to the Sun changes everything.
- For the Moon, moving between the two Earth-based origins changes up to about a degree, which is a great deal.
That last line is the reason the Topocentric option exists at all.
What comes next
The shift in a body's direction when the vantage point moves is called parallax. The next page explains how it works, and why its size depends on distance.
Next: Parallax.