Parallax
Hold a finger up at arm's length and look at it with one eye closed, then the other. The finger jumps against the background. Bring it closer and the jump gets bigger; hold it further away and the jump gets smaller.
That is parallax, and it is the entire mechanism behind the origin setting.
Parallax is the change in an object's apparent direction caused by a change in the observer's position.
Distance decides everything
The two eyes in that experiment are a fixed distance apart, and yet the finger's jump varied. What varied was the distance to the finger.
Same baseline, two objects, two very different results. The near object swings across the background; the far one barely moves.
This is why parallax is a story about the Solar System's near neighbourhood and nothing else. The baseline available on Earth — at most one Earth radius from the centre to the surface, about 4,000 miles — is enormous compared with a room and utterly negligible compared with the distance to a star.
The numbers
Here is the whole of the practical content of this category, in one table. The shift is what you get by moving from the centre of the Earth to a point on its surface.
| Body | Typical shift |
|---|---|
| The Moon | up to about 1° |
| Venus, at its closest | around half an arcminute |
| Mars, at its closest | around a third of an arcminute |
| Jupiter and beyond | a couple of arcseconds at most |
| The stars | far too small to measure, even for the nearest |
A degree is two full Moon-widths. That is not a refinement — it is a visible, consequential difference, and it applies to exactly one body.
Everything else on the list is smaller than the width of the line a chart is drawn with.
Why the Moon is the exception
The Moon is close. About sixty Earth radii away, which means the baseline from the Earth's centre to its surface is a full sixtieth of the distance to the target — an enormous fraction by astronomical standards.
Two observers on opposite sides of the Earth genuinely see the Moon in different places against the stars. This is not a subtlety: it is why a solar eclipse is total along a narrow track and partial on either side of it. The Moon covers the Sun for observers in one strip and misses for observers a few hundred miles away, because they are looking at it from different places.
Any calculation involving the Moon and a specific location on the ground — eclipse circumstances, occultations, precise rising times — has to account for it.
Height contributes too
The surface is not all at the same distance from the centre. Standing higher lifts you a little further from the Earth's centre and adds a little to the baseline.
The contribution is proportional, and the proportions are unforgiving: Denver's mile is about one part in four thousand of the Earth's radius, so it adds just under an arcsecond to the Moon's shift. This is the arithmetic behind the claim made back in Height Above Sea Level — that height changes almost nothing.
Almost nothing is not nothing, which is why the field is there.
What comes next
Three origins, three vantage points. The first is the one nearly every chart ever cast has used, and the reason it is the default is not laziness.
Next: Geocentric.