# Pluto's orbital inclination

1. May 7, 2007

### DaveC426913

I read in several places (including thenineplanets.org) that Pluto's poles are "almost perpendicular to its orbit". But the actual number I see is 122 degrees. So its polar axis is actually 32 degrees below its plane, right? (meaning that Earth sees the North pole of Pluto in its Southern limb) .

Add that to the plane of its orbit, and we get an angle of Pluto's polar axis compared to the axis of the solar system of more like 139 degrees.

Am I right?

And isn't it a bit inaccurate to say its axis of rotation is almost perpendicular to its orbit?

2. May 7, 2007

### cesiumfrog

Perhaps the first issue would be that pluto's orbit is very slow (the pioneer analomaly people would love to have recorded far more of pluto's motion), so it's necessary to distinguish statements about pluto "right now" from statements about pluto's "complete orbit".

Note the language is slightly ambiguous: At some point in it's orbit, it seems the Pluto's rotation axis will (in context) be exceptionally near parallel to orbital velocity, while earth's polar axis is always "almost perpendicular" to its orbital plane and orbital velocity.

Compared to other planets (the plane of earth's rotation is so well aligned with the plane of earth's orbit, you'd attribute it to solar system formation rather than coincidence), I think it is indeed reasonable to point out that those two planes are comparatively very perpendicular in the case of pluto.

That and astronomers know how to do proper uncertainties/statistics (often with small data sets: eg. 8 planets), and they're very accustomed to only expecting order-of-magnitude-ish precision.

3. May 8, 2007

### tony873004

Depending on which way the orbit is tilted relative to the polar axis, you might want to subtract the two instead of adding them. So it could be 122+17=139 degrees like you said, or 122-17=105 degrees, or any answer inbetween 139 or 105. "Almost" is a subjective term. I guess 105 would be "almost perpendicular".

4. May 9, 2007

### DaveC426913

I don't think this is so.* See attachment.

*(I can't be sure. I think I'm simplifying the number of planes.)

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Last edited: May 9, 2007