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Le 25/09/2018 à 07:39, Mike Horvath a écrit :
> Found a document with a table of values:
>
> https://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2009reprint.pdf
>
>
> The pertinent variables are alpha, delta and W. What do I do with these
> variables? Dunno for sure. This document provides a formula:
>
> https://depositonce.tu-berlin.de/bitstream/11303/6237/4/burmeister_steffi.pdf
>
Read start of 1.3, alpha, delta & W are defined there.
>
> I /think/ I need to rotate around the z-axis by W, around the x-axis by
> (90-δ), and around the z-axis again by (90+α), in that order.
W is the "day" part of the planet, adjusting the prime meridian.
Right-handed system, so, yes, W is for z-axis when applied first.
Otherwise, it would be applied along the rotation axis of the planet
once transformed from z by alpha & delta.
Figure 1.3 , page 11
From the point gamma on ICRF equator, starting sphere, the axis of
rotation of the planet is located at 90°+alpha, for an amount of
90°-delta. (a single tilting of the planet axis, along a single
perpendicular axis)
If I assert +x is gamma at start, +z north, you have to apply a rotation
along v_rotate( y, alpha*z) of 90°-delta, then a rotation of W along the
new pole axis.
You can of course transfer the W part before the other rotation, as it
is simpler to use z.
> Lastly, an
> additional rotation (around the x-axis?) by 23.43928 degrees must be
> done to get the body out of the ICRF frame and into the ecliptic frame.
That's a change of referential, the matrix should be well-known. (i.e. I
have no clue)
I do not know the x,y,z of ICRF(2) compared to the ecliptic plan and the
vernal point.
>
> But I still don't know the starting conditions. I.e. before applying the
> transformations, should the globe's North Pole point upward? Should the
> intersection of the Prime Meridian and Equator lie along the x-axis?
Yes. x2.
> Also, am I wrong, and should I perform the inverse matrix calculations
> instead? Either way, I have not gotten results that match what I see in
> Celestia for the same Julian Date.
>
>
> Mike
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