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

Post a reply to this message