[Kstars-devel] Fwd: Plans for threading in KStars

[Aleksey Khudyakov]

> There is no answer in Meeus, he only presents simplified versions, but
> precise enough for practical purposes. Precession has the main component
> a rotation in the ecliptic plane, but also a much smaller component of
> tilting the ecliptic plane. Nutation is a little more complicated, it
> has oscillatory terms.
>
In the end it's just a sphere rotation so it could be represented as quaternion
and it'll be same for all objects. It's surely non-trivial task to calculate it.


> The final question is the map drawing - do you need a final transform to
> spherical coordinates, or you can modify the drawing algorithm to use
> directly 3D Cartesian coordinates? On the plus side, it still avoids a
> lot of direct/inverse trig calls in the intermediate steps.
>
No. Most of projections are easier to compute from Cartesian coordinates.
Only equirectangular projection require trigonometry calls (asin, acos).

asin/acos are slower than sin/cos, I beleive they solve equation, but we may
take advantage of the fact that we don't need of full 15 digit precision as long
as deviation of projections from true location are less than say 0.1
pixel it's OK