Re: precession
Posted: 30.01.2002, 10:05
[ this is in reply to a post on the old board ]
dramsey wrote:
> Ayup, it doesn't do precession. For the simple reason that continually recomputing the position of 100.000 * stars for every redisplay we do (25 a second if you're > > getting a good screen rate), is NOT something you want your computer doing.
I do not think this is correct. After all, the program is not calculating a geocentric model
of the solar system. (which would be an interesting, if academic, exercise: think epicycles)
Of course, the "position" of the stars, at least the ones close enough
to be visible has to be recomputed any time you press a cursor key, but this is something
I very much do want my computer to be doing. The accounting for precession would be one
simple operation, namely aligning the earth axis with a point on a circle. And this only if
earth is close enough to be rendered as more than a dot or if "sync orbit earth" is on.
>Also, over that timeframe, stars also move acoss the starfield, and computing those positions is NON-trivial to impossible as the motions aren't really known very
>well. So, Celestia just doesn#t even attempt to do it.
this on the other hand is in agreement with what I have supposed: I have combined the two
issues in one post because I was thinking about long-(millennia)-time issues. Here, I very
much agree it cannot be done because the vectors are not known. I don't think it would
matter very much performance-wise, since the positions of the stars would only need to be
updated very sporadically.
cheers,
DAB
dramsey wrote:
> Ayup, it doesn't do precession. For the simple reason that continually recomputing the position of 100.000 * stars for every redisplay we do (25 a second if you're > > getting a good screen rate), is NOT something you want your computer doing.
I do not think this is correct. After all, the program is not calculating a geocentric model
of the solar system. (which would be an interesting, if academic, exercise: think epicycles)
Of course, the "position" of the stars, at least the ones close enough
to be visible has to be recomputed any time you press a cursor key, but this is something
I very much do want my computer to be doing. The accounting for precession would be one
simple operation, namely aligning the earth axis with a point on a circle. And this only if
earth is close enough to be rendered as more than a dot or if "sync orbit earth" is on.
>Also, over that timeframe, stars also move acoss the starfield, and computing those positions is NON-trivial to impossible as the motions aren't really known very
>well. So, Celestia just doesn#t even attempt to do it.
this on the other hand is in agreement with what I have supposed: I have combined the two
issues in one post because I was thinking about long-(millennia)-time issues. Here, I very
much agree it cannot be done because the vectors are not known. I don't think it would
matter very much performance-wise, since the positions of the stars would only need to be
updated very sporadically.
cheers,
DAB