Attn: Fridger, Guillermo & Other Celestia Physicists

[VikingTechJPL]

Having just learned that the "Capitals is screaming" convention extends to this Forum, I am duly editing the title of this thread.

I do not mean to scream at anyone—except maybe Fridger every now and then!

Just joking, Fridger.

[Fenerit]

Surely. Need more a certain sort of "advertisement" than a new metric. So to say: "is Venus in conjunction with Jupiter? And then why you want select Jupiter?"

[Fenerit]

I am not convinced about more complex, (i.e. more time consuming) versions of Light Travel (LT) delay...

Fridger

Bold mine.

FYI, whether it can interest someone, I call that "tempo luce", which in english is "light time", because its translation should be "percorrenza luce" which is both grammatically ugly and a no-sense in this contest. Since here is matter of relativity, and as the berliner philosopher Hans Reichenbach pointed out, what that is a measure, relativity do conceptualize it. Thus the language must express a concept; then "light travel" is "the time that light spend in travel a certain distance" which in italian is "il tempo che impiega la luce a percorrere una certa distanza". Too long to stay on screen.

[VikingTechJPL]

Greetings All,

Massimo, the question you asked two posts ago got me thinking about why I had applied Light-Travel-Delay to occultations involving two planets. And, because the times didn't agree, was the choice of one planet accurate and not the other? Now, I think I was wrong to apply Light-Travel-Delay at all for occultations for the following reason:

Since VSOP87 returns quite accurate planet positions (Right Ascension and Declination) "as seen over time from Earth", the theory must be taking into account the Light-Travel from each planet to Earth. If the positions of the planets appear to be correct "to our eyes" it should make no difference how far light had to travel. The theory inherently seems to correct for that, specifically because it is referencing an Earth-based observer. If this reasoning is correct, then:
The Good News: occultations "viewed from Earth" should be as accurate as VSOP87's truncation in Celestia allows, as long as we "do not" use Light-Time-Delay to view them;
The Bad News(which is not so bad really): due to how VSOP87 plus Celestia's "Instantaneous-Light-Travel" places planets slightly inaccurately in 3-D space, occultations viewed from another planet will not appear to occur at their proper times. This would not seem to be of a great concern unless: 1) someone wishes to simulate extraterrestrially-viewed occultations with extremely high accuracy and 2) you were trying to stage a satellite flight between two planets. As Chris says, if you need that kind of accuracy you would need SPICE kernals anyway.

Unlike occultations, viewing extraterrestrial eclipses (like Galilean ones) from Earth's surface "does require" using Light-Travel-Delay which, as Fridger confirms, does yield quite accurate eclipse times. Though the events in 3-D space (x, y, z) are slightly off due to a planet's slight misplacement, "time-wise" they are very close to actual because of the use of each planet's frame to adjust its moons' positions. Similarly, they "look right" relative to their parent planets.

If the above reasoning is correct, I'm glad that Global-Light-Travel-Time is not an issue for vieweing planet occultations from Earth. Though we now know that planet locations are off slightly in 3-D, in the total scheme of things GLTT may not return to become an issue again until more Celestia users need even more accurate 3-D space representations for increasingly sophisticated spacecraft orbits.

The bottom line for now seems to be:
1) for extra-terrestrial eclipses, turn LT on;
2) for occultations, turn LT off;
3) for GLTT, wait and see.

Regards,
Gary

[Fenerit]

I agree. Basically, who travel in search for optimal occultations/conjunction/oppositions displacements is the observer, not the light.